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My take on the Nikon D90 AF modes

The D90 uses Multi-CAM1000 which is the same AF module used in the D80, a technology that has trickled down to newer Nikon models such as the D5000 and D3000.

It offers 11 AF points, one of which is a cross-type which is the center.

I wrote this as a guide to myself and hopefully would be beneficial to others new to the D90.

D90 Autofocus modes

First, let me describe the different Autofocus modes available with the D90. The descriptions were taken literally from the D90 manual.

AF-S mode (Single-servo AF)

The D90 user manual describes this mode as "For stationary subjects. Focus locks when shutter-release button is pressed halfway. Shutter can only be released when the in-focus indicator is displayed."

AF-C (Continuous-servo AF)

This is described as "For moving subjects. Camera focuses continuously while shutter-release button is pressed halfway. Photographs can be taken even when in-focus indicator in not displayed."

AF-A (Auto select)

This is described as "Camera automatically selects single-servo autofocus when the subject is stationary, continuous-servo autofocus when subject is moving. Shutter can only be released if the camera is able to focus."

D90 AF-area Modes

Single point

This AF-area Mode is available in AF-A, AF-S and AF-C modes.

In AF-S mode, I can use any of the 11 AF points to compose my photos. Once locked in to the focus area, changing the composition will not cause a shift in focus.

In AF-C and AF-A mode, any of the 11 AF points can be used to compose the photo. However, if I recompose and the selected focus point "sees" a focus area closer, or farther than the original, it will shift focus to that area.

Shifting to a new focus area is faster in AF-C mode as the D90 seems to "figure out" if it is needed in AF-A mode.

Dynamic area

The focusing behavior in this mode, coupled with AF-S mode is exactly the same as Single point. I'd rather have this option not available in AF-S so as to avoid confusion, as there's no functionality provided here.

In AF-C and AF-A mode, I can select the focus point and should the subject move, the camera will focus based on information from the other focus points.

Auto-area

In this mode, the user does not have control of the focus point. The camera will select the focus point on the detected subject, and if using type D or G lenses, the camera is supposed to distinguish from human subjects and background.

In AF-S mode, once the camera acquired focus on what it believes to be the subject, recomposing will not cause re-focus, even if the original subject is already outside the viewfinder.

In AF-C and AF-A mode, the camera will acquire focus on what it believes to be the subject, recomposing will cause a re-focus on a new subject.

3D-tracking (11 points)

This mode is not available in AF-S, and it makes a whole lot of sense. If the 3D-Tracking was currently active in AF-C or AF-A mode, but then switched to AF-S mode, the D90 will select the Dynamic area mode.

In both AF-C and AF-A modes, the desired focus point can be chosen, and should the focus area shifts (moves) or I recompose within the coverage of the 11 AF points, one of the AF points will pick up the focus.

One interesting observation is that if I acquired an initial focus area using the center AF point for example, recomposing which would cause the original subject to be outside of the viewfinder, causes any of the 11 AF points to acquire focus on the new subject. It will not reset to the center AF point, which makes a lot of sense. Once 3D-Tracking has engaged, any available AF point will now be active to acquire focus.

As pointed out in the manual, 3D-Tracking is not effective in tracking a subject whose color is similar to that of its background. Information available elsewhere mentioned that the camera will use the metering sensor in the viewfinder to differentiate the subject from the background. Hence, a subject with a different color or contrast than the background, is best for 3D-Tracking.

One of my practical experience in using 3D-Tracking was when I shot migratory birds at Candaba Swamp. I was able to acquire a focus on one of the three birds in the group, and while they were flying above me, the D90 kept on tracking the bird in AF-C mode.

I have yet to get to grip with this mode for sports though, as shutter can be released (AF-C mode) even if the subject is not in focus. So in some situations, I get a blurred shot.

Another use for 3D-Tracking is for portraits. Say I used the center AF point to focus on the eye, recomposing will cause one of the AF points to keep the focus on the eye. With this method, I no longer have to manually select a focus point and acquire focus.

To show the differences between the AF area of the D90, I used my AF-S 300mm f/4 IF-ED. The D90 is set to aperture priority at f/4 and AF-C. I am not going to discuss Auto-area anymore, because it will not give me a choice as the camera will obviouly, automatically pick the focus point.

Figure 1: Single-point

Initially the focus point was the lock button of the door knob. Then focus was deliberately veered to the left, with the subject leaving the center focus point, a new focus point was acquired, making the original subject out of focus. Focus area indicator [] now points to the newly acquired subject.

Figure 2: Dynamic area

Similar to above, focus point was initially acquired by the center focus point on the lock button of the door knob. Focus veered to the left, but the subject is still within the right-most AF point. The camera did not lose focus on the subject, even if the focus area indicator [] is no longer pointed to the subject.

Figure 3: 3D tracking

Again, focus point was initially acquired through the center focus point on the lock button of the door knob. Focus veered to the left, but still within the right-most AF point as visibly displayed on the viewfinder with a []. The camera kept focus on the original subject.

So what's the difference between Dynamic area and 3d tracking? You simply see which of the 11-point AF focus points has a lock on the subject. Result will be the same. In fact, Nikon Capture NX 2 will show AF-area Mode as Dynamic, 3D in the camera info section of the Metadata.

Pretty neat isn't it?

However, these modes are affected when you slap a teleconverter in. To verify the behaviour, I added my Nikkor TC20E-II 2x teleconverter on the AF-S 300mm f/4 IF-ED making the effective aperture to f/8.

Figure 4: Single point, AF-S 300mm f/4 + TC20E-II

Figure 5: 3D tracking, AF-S 300mm f/4 + TC20E-II

As long as the effective aperture is f/5.6 or better, the AF-area behaviours are normal as described above.

Avoid buying lenses with an effective aperture of f/6.3 on the telephoto end as the AF-area behaviour described above will no longer work.

Conclusion

The D90's Multi-CAM1000 is a very good AF module, and one just need to understand the combinations that will work for the particular scene they're shooting.