AREN'T
THE AF SENSORS THE SAME SIZE AS THE VIEWFINDER'S AF BRACKETS?
In fact, the AF sensors on a Nikon D2x are
significantly wider (or taller) than the viewfinder's AF brackets.
You should be aware of this fact, use it to your benefit,
and guard against situations where it might affect the focus
accuracy of your Nikon D2x.
If you're shooting a subject that is smaller
than the sensor, you may well have difficulty maintaining
focus on the subject if a higher contrast object is sharing
the sensor. Since the underlying sensor is significantly larger
than the visible AF bracket, the subject could actually be
larger than the bracket and the AF system still could be fooled
by higher contrast objects sharing the sensor.
A good example of this might be a portrait
shot. You might think your camera is focusing on a person's
eye, when in fact the eye and ear are both under the sensor.
If the ear has more light on it than the eye...guess which
will be in focus? Yes, the ear! Do you see why some may feel
that their camera has a backfocus issue, when in fact it is
working as designed.
AF SENSOR REVIEW
The Nikon D2x has a total of 11 autofocus
sensors, as represented by the brackets in the viewfinder.
The 9 sensors in the center of the viewfinder are "cross-sensors,"
which means they work in both directions, left-right and up-down.
The two sensors on the outside, far left and right, are not
cross sensors and only work in one direction, up or down,
if you hold the camera horizontally. So, 9 of the 11 sensors
are cross-type, and two aren't.
The actual size of the AF sensor does not
vary on the D2x. But, the positioning of the sensors, in relation
to the viewfinder brackets, might. That is why we want to
run this test.
Other than the two outside sensors on the
left and right, the normal cross pattern and width of the
underlying AF sensors look like the image below, in relation
to the viewfinder focus brackets. (bracket is red, sensor
is white):
HOW
TO TEST SENSOR WIDTH
A
good way to test your camera's sensor location is to use Digital
Darrell's patent-pending Featureless Blank Wall Method.
Here is a summary and detail of the process:
Summary
* Autofocus on a blank wall, and let it rack in and out once.
* Keep the shutter button down half way so
that the AF system keeps seeking a subject.
* Slowly move the camera toward a window
frame until the sensor detects it and focuses.
* Look to see exactly where that happened
in the viewfinder.
* Repeat for all the sensors.
Detail
Since there is no contrast in a blank wall,
the autofocus will simply rack in and out and be unable to
focus. After it has racked in and out once, it will stop racking.
Now, without letting up on the shutter button, slowly move
the camera toward an object like a window frame or lamp, and
carefully note when and where the autofocus system senses
the object and focuses. You will find that autofocus happens
well outside of each bracket, and reveals the true width of
the AF sensor on your camera.
Now, try this method on each of your 11 AF
viewfinder brackets by using the thumb rocker switch to move
to a different sensor, and re-testing. By doing this, you
will learn the exact location and apparent width of each AF
sensor. Some will be shifted more than others.
By careful testing, you will find out whether
your viewfinder brackets are centered well within the actual
AF sensors, or are more to the left or right (or up and down).
By knowing this, you can adjust your focus techniques to match
the "personality" of your unique camera. If the
focus sensors are way out of alignment, you can have them
adjusted, often within warranty service.
Be sure to test the 9 cross-sensors in both
directions by holding the camera horizontally, then vertically,
while testing. Also, test the sensors with all your favorite
lenses, so you can determine whether the lens in use changes
the apparent position of the sensors.
If possible, try to keep your camera lens
face parallel to the wall while testing. Don't stand too close
to the wall, either. Try to test from 8 to 10 feet (~3 meters)
away.
Normally,
the AF brackets will be in the center of the sensors, and
will work as designed. Your knowledge of the sensor location
on your D2x will improve your autofocus technique.
Keep
on capturing time...
