| Pushing the shutter button halfway down
activates the autofocus in both single servo and continuous
servo autofocus modes. In the continuous mode, simply pushing
the shutter button the rest of the way down trips the shutter.
It is certainly possible to push when the subject is not properly
focused. It’s your job to decide when to take the photo
by checking focus in the viewfinder. When in the continuous
mode in the 80088 and later cameras. The computer will track
the subject and attempt to predict the position of the subject
at the actual time of shutter release, when the camera-to-subject
distance is changing too rapidly for the camera’s autofocus
motors to keep up—a sort of "head ‘em off
at the pass" approach. Sometimes, it even works. When
using continuous servo autofocus, it always possible to get
an out-of-focus image by simply pushing the button all the
way in when the subject is out of focus. This will happen
if your own movement, camera movement. Or subject movement
causes the sensor segment of the finder to come off the subject,
or if the lens is searching for focus and is not locked on.
In the single servo autofocus mode, the computer in the
cameras will decide when the shutter will trip once you depress
the shutter button. The autofocus will go into search mode
when you press the shutter button down and will continue to
search until maximum contrast (focus) is achieved; the shutter
will then trip. If focus is not sharp, the cameras will refuse
to release the shutter. Theoretically, you shouldn’t
be able to take an out-of-focus frame in this mode. If you
depress the button halfway in single servo, the camera will
select a plane of focus and then disengage the autofocus entirely.
If you don’t trip the shutter at that instant and continue
to keep the shutter halfway depressed, any significant change
in camera-to-subject distance will result in an out-of-focus
image. Refocusing just before shooting can help insure you
against this; however, you invite the possibility that your
decision to expose the frame will be vetoed. If you keep a
constant pressure on the shutter release, holding the shutter
button fully depressed, you will only rarely have an out-of-focus
shot in single servo, as long as the sensor remains on the
subject.
The mere use of autofocus doesn’t guarantee a sharp
image. Practice makes you better at using either form of autofocus.
It’s a skill—kind of like playing a video arcade
game.
Single servo is usually my choice. I feel that I have the
most control in this mode. I use continuous when things are
so crazy that the damned autofocus keeps seizing up and refuses
to fire in single—an experience with which you are probably
quite familiar. Continuous is my "punt" option.
The bottom line is your best bet is to always visually confirm
focus in the viewfinder before you push the button.
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