The reciprocal rule in photography is that when shooting a photo handheld, your shutter speed should be the reciprocal of the focal length that you are shooting at (1/focal length) in order to avoid blur caused by the natural shaking of your hands.
This means that the wider focal length you are shooting at, the slower shutter speed you can use while still maintaining a sharp image.
Calculating The Crop Factor
Crop sensor lenses have different “effective” focal lengths than what’s actually written on the lens and you need to that that into consideration when calculating the reciprocal rule shutter speed.
The focal length indicated on lenses is based on a full frame sensor, but crop sensor lenses are smaller and give an equally smaller field of view which results in them behaving like longer focal length lenses.
In order to calculate the appropriate shutter speed, you need to know the crop factor of your specific camera. Here are some of the most common ones:
- Canon Crop Sensors – 1.6x
- Nikon/Sony Crop Sensors – 1.5x
- Micro 4/3 Cameras – 2.0x
Then all you have to do is multiple the listed focal length of the lens by the crop factor. For example, on a Canon crop sensor camera, a 50mm lens has the approximate field of view as a 80mm lens. So in this situation you would want to shoot at a shutter speed of 1/80 or faster.
Many modern cameras have image stabilization (sometimes called vibration reduction) built into the camera body, the lens, or both.
This function will help to stabiize the image hitting the sensor so that any slight movement by your hands will be eliminated enough to shoot at slower shutter speeds than the reciprocal rule would otherwise indicate.
Some image stabilization systems can give you up to 5 stops of camera shake reduction and most are in the 3-5 stop range.
That can make a huge difference in the shutter speeds you can shoot at.
For example, if you were using a 200mm lens with no stabilization, then a shutter speed of 1/200 sec. or faster would usually be necessary to eliminate blur from camera shake.
But 5 stops slower means you could shoot that same shot at 1/6 of a second and (theoretically) not see any blur from camera shake.
Even if you have a camera, lens, or both with image stabilization like this, be sure to test it out before you start using it on shots that you don’t want to miss.
This Doesn’t Work When Shooting Movement
The reciprical rule is all about controling the motion blur caused by hand holding the camera, not the movement of the thing you’re shooting.
If your subject is moving than you need to use a fast enough shutter speed to freeze them based on how fast they are moving. You may also choose not to freeze the subject if you want to visually show their movement in the frame.
Whichever you choose to do, consider the reciprocal rule as well for anything in the frame that is stationary.
Using The Reciprocal Rule
Keep in mind that this is more of a guideline than a rule.
I tend to be overly cautious when I am shooting for a client and go with a faster shutter speed than might be necessary. But in that situation, ensureing perfect sharpness is critical and I don’t want to miss the shot.
In addition to hardware variations like image stabilization, you also have to consider that every person is going to have different degrees of camera shake and even that can vary depending on the time of day or other factors like fatigue or having an extra cup of coffee!
So the best thing you can do is test this out as much as possible when you aren’t shooting “must have” shots. That way, when you do find yourself in a situation where you can’t miss the shot, you know exaclty how you and your camera bahave and can asjust accordingly.