The Benefits of Pre Focusing
A focusing technique that helps lens focus speed
Pre Focusing has pretty much been covered in the examples already given. By being a bit smart and thinking about what you are shooting, pre-focussing can save you a lot of time and missed shots.
It is mainly useful for sports where the action is fast and you need to give your lens' autofocus as much help as possible. Take a simple point and shoot camera as an example.
When you use a point and shoot camera, do you sometimes get what is called shutter lag? This is partly due to the lens hunting for focus as it tries to determine what it is you are focusing on.
The same can happen with some DSLR lenses. I know I have one, the Sigma 105mm 2.8 macro, that takes an age to focus. It can sometimes be infuriating as it will sometimes hunt through the entire focus range before finally locking on to the subject.
I can reduce this amount of time substantially by pre focusing on the subject as best I can prior to attempting to take the shot. This gives the lens less "hunting" to do and should lock on a lot quicker.
Pre Focusing Practice
Try to "Pre-focus" on a fast corner of a Grand Prix track so your lens doesn't waste time searching for the right focus point. Pre-focus on the point of exit on a snow-boarder or skiers jump. Pre-focus on the point on a playground slide where you wish to photograph your child…I think you get the point!
When shooting using this technique, and once you have the focus point determined and locked, switch to manual focus on the lens to ensure consistent focussing but bear in mind how much depth of field you will have by using certain focal lengths and apertures.
As an experiment, spend an entire day out and about photographing, using nothing but manual focus and see how you get on. It will hone in your skills and when you get back to autofocus, you may have learned a thing or two. Again, watch the apertures and check results to see which work best.
Many modern lenses allow you to "finely adjust" manually even with the lens on autofocus so you can really work "with" your lens rather that it working against you.