The article below is written using the D2x camera as an example, and talks about the eleven AF sensors in that camera, but this information, except for sensor count, applies also to virtually all the Nikon digital cameras. So, if you have a D2H, D200, D100, D70, D50, or even an F6...read on.

AREN'T THE AF SENSORS THE SAME SIZE AS THE AF VIEWFINDER'S 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, where 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 width is to use Digital Darrell's patent-pending Featureless Blank Wall Method. Here is a summary and detail of the process:

Summary

1. Autofocus on a blank wall, and let it rack in and out once. (It won't focus...no contrast)
2. Keep the shutter button down half way so that the AF system keeps seeking a subject.
3. Slowly move the camera toward a window frame until the sensor detects it and focuses.
4. Look to see exactly where that happened in the viewfinder.
5. 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 but continue silently seeking a focus point. 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 and 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...