Nikon focus modes

When you set your camera lens to AF (auto focus), digital SLR camera's offer the photographer a choice between interchangeable modes. Auto focus modes can vary between different digital camera brands. In the past, Nikon's two most popular modes were called Continuous Servo AF and single area AF. Nowadays, they are called AF-C (short for auto focus continuous) and AF-S (short for auto focus single). Many Nikon models also have AF-A and MF (manual focus) modes to choose from as well.

What are the differences between AF-C, AF-S, AF-A and MF focus options?
AF-C (AF continuous, sometimes called continuous servo) is good use when photographing moving objects. When your camera is set to AF-C and you focus on a moving subject, for example a dog running towards you, the focus will stay on the animal so long as your shutter button is held half way down. In other words, the camera will keep re-focusing as the animal moves. That is, so long as you keep your shutter button held half way down.

AF-S (AF single, sometimes called single area AF) mode, is good for photographing subjects that don't move, such as flowers or portraits etc. It locks the focus on the non moving object that you want to photograph. You can then recompose the shot and take the photograph.

AF-A is where the camera selects and goes between the last two modes. If it thinks the subject you are photographing is stationary, then it will automatically use AF-S focus mode. If it picks up that the subject you're photographing is a moving subject, then it will automatically use AF-C focus mode. This is also the default camera focus mode, unless you change the setting to AF-C or AF-S.

MF is the manual focus mode. It's unclear why this focus mode is included, because nowadays there is usually an option to switch between auto and manual focus on the side of the actual lens. Maybe it's in case you come across a lens that doesn't give you that option.


Why would photographers change between focus modes?
The default camera setting is AF-A. As mentioned earlier, this mode guesses if the subject you are photographing is stationary or moving, and accordingly sets the focus to either AF-C or AF-S. Photographers don't always trust the camera to pick the correct setting, preferring to make the choice themselves.

http://www.slrphotographyguide.com/camera/nikon-digital-slr/focus-modes.shtml