Have you ever "just missed" the moment in a shot, even though you were sure you caught it when you pressed the shutter button? You know the scenario; you waited for the peak of the action and clicked, only to find when you looked at the result that you were a fraction of a second too late. Shutter lag gotcha' again, huh? Maybe.
What is Shutter Lag?
The definition of shutter lag is a little bit sketchy these days, particularly in digital cameras in autoexposure mode. When you press the shutter release, a lot happens. Light is metered, adjustments are made, components power up, and on a DSLR, the mirror swings up out of the way. If you're shooting in AF mode, the lens focuses, too. All of that happens in milliseconds, but those milliseconds can be enough when the action is happening. No matter how you define it, that can be considered shutter lag.
Why Prefocusing is Important
You probably already know that pressing the shutter button halfway lets your lens focus before you take a shot. That's prefocusing. If you just press the button all the way, however, the lens will still focus before the shutter activates. That's why in those "off the cuff" situations, it's easy to just raise the camera and press the shutter. If you're shooting on a tripod, it's even easier to just decide skip the prefocusing. The results aren't always what you expect.
Here's what you may not know: When you press the shutter halfway, your camera's components also power up and the exposure settings are calculated. Everything is ready for the mirror to swing up and the shutter to open. That means that the solution to "shutter lag" is simple. Prefocus before every shot, even if you're focusing manually. Frame your shot, then press the shutter button halfway and hold it until the moment you want to capture comes.
There will be many situations where you'll be able to frame the shot and prefocus quite a while before that moment arrives. Take advantage of those situations, especially when you're shooting moving subjects in AF Servo mode. Make prefocusing a habit, even for grab shots. You should soon start to see a lot fewer missed moments, and that will be worth the effort.