II. Three Focus Modes
These modes are selected via a lever on the front of the camera, next to the lens mount:
|S||(AF-S)||Single-servo AF (Camera focuses, then locks focus)|
|C||(AF-C)||Continuous-servo AF (Camera continually adjusts focus, does not lock)|
Sample image made with a Nikon D2X
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.