Single Servo Versus Continuous Servo Autofocus - Nikon D300 Complete Manual

Single Servo versus Continuous Servo Autofocus

Let's take a closer look at the differences between the two
primary autofocus Focus Modes:
• Single Servo AF: the camera finds focus once and locks
on that as long as the shutter release is held partway
picture is never taken until focus is achieved. Put another
way, when you fully press the shutter release the actual
shutter opening is delayed until focus is achieved. In low-
light or low contrast conditions where the camera has a
hard time detecting focus, there may be a lag between
pressing the shutter release and the taking of the picture.
In practice, the D300's autofocus sensors are so good that
such lag rarely happens (usually only for off-center
subjects). About the only time I've seen it is in low light
situations where there is also very little contrast in the
subject.
• Continuous Servo AF: the camera looks for focus the
moment the shutter release reaches the halfway point
and continues to monitor focus as long as the shutter
release is held partway. If a subject starts to move after
focus was established, focus still follows the subject.
However, a picture is normally taken immediately when
the shutter release is fully pressed, even if focus hasn't yet
been achieved. That doesn't necessarily mean that the
image will be out of focus, though. Remember, the
camera has a precise calculation of where the focus point
is and has instructed the lens to get there. Due to the brief
delay in the time between pressing the shutter release and
the opening of the shutter, the lens may have finally
gotten to the right spot during that time. Indeed, for the
fast focusing AF-S lenses, that's often the case in bright
light.
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Just a reminder: you can assign focus initiation to controls other than the shutter
release, such as the AF-ON button. This is done via the options on the Custom
Settings menu.
Thom Hogan's Complete Guide to the Nikon D300
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