At the time I wrote that article, there was also a tool available from Helicon focus that would do the same thing automatically and then use a contrast detection algorithm to determine whether the image was maximally sharp (i.e. dead on in focus). Unfortunately they seem to have removed that feature from their software.

Likewise, a new app has shown up on the scene Reikan’s FoCal. FoCal seems to be doing the same thing that Helicon Focus’s software did, though it seems to do it using a special target. Do note, as of the writing of this I haven’t actually tested FoCal.

I’m not one to get in the way of other people’s businesses, but with the economy and my business the way it is, I have more spare time than spare change; if I can work out a solution that costs me time and not money well that’s what I’m going to do. Which brings me to this post.

Theory

The traditional AF calibration process goes something like this:

1. Set up target
2. Align camera
3. Manually focus to infinity
4. AF on the target take an image
5. dig through menus to get the the AF settings, change the setting slightly
6. repeat 3-5 until you’ve either covered all the necessary adjustments or until you find one that works best

However, the AF micro adjusts are essentially just saying to the camera, “when you think it’s in focus, move the lens x steps more”. The whole refocus to infinity and repeat is largely a product of not being able to simply command the AF system to move in small steps from the camera, which is what the live view solution does.

The unassisted live view method is a slight improvement over the unassisted method, in that the live view method only requires one manual focus and AF operation, but requires you to manually advance the focus in EOS Utility (or whatever camera control software you use) and keep track of the number of times you click the button.

Automating Live View Capture

My fist thought was, hey I’m not a useless programmer, I can grab the SDK from Canon and just write up a nice little C# app that does all the camera control stuff for me. Then I realized that the Canon SLR SDK is written in C not C# so instead of just getting down to business, I would have to write a wrapper then write the app. Okay scratch that idea.

Then I remembered AutoHotKey. AutoHotKey is a generic macro program that mimics keyboard and mouse inputs following a script. If it sounds familiar to readers of this site, it’s what I used to work around rather annoying problem I was having with my Wacom tablet drivers crashing. While AutoHotKey can’t do any form of image analysis, it could at least remove the work in capturing the images, and that’s enough for me for now.

Caveats and Prerequisites

There are a couple of caveats with this whole solution.

1. It only works with Canon cameras, specifically those with Live View.
2. You must have Canon’s EOS Utility installed.
3. You need AutoHotKey_L installed (tested with version 1.1.05.06).
4. You can’t do anything else while the process is running.

The first 3 are pretty straight forward; number 4 is a gotcha. Because this script emulates keyboard and mouse actions, if you start using the keyboard and mouse while the script is running it won’t be able to send the right actions to the right program. I’ve tried to make it as fool proof as possible, but there’s only so much I can do with these tools. As it stands it shouldn’t take more than about 2-6 minutes depending on whether you run the fine or course capture process.

Also while you can’t calibrate those cameras with Live View but without AF Micro-adjusts (like the EOS 40D, or the newer Rebels) you can generate the image stack to see if they are focusing correctly.

Procedure

After insuring you meet the prerequisites and having the software installed, download the Zip file from the link in the box to the right, and extract it somewhere you can get to (like your desktop). Then follow the steps for building the image stack.

1. Connect the camera to your computer via the USB cable.
2. Setup and align your camera and autofocus target as you normally would.
3. Set the camera to manual exposure mode.
4. If testing a zoom lens, zoom to the most common focal length you use, alternatively you can repeat the procedure for various focal lengths and use the micro-adjust that best matches all the focal lengths.
5. Set the aperture to it’s widest setting (i.e. f/2.8 on an f/2.8 lens)
6. Set the ISO to 100
7. Adjust the exposure using the shutter speed so that the camera meters correctly (I usually meter at +1/3 to +2/3rds given my environment tends to be on average lighter than 18% gray.
8. Start Canon’s EOS Utility
9. Make sure that EOS Utility has brought up the remove control panel and is communicating with the camera.
10. Double click on the AF Test.ahk script you extracted from the zip file.
11. Follow the onscreen prompts from the script.
12. When everything is done, the script will automatically close the EOS Utility live view window and display a notice that capture is complete.

Again, while the capture is running it’s important that you don’t try and do something else on the computer.

The output of the capture will be stored in the director you’ve configured EOS Utility to save images to. The first image will be the one made to set the AF system, you can delete this if you wish, it’s extraneous. The remaining 11 or 41 images are the focus stacks.

If you chose to use the course steps, you’ll have 11 files that correspond in order to micro adjust settings of -20, -16, -12, -8, -4, 0, 4, 8 12, 16, and 20. If you chose fine you’ll have 41 images corresponding to -20 to +20 in 1 step increments.

Interpreting the Results

The easiest way to interpret the results is to load the images into your favorite RAW viewer and page though the images. The one that looks most “on” will be the image that you want to make note of. You just have to count what image that is and convert that to the correct AF micro-adjust step. You can use the tables below to convert the image’s sequence number to an AF Adjust. (Note, this assumes you don’t count or deleted the first image taken.)

After dialing in the AF adjust, take a few manual test images to insure that it is correct. Again, due to the way this script works, it’s possible that a command may not be relayed properly for any number of uncontrollable reasons.

As always, if you find a bug, please leave a comment here, or email the contact address in the script, and I’ll try and address it.

Instead what I’ve found works best is to shoot shorter exposures of each of the various elements, then composite them together in Photoshop.

Shooting the Display

In my experience, there’s no trick to in shooting a fireworks presentation. In general:

• Use a Tripod
• Use a cable release
• Set the camera to the lowest ISO, 100 or 200
• Use an aperture between f/8 and f/22 depending

The only real trick is balancing the aperture and exposure time, with the brightness of the actual fireworks. This is something that usually takes me the first couple of frames to get right.

Too wide of an aperture and bright fireworks, or most if they’re near buy, will blow out. Additionally, it will make any smoke illuminated by the fireworks brighter; which, in my opinion, is undesirable. Too narrow of an aperture and the dimmer, usually blue, fireworks will not reproduce well.

Usually it takes me a couple of shots to get the exact settings dialed in, which is usually okay when you consider  that the first couple of bursts typically aren’t the meat of the show.

The other major consideration is if you’re trying to expose for a foreground element, like a building. In which case your range of viable exposure values is considerably more limited. Then again, if you composite the shot like I do, you can make a proper exposure for the foreground before or after the display and just composite it in.

Processing

My workflow starts by bringing the images I shot into Lightroom. I’ll make minor exposure tweaks, usually bringing the blacks up to remove sky glow from the horizon. At this point it’s less than ideal to make too many changes in develop, though it’s possible some additional changes will be necessary before the image is complete, many things can be fixed up in Photoshop without serious issues.

After I’ve made my minor corrections and decided on the images I want to composite, it’s time to start the compositing.

1. Using control (command) select the images in the grid that you want to composite.
2. Right click on one of the images and select Edit In→Open as Layers in Photoshop
   
3. Set up the base layer
   1. For an image with a background/foreground,
   1. Move the best exposed background/foreground image to the bottom most layer
   2. For an image with no background/foreground
   1. Make a new layer
   2. Fill it black
   3. Place it as the lower most layer in the document.
4. Set the upper firework layers’ blend modes to screen.

At this point, if smoke wasn’t too bad to start with, or minimizing it isn’t desirable, you’re pretty much done.

However, in my experience there’s usually quite a bit of smoke that needs to be removed, or at least reduced. This is especially true on calm days or for demonstrations that have a large proportion of low-level material.

In this case, I’ve found the easiest way to do that is to adjust the layers blend mode, specifically the black side of the “This layer” slider.

The finished results, with and without the added step to reducing the smoke in the scene.

Who knew?

Apparently, not me, that’s for sure.

That’s not to say that I didn’t know about focus stacking. I’ve played with the product Helicon Focus, and while I didn’t ultimately find it worth the money based on my needs, it does have some nifty features. Nevertheless, that’s not what this is about…

Why Choose focus stacking over stopping down?

Three reasons:

• More depth of field than you can get even when stopped down as far as the lens will go.
• More control over bokeh than you can get by stopping down.
• More depth of field without giving up optimal sharpness.

The first case is the typical case presented for using focus stacking. The you’re stopped down to f/22 but you still don’t have enough depth of field for the shot. Typically, this is combined with macro photography, where depth of field, even at tiny apertures, is incredibly shallow.

The real interesting case, for me, is the second one. This is the case the partially replaces the need for a tilt shift lens, but lets you keep smooth out of focus backgrounds where you want them.

While most people think of depth of field as how much will be in focus, you can also think of it as controlling how fast something will become out of focus as the distance between it and the point of focus increases, and simultaneously controlling how out of focus something will be.

I think the photos below demonstrate this quite nicely. The objective, as far as I’m concerned, is to get a completely in focus flower on as clean of a background as possible.

Shooting at f/16 yields enough depth of field to get the flower completely in focus, but the background becomes extremely busy and distracting.

Conversely, shooting at f/2.8 the background has been cleaned up, but there isn’t enough depth of field to get the whole flower in focus.

Combining multiple images shot at f/2.8 with slightly shifted focus points, however, has the effect of increasing the depth of field on the subject while simultaneously keeping the bokeh quality of a fast aperture shot.

Problems with Focus Stacking

Like everything in photography, focus stacking is a tool that can help in some situations and may not work at all in others. It’s simply not the end-all be-all solution to depth of field problems.

Much like stitched panoramas, the stacked images are brief snaps in time taken over a much longer time than even a single stopped down exposure. As a result, moving subjects can be repeated, broken, or the whole process may not be possible. Ultimately, this means that like stitched panoramas, focus stacking is inherently limited by the subject.

Additionally focus stacking costs some resolution. This is due to the need to align and correct images slightly to account for the shifts in position. In fact, these shifts are completely unavoidable; as shifting, the focus will slightly alter the subject’s magnificent. How much resolution is lost depends on the image, the lens used, and whether the background is conducive to repair or extension in post processing.

For example, the images in this article were shot with a Sigma 150mm f/2.8 macro lens on a Canon EOS-1D Mark 3. After alignment and cropping, the 10.1 MP source images ended up producing a 8.9 MP file.

The final consideration is that if you’re doing this in the field, there’s no way to automate the image collection with current SLRs.

The 30s Tutorial for Lightroom and Photoshop Users

After importing your images into Lightroom, select the images that you want to stack, right click one of them, and select “Open as Layers in Photoshop…”

Once the images have been loaded into Photoshop there are 2 steps. First select all the layers and click Edit -> “Auto-Align Layers…” when the Auto-Align Layers dialog box comes up you make sure that Geometric Distortion is checked. Click OK and let PS do its thing.

After PS has aligned the layers, go to the Edit menu and pick “Auto-Blend Layers…” This will bring up the layer blend dialog, which has all of 3 options, create a panorama, stack images, and a check box for seamless tones and colors. Since this is a stack, we want Stack images. Additionally, it’s probably a good idea to keep the “Seamless Tones and Colors” option checked, as that will correct any minor issues in color variations between the stacked images. Again, click OK, and let Photoshop do its thing.

The catch, if you want to call it that, is that the rule only applies to 35mm film, and by extension full frame or FX digital. For crop cameras, the rule’s shutter speed will be too slow. Instead, you need to use the 35mm equivalent focal length. Unfortunately, that also means doing rather complicated math in your head while shooting. If you ask me, that’s not an ideal situation.

Doubling the Focal Length

One solution is to simply double the focal length. Doing this will result in a safe shutter speed for every major brand of SLR from Canon’s APS-H sensors at 1.3x crop to Olympus’s Micro-4/3rds sensors at 2x. It has to main advantages:

1. It makes the math easier. Doubling a number is a lot easier than trying to multiply by 1.5 or 1.6.
2. It builds in some extra cushion for most cameras, making camera shake even less likely.

On one hand, it’s safe. Since, in all cases but micro-4/3rds, the shutter speeds are higher than needed as prescribed by the rule there’s even less of a chance for camera shake to be an issue.

However, you’re giving up precious shutter speeds for the sake of “safety”. This is doubly the case if you know from experience you’re stable enough to shoot at or slightly below the suggested speed.

This brings me to when we really care about the hand holding rule of thumb. Let’s be honest, it’s not when we’re metering 1/500^th with a 50mm lens. It’s those cases where we’re trying to get every bit of shutter speed in the dark and still have a sharp picture. In these situations loosing 1/3 to 2/3rds of a stop using an overly save calculation can be a problem.

There is a tidy solution though…

Use the Camera as a Calculator

Due to an amazing twist of luck, there’s a way to use the camera as a calculator and get numbers that are far more accurate without doing any math at all. The trick works because sensor sizes correspond very closely to fractional-stop exposure increments. Whether that was a consideration when the camera companies were picking sensor sizes, I don’t know, but it’s certainly very useful.

By dialing in exposure compensation to adjust the meter exposure, you can quickly see if the meter exposure is fast enough to meet the suggested rule of thumb.

Here are the adjustments:

Working this way you can quickly check when you’re in the danger zone, without spending a lot of time trying to figure out what the exact numbers are. Even better, it becomes second nature to do it in your head, once you become familiar with 1/3^rd stop shutter speed steps between say 1/30^th and 1/250^th and you’re not memorizing anything that isn’t generally applicable anyway.

Putting it into practice:

1. Meter the scene.
2. Dial in negative exposure compensation from the list above.
3. Read the shutter speed listed:
   1. If the shutter speed is faster than the focal length, you’re good.
   2. If not you’re in shaky image country.

That’s it, no math, no multiplying, and no memorizing equivalent focal lengths.

For example, suppose you’re using an EOS-1D and have a 85mm lens on it. Looking though the viewfinder and you see the metered shutter speed is 1/100^th. One click of negative exposure compensation and the camera meters 1/80^th now. 1/80^th is right on the line and I know from experience not quite enough for me to be comfortable assuming the image will be shake free.

As another example, suppose you’re using a 7D with a 24mm lens and the meter is showing a 1/40^th shutter speed. Since it’s an APS-C (1.5/1.6 crop) camera, dial in -2/3rds or 2 clicks on the rear dial. The meter will show 1/25^th, which larger than 24, so you’re good shake wise, if only just.

The nicest part of this technique to me, though, is that it doesn’t require doing mental math or trying to memorize something that isn’t generally applicable (i.e. 35mm equivalent focal lengths). Shutter speeds are something that you either can see in the viewfinder, or will eventually memorize simply though experience using and seeing them. Once you’re familiar with the shutter speed steps, it’s possible to check yourself without even adjusting the camera’s settings and do so intuitively.

Finally, this technique is applicable as long as you can translate a crop factor into fractional stops. For example, if you ever find yourself shooting medium format, you can take the rule with you, using +2/3rds for 645-format (~1.6x larger frame than 35mm/FX digital) to the same effect.

Simple put a scrim is a sheet of diffusing material stretched over a frame. Simple enough; however, it’s one of the most versatile light modifiers you can have. Better yet, when it comes to bang for your buck, you can build one for less than $20 and about 30 minutes.

Scrim Material

Good scrim material should be translucent fabric or plastic that is of a uniform density. Commercial scrims typically use lightweight white rip-stop nylon or some similar material. Rip-stop Nylon can be had from most fabric stores in 58-60” widths and as long as you need, though compared to other options at $5-10 per yard, it’s not the cheapest solution.

A cheaper alternative, at least if the scrim needn’t be larger than about 70×70 inches is a while shower curtain liner. A 70×70 inch white shower curtain liner can be had for $5-10 depending on where you buy it, which is big enough for most DIY scrim needs.

The largest shower curtain liners I’ve been able to find are 108-inches wide. It’s also should be possible to get 108-inch width bolts of rip-stop nylon some places (though I’ve not haven’t had any luck or need for it yet). A 108-inch wide bolt of rip-stop would give you the ability to build a scrim 9-feet by however long you needed. However for a scrim that size, neither the 3/4-inch PVC used here or the simple design will likely work.

The Frame

Parts List

• 2-3          10 foot stick of 3/4 inch schedule 40 PVC pipes
• 1             10 foot stick of 3/4” class 200 PVC pipe (clips)
• 6-            3/4 inch 90° elbows
• 2-            3/4 inch Tees
• 2-            3/4 inch couplings

Tools & Materials

• Hacksaw or PVC Pipe Cutter (Order one from Amazon.com)
• PVC Cement (Save a couple buys, buy it from Amazon.com)
• Heavy duty scissors (to cut the clips)

Design

The overall dimensions for the scrim frame are set by the size of the material available minus 2 inches for clipping area. For example, a 70 x 70 inch shower curtain liner would limit the frame size to 68 x 68 inches.

The overall frame design with dimensions needed to cut the main tubes is shown below. All 4 tubes that make up matching sides are cut to the same length.

The dimensions W and H are 2 inches smaller than the scrim material used. Following the example of a 70×70 inch fabric, the 68×68 inch frame would have the top and bottom pipes cut to 32-1/2 inches, and the left and right pipes cut to 33 inches.

I partially glued my scrim frame together in order to reduce the risk of losing pieces and to add some rigidity. The diagram below shows how I glued my frame together. The couplings on the side rails allow the overall collapsed length to be kept under 3 feet.

If instead, you elect to use a single pipe for the sides, you’ll want to follow the alternative gluing plan show below. In all cases, aligning multiple elbows on the same pipe section should be avoided due to the difficulty of doing so before the PVC cement permanently bonds the parts.

The final two pieces are the “stand adapters”. These fit into the Tees on the short side and add two bits of functionality.

When the long end is fitted to the Tee, the stand will largely stand on its own. There are of course some limitations, both with respect to the maximum size scrim that can be supported this way (I’ve only tested up to 3 by 5 foot scrims with this system) also it won’t do so well in really windy environments.

The second method is when the short end is fitted to the Tees. In this configuration, with one “stand adapter” on each tee, the long pipes can be slipped over the top of a pair of light stands allowing the scrim to act as a suspended shade.

Scrim Clips

There are a number of ways to attach the scrim material to the frame, the two best solutions are either sewing pockets into the corners of the scrim material or using clips. I choose clips since it involved the least sewing work and I wouldn’t have to go looking for more material. The clips are made from ~2” lengths of class 200 PVC pipe that have had about 1/4 of them cut out.

A brief overview of Auto Focus

If you’ve ever used a manual focus camera with a split prism screen, you’re already familiar with the way autofocus works, even if you don’t realize it.

The exact details are somewhat outside the scope of this article, but at a simplified level, autofocus works something like this:

Light from the lens is split and sent though a pair of prisms, the prisms converts focus errors into laterally shifted images. The shifted images are projected on to a pair of linear photo-sensors—think of this as a single row of pixels from the camera’s main sensor. The camera then looks for patterns on these sensors and then tells the lens to focus so that the sensors both show the same pattern.

The diagram attempts to illustrate the “AF system’s” view of a high contrast low frequency target, i.e. a black line on a white background. The rows of boxes above and below the images represent the AF sensor’s photo sites, and are colored to represent what the sensor would output. This is the simplest best-case scenario for an AF system, and it’s very easy for the camera to deal with.

The ultimate design objective is to make sure that the above simple scenario is the only scenario that the camera can experience across a wide range of conditions.

Potential Design Issues

The design of the autofocus sensor poses 3 broad problems, alignment, contrast, and image confusion.

Alignment

The linear nature of the sensor means that its ability to detect “lines” is directional. Moreover, if the line and sensor are aligned, in other words a vertical line and a vertically oriented sensor, the sensor will be unable to find anything to focus on at all.

Fortunately modern AF system design has reduced the severity of this problem with the wide spread use of “cross-type” AF points. These AF points place two sets of linear sensors at right angles to each other. This eliminates the possibility of having the subject and sensor aligned in such a way that focus cannot be achieved. What is perfectly aligned to not work with one sensor pair, will be in perfect alignment with the second pair.

From the AF target design perspective, it’s simple enough to insure that the target is functional regardless of sensor orientation and availability of cross type sensors, that it’s ultimately pointless to worry too much about the sensor type. Instead, a target pattern consisting of lines crossing at 90° angles is sufficient to cover the possible alignment cases.

Contrast

Due to their design and the requirements they must operate under, autofocus sensors tend to be significantly less sensitive to contrast than the actual imaging sensor. The simple, and obvious, solution to this is to insure the autofocus target has the highest possible contrast. To that end, it should be printed using fully saturated black on white paper.

Resolution/Image Confusion

The ultimate design objective is to insure that the target cannot “confuse” the camera’s autofocus system. Insuring high contrast and coverage is one thing, but going overboard with providing a target field can lead to accuracy and repeatability issues.

The mechanism is simple. When there is a high frequency pattern (like repeating lines), it’s possible for the defocused image align in such a way that the autofocus system things that focus has been achieved when it hasn’t. The image below shows this condition occurring in a split prism on a manual focus camera.

Of course, in a manual focus system we know that focus hasn’t been achieved, but such “outside” observations are not available to the computer in the camera. However, to the auto focus system, all it can “see” is what’s in the inner most circle. As far as the AF system knows, this could be an in focus image of a “soft” set of parallel lines, not the out of focus image of a set of lines. Though, it’s worth pointing out that SLR AF systems are more accurate than this simple demonstration implies.

This too has a simple solution; don’t use repeating patterns in the target design.

Sound Target Designs

If there’s anything that should be obvious about designing an AF target, it’s KISS (keep it simple stupid). What you clearly don’t want it something fancy that could ultimately lead to inaccurate focus. As such there are two target designs I tend to favor. The first is a simple pair of wide lines crossing at right angles running horizontally and vertically across the target.

The second is alternating quadrants of black and white.

Both of these targets are high contrast, suitable for both vertical and horizontal autofocus points, and have no high frequency patterns that could accidentally induce error.

In my opinion the key elements are clouds, color and foreground, though not necessarily in that order. Beach sunsets pose a special problem since a foreground of interest is usually much harder to come by unless you’re lucky enough to find a good-sized bit of driftwood. Clouds are always problematic, especially in the winter and spring in Florida where the skies are usually clear and completely devoid of clouds.

Last weekend I spent the day on Sanibel Island on Florida’s Gulf coast. The following image is one of many images shot during the not all that impressive sunset.

For all the image lacks, there is a serene quality in its muted pastel colors and evenness of light. However, it lacks pop and given the lack of interesting clouds and a strong foreground it could just as easily end up on the cutting room floor. Fortunately, the lack of pop can be easily fixed with some minor adjustments in post processing.

Contrast

Contrast affects many things in how we perceive images. Local contrast and apparent sharpness are directly related; the higher the local contrast is the more sharp an image appears. On the other hand, the overall contrast of the scene really affects the pop and mood.

Just looking at the image it’s clear that it lacks contrast. It’s even more obvious when looking at the histogram, where the bulk of the image data is in the center and doesn’t actually extend out to the edges.

Naively the lack of contrast could be addressed by adjusting the contrast slider. Doing so will darken the darker areas, which is what we want to happen, however, the contrast slider will also brighten the highlights. The contrast slider can be thought of as controlling the wideness of the histogram, adding contrast stretches the darks towards black and the lights towards white. Reducing the contrast pushes the darks towards white and the lights towards black—making everything grayer.

However, in this image specifically, the real problem with the contrast is the lack of blacks, not so much the highlights. Moreover, if you look at the peak in the reds and oranges in the histogram, which incidentally corresponds with the sky, it’s not necessarily desirable to push those further to the right, brightening them.

While not precisely the same, the blacks and exposure sliders can be thought of as acting like halves of the contrast slider. The Exposure control pulls the histogram towards white, and the black control pulls the histogram towards black.

In this case, increasing the black level will increase the overall contrast. It will also stretch the entire histogram towards the left. The whole histogram shift has a secondary advantage, as it will increase the saturation in the sky and ocean.

Saturation

Saturation sells. Okay it’s considerably more complicated than that, but by and large, most people looking at images will perceive a more saturated image as better, I’m no exception. That said; don’t take this to mean that cranking the saturation to 11 is a desirable thing. In many cases, it’s not.

Ultimately, I think I like the slightly more saturated sky, as it feels that it brings out the pinks and oranges much better. The final rendition with the simple tweaks to blacks and saturation has much more impact than the image did right out of the camera, or even what was visible at the actually being there.

The key idea here, I think, is to understand the differences between how contrast, exposure, and blacks affect the image and when to apply them. Unfortunately, like most things in photograph there’s no hard and fast rule that can be given or followed, instead the one has to use their best judgment when looking at the image and the histogram. In fact, an additional point may be to look at the histogram, as it’s a useful tool when deciding on what adjustments need to be applied.

Like always, my primary concern when shooting in bad weather is the quality and direction of light. However, unlike on a clear day, the weather adds an additional consideration, a more nebulous “seeing.” I’m not talking about not just how far you can be from the subject, but how the atmospheric conditions affect the quality of the images produced.

Overcast Skies

I find overcast skies are a blessing and a curse. They act like a huge soft box you don’t have to carry or setup. Who doesn’t want that? However, I find they also tend to make poor backgrounds for anything that has sky in it. The key to optimizing your images is to focus on subject matter that performs maximally under soft light.

Overcast skies reduce the dynamic range, bringing up the shadows, reducing harsh shadows, and remove many distracting specular highlights in backgrounds.

For images that don’t include the sky, I don’t make any special setting changes on my cameras and enjoy the soft light.

The real trouble with overcast skies is when you start including sky a large amount of sky in the frame. Clouds are dramatically brighter than blue sky, and well brighter than the 12-18% reflectance a camera’s meter expects. As a result, I find it’s necessary to add quite a lot of exposure compensation, typically between 1 and 2 stops.

There is also a directionality consideration, even though the sky is acting as a big soft box. This isn’t as much of an issue when shooting images without a lot of sky in them, but becomes a bigger deal when a large part of the frame is sky. I find that even though the sky is acting as a large diffuser, it’s still necessary to keep the sky behind you, and I will still try to position myself in such a way that the sun is behind me, regardless of whether the sky is clear or overcast.

Ultimately, I find shooting towards the sky tends to produce high key images they lack the sense of intimacy that fog or rain can produce.

On a more gear related note, purely overcast days typically pose the fewest number of technical issues. There’s no rain, or significant moisture in the air so camera covers and rain sleeves aren’t necessary. Nor does the cloud cover at altitude impact the use of a flash or strobe the way rain or snow can.

Fog

Fog is one weather condition I’m increasingly enjoying shooting in. Maybe it’s due to the limited amount of fog I actually see, but I find that when there is fog, my images tend to be more intimate, mostly out of necessity. However, the reduced “seeing” also makes producing images with complex layered elements easier in some cases.

The down side is that fog brings two challenges that need to be dealt with.

First, the quality of light is similar to an overcast sky. There is still a level of directionality, though to a much lesser degree both because there is no surface emitting the light (the bottom of the clouds) and because fog can be buried under normal cloud cover (double diffusion).

From a brightness standpoint, fog typically isn’t as overwhelmingly bright as an overcast sky. When shooting in fog, I’ll start by opening up 1 stop; sometimes I’ll go a bit more if I need to but never as much as I will when shooting towards an overcast sky.

The real strength to me is that fog not only hides backgrounds, but also adds a layer of mystery to images as objects quickly go from distinct objects to dark outlines in the background.

Adding flash can be a bit trickier, depending on how dense the fog is. At a minimum flash range will be reduced, likely significantly. In the worst case, the fog will reflect a lot more of the flash back than reaches the subject, further washing out an already low contrast scenario. Personally, I don’t use a flash in fog. I’ll compensate for the reduced light levels by opening up a bit or using a slightly higher ISO.

The more pressing issue is dealing with the lack of contrast and color vibrancy that is prevalent in fogy scenes. Fortunately, this is something that digital capture makes considerably easier to work with.

An optimized frame shot in the fog will look washed out, and will likely have very little information in the left (darkest) most quarter of the histogram. That’s okay, if not desirable. Maximizing the exposure, will minimize noise at capture time, which is beneficial due to the increased manipulation requirements. Processing, at a minimum, will require increasing the black levels, contrast, and color saturation.

Straight out of the camera, images shot in fog will be flat, lacking both contrast and color vibrancy.

After processing by bringing up the black levels and adding contrast and color saturation images shot in fog can be just as "good" as images shot in any other conditions.

The final aspect of shooting in fog is the reduction in usable working distance. What seems like a perfectly workable shot to the eye or through the viewfinder can quickly result in a useless frame to the sensor. Gaining an appreciation for the differences between how the mind perceives a scene in fog and how a camera captures it takes some getting used to.

Moreover, telephoto lenses will multiply the shrouding effect of the fog. In extreme cases, dense fog coupled with a telephoto lens can result in soft images due only to the increased diffusion from the water vapor in the air.

Image challenges aside, fog doesn’t present much of an issue with respect to protecting your gear. While I personally prefer to have weather sealed lenses and bodies when working fog, it’s not strictly necessary unless conditions are such that water is actively condensing on your gear. For an extra bit of protection, you could certainly use a rain jacket for your gear. OP/TECH USA makes a series of lightweight, inexpensive, disposable rain covers that would work well if you’re concerned about working in fog.

Rain

Rain presents the biggest challenge for photography, namely getting everything wet. Even with weather-sealed equipment, I am less than thrilled with the proposition of taking my gear out into even moderate rain without some form of protection. More importantly, raindrops landing on the front element or protective filter will seriously distort the captured image—this can be a major problem in surf, spray, or mist conditions as well. In addition to a rain cover, a lens hood is practically a given.

Rain presents the same limitations in visibility that fog does. However, rain, like snow, adds an additional twist in that the droplets can be big enough to reflect a flash burst back to the camera and show up in the image as bright spots. Like fog, if I am, shooting in the rain, I will avoid using a flash, never mind it’s more of a potential hazard due to the voltages involved in making a flash work, than I’m willing to deal with.

The more pressing issues are the limited visibility, low light levels, and limitations imposed on a flash by the falling droplets. Truth be told, rain becomes a very complicated subject discuss and work in and more than anything else, the situation dictates what you can and can’t do and it’s hard to give any general advice.

If the rain is light enough and there’s enough light, you can work much as you would under overcast skies. For example, flowers will benefit from the soft light and have the added benefit of having naturally placed water droplets on them.

Heavier rain is more problematic, and for the most part something, I simply avoid being in and shoot around it. In fact, these situations tend to be my preferred scenario as the clouds, and even rain itself can add a significant amount of drama to an image.

When dealing with rain I find it’s usually not necessary to adjust the exposure compensation, as the sky tends to be dark.

As for protecting your gear, if you are going to be shooting around anything that could produce rain while you’re there, I’d carry rain gear for your camera at a minimum. Preferably, I’d aim to work from inside a cover (like a car or overhang). That said in a light drizzle, most weather sealed cameras and lenses are good enough. I’ve on occasion run outside with an unprotected 1D and weather sealed L lens to get a quick shot while it’s still drizzling.

The one final aspect of rain is the storm itself. I’ve spent many a night with camera and tripod watching a storm pass trying to capture lightning bolts. A dramatic building storm can change the entire feel of a landscape or architectural image. Finally, the storm clouds themselves can be an interesting subject on their own.

Final Thoughts

Less than ideal to bad weather is one of the few things that can get me fired up and out the door with a camera. I’ve made too many successful images in those situations that seeing fog or rain on the horizon has long since stopped being something that gets me down, and I don’t think you should let it bother you either.

Every image in this post was shot in less than ideal weather. In some cases, the Great Egret and the Sandwich Tern specifically, the weather, light, and timing produced some of my favorite images over anything shot in better conditions.

Next time there’s fog or rain in the forecast; don’t write the day off as a photographic dead end.

Cleaning your Gear

This is a bit of a both sides of the equation thing, some of it is related to dust and some of it is related to maintaining the optimal functionality of your gear. I clean my gear every time I get back from the field. If you don’t currently clean your gear and want to know how, Moose Peterson has a very solid 4-part video guide at the top of his video archive page. It’s well worth the watch.

That said, there are a couple of things I want to call out as being important and a couple of additions I’d like to make.

Using a Cotton Swab to Clean the Mirror Box

Moose suggests using a Q-tip to remove fine metal shavings and dirt from the lens mount and mirror box. I had never thought of doing this, and I really like the idea. However I’m not sure if a cotton-swab is the best solution, though I don’t know anything better. I’ve found that it’s not hard to get fibers snagged on something and pulled from the cotton. Fibers, that in turn have to be cleaned out carefully with tweezers one at a time.

Also, keep in mind that many digital bodies now have tacky pads in the mirror box to catch and retain dust and you don’t want to go rolling a q-tip though one of those-though you shouldn’t be putting a q-tip that far into the mirror box.

Clean the Body and Lens Caps

I wash my body and sometimes lens caps—yes, with soap and water—depending on how dusty they are. Since Canon doesn’t make, to my knowledge anyway, metal body caps at all and the plastic ones will collect dust in hard to reach crevices it’s the easiest way to get the dust out of them. Just make sure they dry thoroughly before putting them back on the camera.

Also, don’t forget to clean the lens mount on the lens. A little rubbing alcohol on a cotton swap here will turn up all kinds of crud that can get into the mirror box eventually.

Wipe down Your Gear When You’re Done

Not sensor dust related but I just wanted to re-emphasize the importance of this especially when shooting in hostile environments. And just an added note, when I get back from somewhere like the beach I will clean the front filters with lens cleaner to insure there isn’t any salt residue left on the glass even if they appear clean.

This is doubly important when shooting in environments that are aggressive to camera gear and the coatings used on lenses. The sooner you do this the better as well.

While you’re cleaning your gear in general, don’t forget everything else, especially if you have a good quality tripod. In a harsh wet environment, aluminum tripod legs (and the aluminum in Gitzo leg locks) can corrode and seize, freezing the legs in whatever position they were left.

Cleaning Your Sensor

If there isn’t a way to start a debate, this is probably it. Yes, cameras are precise instruments and the last thing you want to do is damage the sensor, god only know how much you’d end up paying to have that repaired.

The official line is that it’s safe to blow the sensor clean with something like a Rocket Blower, if that doesn’t remove the dust you’re supposed to send the camera to the manufacturer and have them clean the sensor. In practice, the camera isn’t nearly as fragile as it sounds and there are several approaches to cleaning the sensor.

There are 3 major methods, blowing, brushing, and wiping, loosely named based on how they work.

Blowing away the Dust

Blowing is the simplest, safest and often least effective. You will need a proper blower, canned “air” dusters can and likely will leave a residue on your sensor that will require a more thorough cleaning to remove. I use a medium Rocket Blower, the small works well enough if you need to pack it in your bag for the road. Visible Dust, the makers of the Arctic Butterfly, makes a “Zeeion blower” that they claim is even better because it statically charges the air and filters it before it blows it in your camera.

Blowing will only remove dry loose dust particles, not sticky or oily ones. The unfortunate, reality of modern self-cleaning sensors though, is that the camera can already deal with this kind of dust quite well on its own, and overall blowing your sensor clean isn’t going to do much. However, it’s still a good idea to blow the sensor clean before you go into one of the more aggressive steps, there’s just no sense having any more dust there than necessary.

Brushing away the Dust

The second method of dealing with dust is brushing. This typically involves using a soft brush of sort, gently dragged across the sensor’s surface, to lift away the dust particles. The most well-known of the sensor cleaning brushes is the Arctic Butterfly by Visible Dust. There are also some “riskier” DiY solutions that use thoroughly cleaned artists’ paintbrushes or makeup brushes.

Brushing is the second most aggressive cleaning method, it can deal with sticky dust that blowing can’t, but cannot deal with oily residues. Like wiping, there is a level of risk in putting anything inside the mirror box and touching the sensor.

Wiping away the Dust

Better known as wet cleaning, this is the most aggressive and best cleaning solution. It will deal with all forms of dust, as well as any oily residues on the sensor’s surface. It’s also arguable the most controversial and most commercialized. There is a huge verity of products available for this, from reusable systems that you have to assemble at cleaning time to disposable one use pre-wetted pads.

I use a system from copper hill images; it uses readily available Pec*Pads and Eclypse E2 cleaning solution with a custom “spatula.” In the intervening years Photographic Solutions, the makers of Pec*Pads, have introduced their own disposable sensor cleaning solutions, though they are less hassle to setup, they tend to be somewhat more expensive on a per use basis.

General Sensor Cleaning Tips

Regardless of cleaning method and especially with the brush and wet methods where a physical instrument must be placed inside the mirror box, having stable camera power is of the up most importance. I strongly recommend using a fully charged battery (or multiple if you have a battery grip), instead of an AC power adapter, to insure that there is no power interruptions while you’re cleaning.

It’s also a good idea to work in a dust free area. Right, dust free, who am I kidding.

Actually, there are a few ways you can deal with dust in the air. One is to clean your camera in the bathroom after running the shower on hot for a few minutes and letting the room cool a bit. The condensing humidity will trap the dust in the air, at least temporarily. Barring that, if you have a fan in your room; turn it off and let the air settle before you start cleaning your gear.

In either case, it’s also useful to work quickly and keep the camera’s mirror box covered as much as possible.

For example, my cleaning procedure works much like this. I start by putting the camera in manual cleaning mode, then holding it lens mount down, and blowing out the mirror box. I then turn the camera off, and put the body cap on it before returning it to my desk while I assemble the swab. Once the swab is assembled, I put a few drops of eclipse on the swab and I’ll again put the camera in cleaning mode, then place it lens mount up on my desk. By the time I have the camera in cleaning mode, the swab is just moist enough to be ready, so I can pull the body cap off and clean the sensor. When I’m done cleaning the sensor, I immediately reattach the body cap, then go about turning off the cleaning mode.

Minimizing Points of Entry

Use whether sealed lenses and don’t change them in dusty locations, barring that change lenses sparingly and quickly and while shielding the camera from the wind.

Minimize the number of ways foreign particles can enter the camera body when you’re in the field. Not all sensor dust is due to material from inside the camera migrating to the sensor, there is still an environmental component that’s more of an issue in some environments than in others. Places like beaches or sandy areas are a good example; a little wind and there’s a lot of very fine particulate matter flying around almost constantly, couple that with the salt spray in air that tends to leave a residue on everything exposed to it, and you have a recipe for a dirty sensor.

The best way deal with these environments is by using weather-sealed lenses and not changing them at all in that environment.

Barring that, the next best thing is to keep the number of lens changes down to as few times as possible and as quickly as possible.

When all else fails and you have to change lenses in windy sandy adverse environments, try to form a shield against the wind by standing with your back to the wind. Then work quickly and close to your body and try to shield your gear with your hands and body as much as possible instead of out at arm’s length on your tripod or something. This is also a place where being able to change lenses with practiced ease is a good skill to have.

Dealing with Dust in Post

Dealing with dust in post shouldn’t be hard, it’s most readily visible in uniformly colored areas, and that makes it easy to clone out. In fact it’s very easy if you use the manufactures’ software (like Canon’s Digital Photo Professional or Nikon’s Capture NX) there is often a way to capture a dust deletion image in the field while you’re shooting and have the software apply it automatically when you process the image.

Using 3rd party image editors makes things a little more complicated, but many of them make the process streamlined enough that it still can be quite fast.

Tricks for Dealing with Dust in Lightroom

I use Adobe’s Photoshop Lightroom, and I’ve found a few handy tricks to dealing with dust.

Use Home and Page Down to Scan the Image

The first trick is to insure you cover the image at 100% magnification without missing any spots. Lightroom provides a simple mechanism to do this. In develop after you’ve zoomed into 100% magnification, press the “Home” key to move the view to the top-left corner. From here you can press the “page-down” key and the view will advance down the image to the bottom. When you reach the bottom of the image, pressing “page-down” again, will shift the view to the top and far enough right to align with the first pass you just made. By repeatedly pressing “page-down”, you can cover an entire image from top-left, to bottom-right without missing any of the image.

Sync Dust removal between Images

Dust spots are static, so they won’t move between images. Which means, that you can de-spot one image, and sync those fixes to neighboring images using Lightroom’s ability to sync development settings. This makes it much easier to deal with spots in a number of similar images where you can be sure the background won’t change drastically.

Edit and Process only the Necessary Images

This should be somewhat obvious, but instead of trying to remove dust spots from all your images, edit down your images to the ones you really want before you start processing them. With fewer images, you can spend more time cleaning them or verifying that dust really is a problem.

Shortcut Keys

You can jump directly to the spot removal tool in Lightroom by pressing the ‘Q’ key on the keyboard while in any library module or in develop.

Dust will likely always be a problem with digital photography and dealing with it isn’t just about cloning out spots in post-production. Keeping your camera clean and well maintained, especially the lens mount and mirror box, means you have somewhat less to worry about, but you’re not out of the woods.

One strategy is simply to do what I suggested in the previous article and use a tape measure to position the target in the necessary spot. It works, and can be accurate enough to get everything close enough to make for accurate measurements. However, it doesn’t make an easy quick way to setup the sight.

To do that there needs to be a sighting system of some sort. There are several ways to approach the sighting problem, but before I look at them, accuracy and depth of field must be considered.

Accuracy and Inaccuracy as Imposed by Depth of Field

This image shows the effect of defocusing as objects are moved away from the plane of exact focus (center pin). When the spot size is equal to the width of the the object (right most pin) the object blurry but still can be used to sight with. However, when the spot size grows to 2x the width of the object (left most pin) the object is blurry enough to make it difficult to sight with.

Most simple sighting systems work by aligning two or more sight points. When the sight points are aligned, the camera is aligned with the target. This strategy requires placing at least one of the “sights” either in front of, or behind, the other.

Accuracy is dependent on the distance between the two sight points, the longer the distance the more accurate the alignment can be. However, this presents a problem in this application, due to depth of field.

One way to simplify the target design and construction is to use the target itself as one of the sight points, with the second sight point either in front of or behind it. As a result, the useable depth of field is in turn halved.

The question ultimately is how far from the target can the second sight point be before it’s unusably blurred.

Fortunately, several factors allow the design to be simplified further. First, we can do the sighting at an aperture narrower than the lens’s wide-open aperture. In fact, due to the way modern SLR viewfinders are designed, at least in part, sighting thought he viewfinder will act as if the lens was stopped down to f/4 or f/5.6.

Further simplifying things, is that testing is typically conducted at a multiple of the focal length—it doesn’t necessarily need to be 50x either, as long as the multiplier is held constant. This reduces the depth of field calculations to being dependent on just the aperture. In other words, all f/2.8 lenses will have the same depth of field at the test distance regardless of their focal length.

Using both simplifications it’s possible to calculate how far away from the target the second sight point can be.

Calculating the Limits Imposed by Depth of Field

The easiest way to compute the placement of the second target point is work the depth of field process in object space (where things are) as opposed to image space (at the sensor) like it’s usually done.

For this, I use an equation presented by Harold Merklinger, in his book The Ins and Outs of Focus. This is based on the geometry of the scene and nothing else. Moreover, I make a simplifying assumption that the distance from the lens to the subject is equal to the distance from the film plane to the subject. In reality they are slightly different (<10% at the 50x testing distance) but close enough that it doesn’t introduce enough error while greatly simplifying finding the center of the lens.

The formula used is show below. S is the spot size, L is the distance in front of behind the place of exact focus, D is the distance from the lens, and ‘d’ is the aperture diameter.

Further applying the simplifying assumption discussed above, that the testing distance is always a multiple of the focal length—in this case, 50—the formula can be simplified as show below.

Where N is the aperture of the lens as express by the f-number and L is the same as above.

S is simply spot size where the target is unusably blurred. In practice, this works out to be twice the width of the target itself. Using this equation, we can solve the practical maximum sight point distance for any given aperture when the target is 50-times the focal length away from the camera.

Additionally the simplifying assumption based on viewfinder accuracy can be applied here to simplifying things further (reducing N to either 4 or 5.6).

For example, if an Xacto #11 knife blade was used as the front target point, S is 0.04 inches (0.5mm). Using f/4 as the sight aperture, and 50x the focal length as the testing distance, L, the upper limit for the front point is 8 inches.

Thinking about the Design

Knowing the limits is one thing, designing the target is another. If we stop and take a moment to look at the commercial Lens Align products, their sighting system utilizes a hole in the center of target board and a rear placed sight post. This is certainly guaranteed to have you pointing at the center of the target when it’s properly aligned, but it increases the complexity of a DiY solution.

One possible simplification is restricting the number of degrees of freedom the target has to deal with.

The sighting system serves to align things in 3D space where there are 6 degrees of freedom in which the camera needs to be orientated. However, if you begin to constrain those degrees of freedom the alignment problem becomes simpler.

For example, constraining the camera and target somehow would seriously constrain the number of angles that need to be dealt with. One such way is to insure the camera and target are level and that the center of the lens is at the same height as the center of the target. Doing this reduces the problem to a single angular error (yaw) which can be dealt with by a single vertical sight.

The problem of insuring alignment still exists, to some degree however, as I showed in the last section there’s a fairly large amount of room for error in placing the camera.

Another simplification is that the alignment points need not be placed in the center of the target. Removing that constraint removes the difficulty of placing an opening that has to be closed in middle of the alignment surface. Actually, this can be dealt with by having the target itself slide into the “holder” covering the alignment sight.

Fortunately, it’s not necessary to have the alignment marks in the center of the target. Perspective that even off center alignment marks will become centered when the target is aligned. In other words, a set of pins or other sighting marks can be placed around the target such that the line up with marks on the target instead of being centered.

The key consideration here is insuring that the center of the target is clear of things that could throw off the autofocus system.

In my prototype, I used both simplifications I just noted. My alignment pin is a hobby knife blade stuck in the front of the base plate about 6” in front of the target, well inside the DoF limits for an f/4 lens at the 50x test distance. Additionally, I only use a single sight point to align the yaw of the target relative to the camera, relying on both leveling the target and camera, and setting the camera so the lens is centered at the same height as the target.

On my prototype target, when the hobby-knife blade is aligned with the red mark on the target proper, the camera is aligned with respect to yaw.

So far, in the first 3 parts of this series I’ve covered how to calculate the allowable error that can exist in the flatness construction of the target, as well as the alignment (skew) of the target to the camera. This is limit is determined based on the AF adjustment size, testing distance, and fastest lens that needs to be tested and the camera’s sensor format. This part took a more in depth look at the problem of alignment, and some ways to solve it. Next time I’ll be looking at the actual target pattern.