If you’ve ever read any introductory material on photography, you’ve probably seen the hand holding rule of thumb. The rule of thumb suggests that to avoid camera shake, the shutter speed should be at least 1/focal length of the lens or faster. So for example, if you’re using a 100 mm lens, you need a shutter speed of 1/100th or faster.
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:
- It makes the math easier. Doubling a number is a lot easier than trying to multiply by 1.5 or 1.6.
- 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 1/500th 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:
|Camera / Format||Crop Factor||Adjustment|
|Canon APS-H (i.e EOS-1D)||1.3x||1/3-stop|
|Canon APS-C (i.e. Rebels/60D/7D)||1.6x||2/3-stop|
|Nikon, Pentax, and Sony Crop bodies||1.5x||1/2-stop (or 2/3rds stop)|
|4/3rds and micro-4/3rds cameras||2x||1-stop|
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 1/3rd stop shutter speed steps between say 1/30th and 1/250th and you’re not memorizing anything that isn’t generally applicable anyway.
Putting it into practice:
- Meter the scene.
- Dial in negative exposure compensation from the list above.
- Read the shutter speed listed:
- If the shutter speed is faster than the focal length, you’re good.
- 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 1/100th. One click of negative exposure compensation and the camera meters 1/80th now. 1/80th 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 1/40th 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 1/25th, 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.