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.

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.

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.

Bracketing on most Canon DSLRs

For non-EOS-1 bodies, this means all the Rebels (###D and 1000D), the mid-range (i.e. 30D, 40D, & 50D), and even the high-end 5D and 5D Mark 2 bracketing is limited to 3 frames, +/- 1/3 to 2 stops, in 1/3^rd or 1/2-stop increments. The 60D and 7D, maintain the same 3 frames, but increase the range to +/- 3 stops in 1/3^rd or 1/2-stop increments.

The camera will automatically control either shutter speed or aperture depending on mode.

Bracketing Order

The bracketing order can be changed from “0, -, +” to “-, 0, +”, however, because brackets are symmetrical, other than lining up your images in order of increasing brightness there’s little need to change the setting. Changing the bracketing order is done the custom function listed in the table below.

C. Fn-09

C. Fn. I-5

C. Fn. I-5

C. Fn-09

C. Fn. I-5

Bracketing Step Size

Finally choosing 1/3 or 1/2 stop increments is done though the same custom function as changing metering increments.

C. Fn-6

C. Fn-1

C. Fn-06

C. Fn. I-1

C. Fn-6

C. Fn. I-1

C. Fn. I-1

Bracketing on EOS-1 Series, Prior to the Mark 3

Prior to the Mark 3s, the EOS-1D and EOS-1Ds cameras could do 2, 3, 5, and 7 frame brackets, with 1/3- or 1/2-stop increments from 1/3 to 3 stops from the center exposure. The number of bracketed exposures is controlled though personal function 08. However, this can not be set though the camera interface; the camera must be connected to a computer running Canon’s EOS utility software. They could also bracket using the ISO in 1-stop increments, a feature that’s since been removed. Bracketing order could be changed much the same way as in the 30D or 5D, by changing the setting in C.Fn-9.

Like the non EOS-1 bodies, the step size is set by changing C.Fn-6. However, due to a limitation in the camera’s ISO controls, setting the camera to 1/2 stop exposure increments, would disable bracketing using ISO.

Bracketing controls works much the same as it does in non 1-series bodies. That is, in aperture priority, the shutter speed is adjusted; in shutter priority, the aperture is adjusted; and in manual, the shutter speed is adjusted.

Bracketing on the EOS-1 Series, Mark 3 and Newer

With the introduction of the 3^rd generation of EOS-1 digital bodies, Canon has expanded the range of bracketing options and given the user even more control over what the camera can do. Canon’s EOS-1D and 1Ds mark 3 and 1D mark 4 bodies can make 2, 3, 5, and 7 frame brackets, with 1/3 or 1/2-stop steps, with 1/3 to 3 stops between brackets. In sort, you can do a bracket as small as 2 frames 1/3^rd stop apart, or as wide as 7 frames over a total range of 18 stops.

Choosing the Size of the Bracket

Setting the number of images made in the bracket is done though C. Fn I-6: Number of Bracketed Shots. You’re options in order are 3, 2, 5 and 7 shots.

Bracket Ordering

At first glance, it would seem like the order of bracketed exposures didn’t matter anymore than it does on any other body. That would be correct, if there wasn’t a 2-frame bracketing mode.

The order of bracketed shots is set using C. Fn I-5: Bracketing Sequence. The available options are 1: “0,-,+”, 2: “-,0,+”, and 3: “+,0,-”. When the camera is set to either the first or second options a 2 frame bracket will be made with the non-biased “0” frame and a negatively biased “-” frame. When C.Fn. I-6 is set to option 3, the second frame in the 2-frame bracket is biased positively.

This is an important distinction, because it allows you to retain the full +/- 3 stop exposure compensation range when shooting brackets that would be past either end of the scale.

Affected Exposure Control

By default, this is similar to non EOS-1 bodies.

The difference is that in manual mode what the bracket changes can be changed from shutter speed to aperture. This is controlled, rather unintuitively, with C. Fn. IV-5: Tv/Av Setting for Manual Exposure. When C.Fn IV-5 is set to the first setting, the camera will change the shutter speed when bracketing in manual mode, when its set to the second setting, the camera will change the aperture. An easy way to remember this is that when in manual mode, the camera adjusts the same thing the main (front) control dial adjusts.

Important Mark 3+ Custom Functions

The question is, is a faster lens better, or worse than a slower lens with some form of stabilization.

Motion: The enemy of sharpness

There are two kinds of motion that can blur images, camera motion, and subject motion. Camera motion is the little bit of rocking and shaking the photographer makes when standing still, the vibration induced by the mirror flipping up, or any number of small movements and vibrations that happen to the camera while a photo is being made. While the movement may be imperceptibly small to us, to the camera and lens these movements are actually quite large.

Subject motion, on the other hand, is just that, your subject moving around on you. You could also lump into subject motion the relative motion of the subject and photographer if the photographer is moving. While not exactly subject motion, its effects are the same.

Stabilizing Things

Of all the motions that can affect the sharpness of an image, the image stabilizer can only affect one of them, camera shake. Specifically it can only reduce vibrations in the plane of the sensor (i.e. up-down and left-right) and in some cases tilt. Moreover, the stabilization system can only correct relatively small motions.

In short, the stabilizer can reduce some camera shake and that’s it.

Aperture: More Light, More Speed

The aperture on the other hand controls how much light can reach the sensor. In this case, we’re looking at maximum apertures, since they let the most light in. More light, in a similarly light environment, means exposing the film for less time (i.e. increasing the shutter speed).

The practical result of shorter exposures is less motion of any kind.

Looking towards the Future

Ultimately, the question might become a moot point.

Digital camera sensors and noise reduction algorithms are improving every year. Today’s ISO 12800 is as good as yesterday’s ISO 1600. Further, as the technology is refined, the number of lenses that can have stabilization systems is increasing; and that’s not even considering the cameras with in-body stabilizers that stabilize all lenses. It’s certainly possible that in a few years, the only choice will be between a fast stabilized-lens and a slow stabilized-lens.

In Short: Which way to go

Image stabilization is not always a substitute for faster shutter speeds, especially when the subject is moving. While an image stabilizer will help improve sharpness of static objects at all shutter speeds, at least when hand holding, it won’t stop a moving subject at all.

The million-dollar question is do you shoot moving subjects in low light or not?

If you do, you’ll probably be better served with a faster lens, even if it’s not stabilized.

Safety Shift Mode 1: Tv/Av

Safety shift mode one applies to all Canon cameras except the D30, D60, and Rebels (EOS ##0D and EOS 1000D). The manual describes it as follows.

1: Enable (Tv/Av)

This works in shutter-priority AE (Tv) and aperture-priority AE (Av) modes. If the subject’s brightness changes suddenly and the current shutter or aperture becomes unsuitable, the shutter or aperture is shifted automatically to obtain a suitable exposure.

From reading that, one might assume that enabling safety shift isn’t necessarily a good idea. The implication is that even in one of the priority modes the camera will be constantly overriding your shutter and aperture settings as the “exposure changes”.

The reality is that Safety Shift only kicks in when your metered exposure exceeds the limits of one of the camera’s ranges.

In Av mode, Tv/Av Safety Shift works something like this; suppose the camera is set to f/2.8 and the metered shutter speed is 1/16000^th of a second, 1 stop faster than the maximum shutter speed the camera can support. Without safety shift, the image will be 1 stop over exposed. With safety shift, the camera will automatically stop the lens down 1 stop so you’re shooting at 1/8000^th at f/4 and the image will be properly exposed. On the flip side, an exposure of 60s @ f/22 would prompt the camera to open the aperture 1 stop to get an exposure of 30s @ f/16.

In Tv mode, the Tv/Av Safety Shift behaves the same way only changing the shutter speed instead. For a required exposure of f/1.4 at 1/250^th with a lens that can only open up to f/2.8, Safety Shift would cause the camera to drop the shutter speed to 1/60^th to insure a proper exposure. Alternatively, an exposure that required an aperture smaller than the lens could produce would result in shortening the shutter speed until the exposure was correct.

What exposure is shifted?

Safety shift uses the meter to determine the exposure. However, exposure compensation settings are factored into the exposure before the safety shift is applied. Therefore, if one is shooting a high-key scene with +2 stops of exposure compensation set, the compensated exposure will be the base exposure and the Safety Shifted exposure will be equivalent to the compensated exposure.

Safety Shift Mode 2: ISO

Canon’s EOS 1 series bodies since the Mark 3s have a second Safety Shift mode to adjust the ISO instead of the aperture or shutter speed. The manual entry for mode 2 reads:

2: Enable (ISO Speed)

This works in Program AE, shutter-priority AE, and aperture priority AE modes. When the subject’s brightness changes erratically and the correct auto exposure cannot be obtained, the camera will change the ISO speed within 100-3200 automatically to obtain the correct exposure.

Like mode 1, mode 2 only alters the exposure once the shutter speed or aperture has reached the limits of what the lens or camera can do. In addition, like Tv/Av mode 2 factors in the exposure compensation that is set when correcting the exposure.

Faking Auto ISO on a EOS-1D(s) Mk. 3 or newer

Auto ISO is either handy or completely useless, depending on the implementation and photographer. Nikon’s implementation is arguably one of the best. In starts with allowing the photographer to set their preferred ISO, from there the photographer can configure a minimum shutter speed and a maximum ISO for the camera to use.

In other words, a camera could be configured so that the shutter speed won’t drop below 1/60^th and the ISO won’t be set above ISO 800.

If you aren’t providing at least that much, the implementation borders on worthless.

Fortunately, the EOS 1 series bodies don’t provide an auto ISO implementation as such at all. However, mode 2 (ISO) Safety Shift combined with the built in shutter speed limitations (Custom function I-12) can be used to mimic Nikon’s auto ISO to some degree; you can’t register a lowest shutter speed faster than 1/60^th and you can’t stop the camera for shifting outside of the registered ISO limits (Custom Function 1-3).

However, even with those limitations this “fake” implementation of auto ISO provides more control than Canon’s auto ISO as implemented on any other Canon camera.

In my experience, Safety Shift, especially the ISO shift on 3^rd generation EOS-1 bodies, provides the latitude to be able to quickly capture a fleeting moment without having to worrying about overexposing an image because the camera couldn’t reach a high enough shutter speed or narrow enough aperture. Moreover, coupled with the ability to limit the lowest shutter speed to something approaching a generally hand-holdable speed (1/60^th) you have a rather good, if slightly more complex to configure auto-ISO system.

• The ratio form gives the ratio of image size to object size. In other words, if a lens has a maximum magnification of 1:4, then at that magnification the object will be 4 times larger than the image.
• The decimal form is simply the decimal representation of that ratio. For example, a 1:4 magnification can also be expressed as 0.25x.

To convert from the ratio form to the decimal form, divide the object size by the image size.

Some Common Magnifications

• 5:1 (5x) – The highest magnification of the Canon MP-E 65mm f/2.8
• 1:1 (1x) – True macro magnification, the image is the same size as the subject was. Most true macro lens support this magnification.
• 1:2 (0.5x) – Half macro magnification, the Canon EF 50mm f/2.5 compact macro and the Zeiss Macro Planar lenses reach this.
• 1:4 (0.25x) – “Consumer Macro”, most lenses that aren’t dedicated macro lenses but are branded as macro reach this magnification.

However, there has been a solution all along. It just seems that nobody had really thought of it until recently. The solution comes from the ability to remotely control a tethered camera’s focus while looking at the live view image on the computer. Even better, there is now software that automates the process and that software is the AF Calibrator feature in Helicon Remote, part of Helicon Soft’s Helicaon Focus.

What Helicon Remote’s AF Calibrator does is use the camera’s built and its own contrast detection algorithms to determine what the sharpest focus setting is without having the user guess. Even better, it does this automatically and the result isn’t a hard to interpret picture but an actual setting for your camera. Further, because this approach doesn’t require reading a ruler off towards the edge of the frame, precise alignment and target design is much less important and setup is a snap.

A Quick Review of Traditional Method and its Problems

Let’s, quickly look at what you need to perform AF calibrations the old way.

1. Place your camera on a tripod and the test target on a flat surface.
2. Carefully align the camera and target.
3. Focus the lens to infinity and then let the camera autofocus on the target and take an exposure
4. Change the AF Microadjust setting and repeat step 3.
5. After all, of the focus step images have been made load them into an image-processing program and evaluate which one places the focus in the right place.

It doesn’t take doing this more than once to realize that it’s a pain and quite problematic if not done extremely carefully. For starters, the target, scale and film plane must be aligned with a high level of precision. This is necessary since the scale is shifted away from the target.

Further, the target can’t be anything that’s handy; it must be specially designed strictly for focus testing. It also has to have specific design features, like a high-contrast focus point sounded by no other detail.

On top of that, very few AF calibration targets are suitable for use at normal working distances. Canon, for example, recommends that all AF tests be done at 50x the lens’s focal length. If you’re testing a 70mm lens, that works out to 12 feet. A 200mm lens requires more than 32 feet between the camera and target.

Finally, you must manually step though each setting (for most cameras that’s 40 images from -20 to +20) manually refocusing the camera to infinity between each shot and letting the AF system refocus.

At 15' with a 70mm lens (even on a 1.3 crop body) this test chart is almost completely useless. Not only isn't there enough room insure that the camera will lock on the target bar, but the scale is smaller than an f/2.8 lens's depth of field.

All told setting up and shooting an AF test, using a paper target can take a couple of hours or more for a single lens. Even commercial solutions designed for focus testing only improves upon that slightly. Moreover, the results are dependent on the photographer’s ability to interpret what he’s looking at accurately.

The Live-view Method

Almost all of the problems with the traditional technique can be solved quite easily if we can insure that the focus point and measurement point are the same. Unfortunately, doing this precludes easy interpretation of the target by the photographer. Fortunately, this type of measurement is something that can be done readily by a computer. In fact, it’s how contrast detection auto focus systems, like those in point and shoot cameras, work. Additionally, since the focus settings on many cameras can be controlled remotely when they are tethered to a computer, the whole process can be automated in software.

In the broad strokes, for “live view” focus testing you:

1. Place the camera on a tripod and align the camera and target. The two planes should be close to parallel but ultimate precision isn’t necessary.
2. Using the center AF point, focus on the target by half pressing the shutter release.
3. Connect the camera to your computer and fire up your remote control software.
4. Using the smallest AF shift amount move the focus forward and backward while keeping track of how many times you click the button in either direction.
5. Observe the sharpness of the image each time you change the focus. The number of button clicks that made the sharpest image will be the value you enter into the camera’s AF adjustment setting.

This works well for a number of reasons. First, since this method focuses and measures at the same point, any errors induced by misalignment are negligible at best. Even better, there is no guesswork involved in coming up what adjustment to use. This is because the smallest step that the camera can shift focus is the same size as an AF micro adjustment step. Finally, since there is no need for a specialized target, the procedure can be done at normal working distances where it will better reflect the real world.

The only hurdle is judging the sharpest image, and this is where the computer takes over. Helicon Software’s Helicon Remote presents us with, as far as I can tell the first solution to computer aided AF calibration.

Live View Focus Testing in Helicon Remote

As I’ve said, Helicon Remote simplifies the process of Focus Calibration significantly since it eliminates the need to determine which image is the sharpest.

Setup for focus testing is as follows.

1. Attach your target on a vertical surface like a wall or door. In this case the target only needs to be a high-contrast dot or line, in fact the less there is for the computer to process the better.
2. Make sure the target is well lit with continuous lights and the more light the better. Since this system uses live view and doesn’t actually take photographs, a flash won’t work. Also, the more light available the less noise there will be and this in turn will makes test more accurate.
3. Place the camera on a tripod approximately 50 times the focal length away from the target. I use the focal length (in mm) divided by 5 or 6—whichever is easier to do in my head—in feet since it’s easy and close enough.
4. Align the camera and target. I do this by leveling the camera with a hot-shoe level then adjusting the height of the tripod so that the lens and target are vertically aligned. Finally insure that the camera is roughly straight out from the target (having wood or tile floors with the joint lines running perpendicular to the wall is handy for this, otherwise eyeballing it will get you close enough).
5. Hook the camera’s USB connection up to a USB cable (but don’t plug the other end into your computer). What you want to avoid is plugging in a cable at the camera during the test.
6. Focus the lens to infinity and half-press the shutter release to allow the AF system to lock on to the target, make sure the AF lock indicator has come on.
7. Start Helicon Remote and plug the USB cable into your computer.
8. In Helicon Remote, click the AF Calibrator button on the toolbar along the top.
9. Follow the steps in the AF Calibrator dialog.

Conclusions

I’m much happier with this method for AF adjustments than I have been with any other method I’ve tried so far. However, I haven’t yet used it enough to really gain an unshakable confidence with it. In the tests I’ve done so far, the results from Helicon Remote coincide fairly well with what I’ve found from other methods.

The biggest benefit here is the speed and ease of interpretation. With the old way, it would take me up to 30 minutes or more to insure the test was. Even with my best testing procedure, it still took quite a bit of time to insure everything was aligned properly. With this method, I can have the target and camera setup, aligned and ready to test in about 5 minutes. After that, the tests take about 30 seconds per run. In the time it took to get good results out of the old method, I can now triple check a lens at all major focal lengths.

Additionally, the Helicon Remote AF test goes far beyond what can be set in camera, +/- 60 points versus +/- 20 points for almost all cameras, so it can be used to determine if a new lens is out of spec.

Finding the Range

What’s needed is a way to measure the distance to the subject with enough accuracy to allow for correct focusing. A tape measure would do, but probably isn’t going to work well for photography. Even if it did work, in many ways, it would only be marginally better than guessing with out a guide. Just imaging asking the important public figure you’ve been hired to photograph to hold one end of the tape measure while you setup the camera.

There is, however, another way to measure the distance to an object with out physically extending a measuring stick to the object. This works because of the trigonometric relationship between angles and the lengths of the sides of a triangle. It doesn’t take long to realize that using trigonometery as a base, the distance to the subject could be computer with out ever leaving the camera.

Figure 2: Overview of the formed to determine the range to the subject when using a rifle scope to calculate range.

Figure 3: Measuring the angle by reading the scale in the scope.

Figure 4: The equation for ranging. (d = distance to subject, s = subject size, α = angle)

To make this trigonometric solution work you do need to know one thing ahead of time, the size of the subject. In addition to that you need a way to measure the angle of the subject and a formula to convert that into a distance. One approach, and what’s used in long distance rifle shooting, is to use a scale in an viewfinder or scope to measure the angle the subject covers from your position. The scale is marked at positions that correspond to known angles. Commonly, at least in rifle scopes, these marks are made in milliradians (mils) or minutes of angle (MOA).

If the hypothetical reticle in figure 3 is marked using milliradians then the subject covers an angle of 4 mils¹ and the subject is known to be 6″ tall. Solving the equation shown in figure 4 for the values given, and a distance of 41.6 inches² is calculated.

Whether or not this is more practical than a tape measure is probably up for debate, what isn’t debatable is that this still isn’t practical for photography. Accuracy is dependent on how well you know the size of the subject and how accurate you can measure the scale and the whole thing is predicated on being able to work out an equation on the scene as a table is still problematic because of the errors in subject size. Fortunately for people shooting rifles, an error of a few feet has little impact on a shot that’s covering 100s of feet. However, an 85mm f/1.4 lens used to make a portrait has a total depth of field of only 4.1 inches at 10 feet.

For what it’s worth, I’m not aware of this style of ranging ever being applied to common photographic equipment. It’s certainly not accessible to most people. However it does introduce the trigonometric strategy for calculating distance rather clearly and that’s what all focusing aids based on range finding use one way or another.

The Coincident Image Rangefinder

The solution to the having to solve equations is to cleverly build that into mechanics of the rangefinder and present it to the user in a simple, easy to understand manner. The basis for doing that is to turn the triangle around so that the length of the subject in the previous figure now becomes a fixed length inside the camera/rangefinder.

The ease of use problem is solved by presenting the user with a pair of superimposed images where adjusting the alignment of the images corresponds with setting the focus. This all comes together in a device known as a coincident image rangefinder (CIR).

In a coincident image rangefinder, the user knows they’ve found the correct distance because the two images in the viewfinder are perfectly aligned. Even better, because of the way the CIR works, the user knows (not that it’s strictly necessary in this case³), which way the lens needs to be adjusted to focus the image.

Figure 5: Reversing range finding triangle.

Reversing the range finding triangle presents us with two solvable  problems, measuring the angle and combining the two images. The second problem is addressed by a beam splitter used as a beam combiner. A beam splitter, like most optical elements, it can be used in both directions. That is, when light is fed into two of the sides of the beam splitter, it will superimpose them over each other and send them out a third side.

Figure 6: Parts of a basic coincident image rangefinder focused at infinity

Figure 7: Distance equation

Figure 7: Simplified light paths though a coincident image rangefinder

The second problem, measuring the angle formed between the two rays form the subject. This appears more challenging at first, however even that is relatively straight forward.

The law of reflection states that the angle of incidence is equal to the angle of reflection. From this two things become clear, first that when the mirror is rotated 45° as shown in figure 6, the camera will be focused at infinity. Second, we know that the angle between the two light rays is equal to twice the angle the mirror will rotated.

From the second point, the equation shown in figure 7 can be derived; where d is the distance to the subject, b is the rangefinder’s base length and θ is the angle the mirror is rotated. Further, this makes measuring the angle α (in figure 5) directly unnecessary to calculating the distance as it was done in the previous method.

The final piece of the puzzle is coupling the rotating mirror to the lens’s focusing ring, so the whole system can be driven by the operator simply by adjust the lens. The actual math is hidden from the user in this mechanical coupling.

How it all fits Together

When the lens isn’t focused properly, the reflected light from the mirror isn’t aimed at the center of the beam splitter. Thus the combined image is misaligned, the direction of the misalignment is also a queue to the direction of the focus error. Figure 8 shows an exaggerated schematic of a coincident image rangefinder at various focus positions.

In practice it’s not quite this simple and there are more elements included to insure that both images are right side up and to project framing lines into the viewfinder, but the concept is the same for all coincident image rangefinders. Visual alignment is the key here, and what makes a CIR easily accessible to all users. The complicated math is handled by the people designing and building the cameras.

This system is used in cameras that are collectively called rangefinders. Arguably, the most successful example of this type of camera is Leica’s M series of rangefinder cameras.

Now that rangefinder provides an easy way to determine when the lens is focused it would seem like this should be the solution to all focusing needs. Simply stick a coincident image rangefinder on top of a camera and it’s good to go.

Not so fast…

Problems with a Coincident Image Rangefinder

There are a few issues the coincident image rangefinder presents. First since a rangefinder doesn’t work though the imaging lens, calibration of the lens-mirror link becomes very important. Small errors in mounting distance, lens construction or even rangefinder construction can throw the focus off enough to cause problems and they aren’t visible until after the image is made.

The second, and more fundamental, issue is accuracy. The rangefinder’s base length dictates the angle the mirror has to rotate for a given focus distance, because of that it controls the accuracy of the range finder. To make the measurement more accurate, a longer base length is needed to increase the angle the mirror has to rotate though. To give an idea how much rotation is involved, the mirror in a hypothetical rangefinder with a 60mm base length will rotate just over 0.5° from the infinity position when focusing on a subject 3M away; at 30m it will have been rotated just over 0.05°.

The objective for the rangefinder designer is to choose a rangefinder base length that is sufficiently long to provide enough accuracy with the focal lengths and working distances that will be used with the system. On the other hand the size of the camera places hard limits on the maximum length of the range finder base length. In turn the limits of the rangefinder place limits on the maximum focal length that the system can reasonably support.

This method of range finding works well, in practice, for wide-angle, normal and short-telephoto focal lengths because the depth of field at long distances grows quickly enough to mask errors in focus. However, telephoto and super telephoto focal lengths require much greater accuracy at long distances. That, coupled with the fixed external viewfinder makes long telephot lenses unwieldy at best on this type of camera, and in general they aren’t available for rangefinder systems.

Conclusions

The coincident image rangefinder solves some of the problems of focusing a small format camera but not all of them. This is no slight of the design though. One things a rangefinder camera offers that was impossible to achieve, before digital at least, is a way to build a very compact camera that doesn’t have any large moving parts. In fact when the exposure is being made, the only moving part in a rangefinder camera is the shutter. This makes rangefinder cameras very quite, as well as eliminating mirror slap induced vibration reducing image quality.

Next time we’ll look at how to make a rangefinder thin enough that it can be placed in a through the lens situation as used in an single-lens reflex camera.