I specialize in photographing birds in flight and I make a living teaching others how to do the same. One of the techniques I use I call bumping the focus. This technique is often mis-understood and I get alot of questions about it. I just finished a lengthy answer to a question in another thread, so I thought it would be usefull to post my answer here as well where it would be easier for folks to find in the future. Here it is:

There are three different uses for the bump focus technique. I will try to explain each of them below. By bumping I simply mean letting off the focus and then refocusing quickly.

1. The first reason to bump the focus is to prefocus. The first task when photographing a BIF is to aquire it in the viewfinder and focus on it. It is beneficial to be able to do this as quickly as possible. When using long focal lengths, the bird may be so out of focus that you can't see it in the viewfinder even if it's there. Then when you do get it in the viewfinder it may take much longer to focus on it if the foicus is set to a drastically different distance. To overcome these issues, I will prefocus at the approximate distance that I anticipate for my subject. Then when the subject arrives, I can find it and focus on it quickly. I prefocus the camera by pointing the camera at something at the desired distance and then I focus on it. Now I'm ready for a BIF at a similar distance. If I need to switch the distance I will simply point the camera at something at the new distance and bump the focus. This will prefocus the camera at the new distance. Photogs that use a tripod will often prefocus manualy. Since manual focus is difficult hand held with big glass, I use the bump to prefocus.

2. When I am tracking a BIF against a varied bg and I miss and focus on the bg I will bump the focus to quickly return focus to the bird. Bumping the focus overrides the delay set by the tracking sensitivity custom function. Iset this tracking sensitivity to slow to get thelongest delay possible. This heelps when you are focused on the bird and want to avoid focusing on the bg, but it hurts when focussed onm the bg and you wantg to return focus to the bird. Bumping the focus overrides the delay.

3. This is the most important use of the bump technique. Most photogs will aquire focus on a bif and then try to continously maintain foucs while they are tracking and watching the bif in the viewfinder. They tend to focus continuosly waiting for the moment they wish to make a photograph. Often while watching, tracking, and waiting for the moment, the photographer will miss and focus on the bg. This is extremely easy to do when the bif is flying against a varied bg. This is the reason it is so much more difficult to photograph BIF against a varied bg as opposed to smooth sky bg. When the focus grabs the bg, then the photographer needs to re-aquire focus on the bif. This may take too much time causing the photog to miss the critical moment. I try to avoid this by only focusing on the BIF when I'm sure I'm on target and during the critical moments when I'm acualy making images. So, what I will typicaly do is aquire the bif initial and focus on it. Then I will let off the focus and just watch it in the viewfinder while tracking it visualy only. As the distance changes, the BIF will start to go out of focus. When that happens I bring it back in focus by quickly making sure the AF point is on the biurd and then I bump the focus to get it in focus again. I do this repeatedly as I'm visualy tracking the bird. When the BIF gets to the spot I want to start making pictures, I wil focus and shoot all at once. I shoot in short controlled bursts trying to time the critical moments with the best wing positions, etc. Because I have bumped the focus along, the focus is very close to where it needs to be when the moment to make pictures arrives. Then when I focus and trip the shutter it happens very quickly. If I tried to focus constantly while the bif approached I would likely miss, focus on the bg, and miss the critical moment. My goal is to keep the bird close to in focus and in the viewfinder without focusing on the bg and to do this up until the critical moment arrives. Then I try to maintain the focus while making great pictures. Bumping takes lots of practice, but if you develop this skill, it will make your keeper rate go way up.