Sometimes when making candid photographs your subject stays right where you hoped but occasionally something or someone steps into the scene blocking your chance of capturing the moment, all because of shutter lag.
Out here in the real world, there are two kinds of shutter lag: The first is the amount of time that it takes from depressing the camera’s shutter release to when an image is saved onto a memory card. During that time, the camera determines proper exposure and focuses the lens. With digital SLRs it includes the time needed to move the mirror out of the way. Shutter lag varies and with some point-and-shoots there’s enough delay for the subject to completely move out of the frame!
The second type of shutter lag is simply how long it takes for the photographer to “see” the photo and snap the shutter. I see this all the time with people holding cameras up to their faces (or at arm’s length) waiting while the perfect moment to make the picture passes them by. This is a far more common form of shutter lag, usually takes longer than the electromechanical one and more good photos are lost because the shooter just takes too long to make up their mind about when to snap the shutter.
Tip: The secret of eliminating the effects of this kind of shutter lag is to anticipate where your subject may be moving and don’t wait. Press the shutter release! Maybe more than once! Sometimes it’s a good idea to shoot a sequence of photographs to find the one that captures the peak moment of action.