We all know that autofocus is very convenient for everyday shooting, especially on cameras with tiny viewfinders that make manual focus difficult (like my Nikon d3000).
However, just like the exposure settings, there's a reason that dSLRs offer a manual focus option: sometimes the brain behind the camera makes better decisions than the "brain" inside the camera.
What are some situations where it's usually better to switch off autofocus and do it by eye?
Macro shooting. The depth of field is so shallow that there is no way for a camera to know what exactly would you like to show to the world.
Studio shot. You know where exactly the things are and already focused. You don't want the camera to focus back and forth every time you press a shutter release.
Portrait with the open aperture with a very long lens. You need to focus on the eyes, and one of the good ways to do it is to focus with your body, slightly moving back and forth until you see the focus on the eyelashes.
Sports/action/kinds/pets in case you know where the subject is going to be. You pre-focus manually and wait. Some cameras even allow you to capture a shot exactly at the moment when something comes in focus.
Shooting moon or other distant objects. You need to prefocus on hyperfocal distance.
Street photography. You know the approximate distance to your subjects which is about 1.5 meters (about 5 ft) and you shoot from your belly in an approximate direction. If the camera is in AF, it will miss too many times.
Often times at a party or wedding reception, the lighting is too dark for the autofocus to work properly, so an AF-assist lamp is needed. Shining a light on someone distracts them and makes them aware of the camera / self-conscious, and the shot can be ruined.
Others give situations in which it's useful. I'll give one where it's absolutely essential. Subjects behind glass, wire fence etc.
I learned the value of manual focus after bringing a whole lot of photos of dirty glass, cage bars and wire fences from a zoo garden.
Manual focus is great in a lot of situations. AF has some intrinsic limitations given how it tends to work. One of its biggest detractors is that it will usually only autofocus around a general point, or possibly a locus of points. This is not a huge deal when precise focus is not important, such as wide hyperfocal landscape shots. In other cases, such as macro shots, portrait shots, or anything where exact placement of focus is important, autofocus often fails, or does not perform adequately.
Another case where manual focus can be a saving grace is when autofocus has a problem consistently focusing in the same place. It may pick the right thing to focus on most of the time, but every so often it may decide to focus on something else entirely. This can be mitigated by selecting different AF pints or loci, however even that may not be sufficient at times. If you ever run into a situation where your autofocus usually focuses on the right thing, but every so often hunts and focuses on something else, you might find it useful to use AF to focus on the proper target, then switch to manual to prevent further hunting. This can often occur at night or indoors where lighting may be low or inconsistent.
I have found that I like to use manual focus most of the time. I shot a high school football game the other night and found auto-focus was the better choice (for me) in that situation.
All the time... I hate auto focus. But seriously, even the few times autofocus is applicable, I would say stick with manual... I tend to use AF if necessary, rather than have it be the rule...
