AF-A, AF-C and AF-S

Started Jan 14, 2010 | Discussions thread
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Werner Gansz
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AF-A, AF-C and AF-S
Jan 14, 2010

This is a variation on the 3D vs Dynamic Area discussion going on in another thread but different enough that I thought I should make a new thread out of it. Related to the question of which focusing scheme to use for various situations (Single Point, Dynamic, AutoArea, 3D) is the question of which Focus mode, -C (continuous), -S(single), -A (Auto)

After re-reading Thom Hogan's ebook section on focusing and doing some experiments I think I now understand these modes. Of course, I thought I understood them before but now I'm positive that I understand them (until I find out I'm wrong and then I won't be as positive anymore)

In AF-S mode the camera focuses once (for whichever focus scheme is used) and never refocuses again no matter what the subject does until you release the shutter and half press again. Assuming it achieved focus when the shutter was initially half-pressed, the camera will take the picture when you final press the shutter, no matter whether the subject is still in focus or not. Set the camera in AF-S, focus, walk toward the subject, press the shutter to take the picture. It will be garbage. Hogan makes the broad statement that in AF-S the D90 "won't take a picture unless it has achieved focus". This is only narrowly correct. The camera will happily take a grossly out-of-focus image as long as it has initially achieved focus.

In AF-C mode the camera will track the moving subject (more or less precisely using whichever focus matrix is selected) but it will take the picture the instant you final-press the shutter, even if it has not reacquired focus. Set the camera to AF-C, focus, walk toward the subject, press the shutter while moving. The likely result is a poorly focused image (use a high shutter speed, just like BIFs). It is difficult to make a controlled test out of this; you could get lucky and hit the shutter just as it momentarily focused. I don't understand the purpose of this mode unless it is for paparazzi to get the once-in-a-lifetime $1M photo of Tiger's girlfriends running naked down the street, focused or not.

In AF-A mode the camera will track a moving subject, just like -C but will not take the picture if the green light is out. Do the same experiment in AF-A as for AF-C and there will likely be a delay between the final shutter press and the camera's response. This would seem to be the correct mode for any action photo where soft focus is unacceptable, including BIFs.

Since it acts like neither -C nor -S, AF-A seems to be a unique mode, not, as the literature implies, an automatic selection between -C and -S. It is really a hybrid mode, the only mode that actually requires something to be in-focus the instant the picture is actually taken. Whether the object in focus is the one you want depends on which focus scheme you used and, of course, your skill.
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Werner
http://wernerg.smugmug.com

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