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Digital SLR Camera Basics

Focus Mode (Viewfinder Photography)

Autofocus is performed when the shutter-release button is pressed halfway. The photographer can choose whether the camera locks or continues to adjust focus while the shutter-release button is pressed halfway. The setting that controls this behavior is focus mode.

Single-servo AF (AF-S)

In single-servo AF (AF-S), focus will lock if the shutter-release button is kept pressed halfway after the camera focuses.

Focus Lock

Focus lock is used to change the composition without changing focus. If you frame the shot so that the main subject is in the selected focus point, focus, and then change the composition while keeping the shutter-release button pressed halfway to lock focus, you can create compositions in which the main subject is not in a focus point but is nevertheless in focus.

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Continuous-servo AF (AF-C)

In continuous-servo AF (AF-C), the camera will continue to focus if the shutter-release button is kept pressed halfway after the camera focuses. Because the camera continues to focus up to the moment the shutter-release button is pressed all the way down, this mode is a good choice for subjects that are in motion.

Continuous Focus

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AF-C mode
AF-C mode

When Not to Choose AF-S or AF-C

AF-S: Focus Lock

Because focus locks while the shutter-release button is pressed halfway, AF-S is not suited to subjects that are constantly moving toward or away from the camera.

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Shutter released during focus lock
Shutter released
during focus lock

AF-C: No Focus Lock

If you change the composition after focusing in AF-C mode, the camera will refocus on the object that is now in the focus point, making it impossible to lock focus using the shutter-release button.

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AF-C: No Focus Lock

Some cameras offer an auto-servo AF focus mode that automatically selects between AF-S and AF-C. When the shutter-release button is pressed halfway, focus locks briefly after the camera focuses on the subject in the focus point. If the subject then starts to move, changing the distance to the camera, the camera will automatically switch to continuous-servo autofocus and adjust focus in response to the change in distance to the subject.
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Sample Camera Displays

camera information display
camera information display

Focus mode:

Displayed as AF-A, AF-S, or AF-C

AF-AAF-A
AF-SAF-S
AF-CAF-C


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