Camera shake is blur caused by camera movement during the exposure. It happens with indoor shots or outdoor shots in poor lighting or at night, when the low light levels mean the camera uses a longer exposure and hence a slower shutter speed. The slower the shutter speed, the more likely it is the camera will move, even slightly, while the picture is being taken.
This has led to the idea of ‘safe shutter speeds’ for handheld photography, and it is possible to work these out based on the focal length of the lens you’re using.
For example, if you are using a 50mm lens, the rule of thumb is that the minimum safe shutter speed is 1/50sec (or the nearest setting, 1/60sec). If you are using a 28mm wide-angle lens, the minimum safe shutter speed is 1/28sec (1/30sec is the nearest), and for a 500mm super-telephoto lens, the minimum would be 1/500sec.
As you can see, the minimum safe shutter speed is worked out by dividing 1 by the focal length of the lens. This is not a foolproof rule. If you’re very steady at holding a camera, this might guarantee a good percentage of sharp shots for you. If you’re not particular steady, you might need to go to the next highest shutter speed to be reasonably sure of a sharp shot.
This rule for working out the minimum safe shutter speed is based on the focal length of the lens. If you’re using a zoom lens, you just use the current zoom setting to work it out. You can use the focal length directly if you’re shooting with a full frame camera, but if you’re using one with a smaller sensor, you should go by the effective focal length instead.
Using ISO to achieve a safe shutter speed
Very often you’ll find the conditions are so dark that the shutter speed remains too low even if you set the lens to its maximum aperture. This is one of the main reasons for increasing the camera’s ISO setting – to get a fast enough shutter speed to avoid camera shake.
For example, if your shutter speed works out at 1/15sec with the camera set to ISO 100, increasing the ISO by 2EV (or ‘stops’) to ISO 400 will give you a much ‘safer’ shutter speed of 1/60sec.
This is all part of the art of exposure adjustments, and it’s made a lot easier by digital cameras and their ability to change the ISO setting between shots as part of the ‘exposure triangle’.
How image stabilizers help
Image stabilizers can reduce or eliminate camera shake by moving a group of elements within the lens, the sensor in the camera, or both to counteract any camera movement during the exposure.
Manufacturers quote their effectiveness in ‘stops’, or EV values. For example, a 5-stop image stabilizer should, in theory, let you shoot at five shutter speeds slower than you would normally. These figures are often quoted for very specific conditions, however, and in practice you may only get 2-4 stops advantage, depending on the focal length of your lens and the shooting conditions.
Even this can be extremely valuable. Without an image stabilizer, you might have to use a shutter speed of 1/60sec to be sure of a sharp shot (say), but with an image stabilizer you might be able to get a sharp shot at a shutter speed of 1/15sec or even 1/4sec.
Tripods and supports
Camera shake is only an issue in handheld photography. You can eliminate the problem entirely by using a tripod or some other fixed camera support. This won’t prevent any subject movement during the exposure (neither will an image stabilizer) but it’s the ideal way to capture close-up and macro shots, night shots and dimly-lit interiors.
For sports, wildlife and some other kinds of photography, a tripod may be impractical. Here, a monopod can provide the same kind of advantages as an image stabilizer (if you’ve got both, then all the better!). Monopods are especially useful for steadying telephoto lenses in sports photography, with the added advantage that they take the weight too.