In
AF-S mode, the sensor that acquires focus will light up in
red briefly once focus is achieved. In AF-C mode, the focus
sensor that set the focus will light up briefly immediately
after the photo is taken. According to the Nikon D2X manual
(p.77), "Camera may be unable to select focus area containing
closest subject when telephoto lens is used or subject is
poorly lit. Single Area AF is recommended in these cases".
Nikon recommends this mode for erratically or rapid moving
subjects when you know that the subject will be the closest
object to camera.
In
my tests, Dynamic Area AF with Closest Subject sometimes focused
on a more distant object if it was higher in contrast. This
was true whether the lens was wide angle or telephoto and
it was especially true in low light situations. Group Dynamic
AF (in Closest Subject Priority) would usually focus on a
near lower-contrast object more reliably than Dynamic Area
AF with Closest Subject. It is logical that Group Dynamic
AF with Closest Subject priority would focus faster and more
accurately than Dynamic Area AF with Closest Subject priority.
Fewer sensors are active in Group Dynamic AF so less distance/contrast
data has to be analyzed. Theoretically, using the same logic,
Pattern 2 should be more responsive than Pattern 1, since
fewer sensors are active.
Also
in my tests, initial AF acquisition was generally faster with
a user-selected AF sensor than when the camera was in Closest
Subject priority. With a user-selected sensor, the camera
knows which sensor to use for initial focusing; it does not
have to analyze data from multiple sensors (as in Closest
Subject modes) before beginning initial focusing action.
Deciding
which AF Area Mode works best for particular situations requires
testing under different conditions. I honestly haven't worked
with the different modes enough to make definitive conclusions,
but I'll share my opinions.
When
speed of initial focus acquisition is the prime consideration,
the camera's AF responsiveness will be faster with a user-selected
sensor than a camera-selected Closest Subject sensor. So the
reason to use the Closest Subject modes would be for situations
with erratic action in which the camera would probably pick
up the closest subject quicker than the photographer can find
and center the subject on a selected sensor. One caveat, of
course, is that Closest Subject priority can only be used
when the subject of interest will be the closest object within
the field of active sensors. Of the Closest Subject modes,
Dynamic Area AF with Closest Subject would be the slowest
since the camera has to analyze data from all eleven sensors
before initiating AF. Group Dynamic AF with Closest Subject
should be faster since fewer sensors are active, and as mentioned
above, Pattern 2 should theoretically be faster than Pattern
1.
The
logic gets more complicated with the user-selected modes.
Is Single Area AF mode faster than Dynamic Area AF? If it
isn't, why even have a Single Area AF mode—since a single
initial sensor is also selected with Dynamic Area AF (and
you have the added advantage of the camera predictively tracking
the subject to other sensors)? If Dynamic Area AF is slower,
then it must be because the other ten sensors are involved
in the AF algorithms. Following the same logic, one would
assume that Group Dynamic AF with Center Sensor selection
is faster than Dynamic Area AF, since fewer total sensors
would be involved in the AF algorithms, and that Pattern 2
would be more responsive than Pattern 1. At the present I'm
not sure how much of this discussion actually involves practical
vs. just theoretical differences.
*Presently
I'm using Dynamic Area AF most of the time. I've compared
it to Single Area AF in the field several times and can't
really tell any significant difference in initial focus acquisition.
With subjects moving directly toward me, I sometimes use Group
Dynamic Area AF (with center sensor selection). I have been
experimenting with Group Dynamic AF (with Closest Subject
Priority) and Dynamic AF with Closest Subject Priority on
flight shots with birds.
