The first two AF focus modes can be modified through custom
functions CSM a1 and a2—allowing two types of shutter-release
priorities to be set: Focus priority or Release
priority. The Manual focus option won't be discussed
here, to concentrate on autofocus issues, although it is a
powerful option in the hands of an experienced photographer.
Let's
take these lever settings one at a time.
S
(AF-S): Single-servo helps
if you want the camera to focus on a stationary subject, and
then lock that focus.
If
the subject moves before the photo is taken, it will be out
of focus unless you release the focus button and then re-press
to reacquire focus. If the subject was moving when the focus
button was initially pressed, then the camera will predictively
track focus until subject stops. Once the subject stops moving,
focus locks as noted above.
You
can adjust CSM a2 options for your AF-S single servo:
AF-S with Focus Priority is the Default setting.
The camera shutter will not fire unless the subject in front
of active AF sensor is in focus. If you choose AF-S with Release
Priority the camera shutter can be fired even if the subject
in front of active sensor is out of focus.
Nikon recommends AF-S for use with relatively
static subjects. AF-S is often used with the "Focus,
then Compose" technique--generally with the D2X in AF-S
and its default CSM a2 and a5 settings (Focus priority setting
and shutter button activating AF). The selected AF sensor
is positioned over the subject and focus is locked by pressing
halfway on the shutter release. The camera is then shifted
to place the subject in the desired composition within frame
and photo is taken. In Single Frame (S) Advance mode, only
one photo with this composition can be taken at a time, since
the shutter button must be released before a second photo
can be taken. Unfortunately, releasing the shutter button
unlocks focus and, when then pressed again, the camera will
refocus on the area now in front of active sensor (and original
subject will then be out of focus).
There are several methods available to work
around this problem and allow the taking of multiple photos
without having to repeat the "Focus, then Compose"
technique for each shot (with D2X in AF-S, Focus priority,
and shutter button activating AF).
One can press the AF-On button to lock focus
(essentially it acts the same as keeping the shutter button
partially depressed). With the AF-On button depressed, the
focus remains locked as you release pressure off the shutter
button.
The AE-L/AF-L button can be depressed to lock
focus and allow multiple photos to be taken without having
to repeat "Focus, then Compose."
Note: There is no AE-L/AF-L button near the
vertical shutter release; however the vertical AF-On button
can be programmed via CSM a8 to function as an AE-L/AE-L button.
If you switch from Single Frame Advance to
a Continuous Frame Advance mode (CL or CH) you will not have
to release the shutter button to take a second shot. You can
easily take multiple photos as long as shutter button is kept
partially depressed.
*I seldom use AF-S mode any longer, except
in dim lighting and/or low contrast conditions—situations
where the capabilities of any AF system are challenged. If
the AF action slow downs with more hunting and searching,
I want the camera to lock focus once achieved. Obviously this
works only with relatively stationary subjects.
C (AF-C): Continuous-servo AF
If you flip the focus mode lever to C, autofocus
never locks on subject. As long as the focus button is pressed,
the camera continually tracks a moving subject and constantly
adjusts focus. AF-C can be set to Focus or Release Priority
through CSM a1 settings.
Continuous
Servo also has options (CSM a1):
AF-C with Focus Priority: The camera continually
focuses but the shutter will only release when the subject
in front of the active sensor is in focus. Since the camera
will not fire until in-focus confirmation is received, there
is the possibility of a shutter delay between pressing the
shutter button and the shutter release. With fast action,
this delay may not be desirable.
AF-C with Release Priority (actually termed
FPS Rate): (This is the Default setting). Photos can be taken
whenever the shutter release button is pressed, even if the
subject is out of focus. The rationale is that continuing
focus action may achieve focus during the time period that
the mirror lifts and shutter opens. Another possibility is
that there may be adequate depth of field to cover desired
area of focus.
CSM a1 also offers a third option termed FPS
Rate + AF which is basically Release Priority with some emphasis
on focus. When in CL or CH advance modes, the frame advance
rate may slow, if needed, to allow for improved focus accuracy
if subject is dark or low in contrast.
*I
use AF-C almost exclusively, presently using FPS Rate (Release
priority)
When I first started using the D2X, I primarily
used AF-S Focus mode in CL or CH frame advance. I used the
shutter button for activating AF and often used the "Focus,
then Compose" technique. By using a continuous frame
advance mode (CL or CH) I could easily take multiple photos
after composing as long as I kept the shutter button partially
depressed.
When I was photographing action—for
example, birds in flight—I would switch to AF-C Focus
mode in CL or CH. I would continue to use the shutter button
to initiate autofocus.
These AF methods worked well most of the time.
However, there were times when I missed potentially exceptional
images because I was in AF-S mode and could not reach for
the switch and go to AF-C quickly enough when interesting
action occurred, for example, an egret or heron that would
suddenly take flight.
I soon realized that in order to be continually
ready for action photography, I needed to keep the camera
in AF-C mode. However I also wanted to be able to use the
"Focus, then Compose" technique. My solution was
to use the AF-On button for autofocusing (setting CSM a5 to
AF-On Only), and AF-C mode in Release priority (setting CSM
a1 to FPS Rate). The "Focus, then Compose" technique
could easily be performed by placing the active AF sensor
on subject and then pressing AF-ON button to acquire focus.
The AF-On button was then released, the camera shifted for
proper composition, and the photo taken.
**See
next pages for an alternative method to use "Focus, then
Compose" in AF-C focus mode.
