9, 11, 21 or 51? How to decide?

Started May 13, 2012 | Discussions thread
2wheel
Regular MemberPosts: 389
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Re: It's not a simple question
In reply to corneaboy, May 14, 2012

corneaboy wrote:

First, consider a large subject. Take a look at the patterns for the 9, 21 and 51 focus point arrays. The nine point array covers only a tiny fraction of the area in the viewfinder. Most of the time some part of the subject image will cover the entire 9 point array. It seems sort of meaningless to say the camera is adjusting the focus as the subject moves from focus point to point.

When choosing a 9 or 21 point array, the center point is always the primary point and outer points are helper points only. Unlike 51 point dynamic, there is no dynamic action where the points within a 9 or 21 point array are automatically selecting A point within that array and adjusting to track the subject. Your chosen focus point remains the primary point and you should be tracking your subject with that point. The outer points are only assistance points (If you will) in case the subject drifts away from the primary point (But within the area on the 9/21 points) or if you over/under pan the subject.

A large or small target has little to do with the above. One may be choosing a small target on a large subject - i.e. the face of a basketball player - and the smaller arrays will better track that target as long as your panning technique is solid.

51 point Dynamic AF selection, for lack of a better descriptive, hands over the focus area selection to the camera. Which, may be fine if that is your desire.

If one MUST get the shot of say, a BIF on a mixed background, one might choose 51 point dynamic. You may get the shot of the bird but depending on other settings, it's possible that the wings or tail of the bird are in-focus. Or, if there are other birds in the frame, the system may jump to another bird instead of the intended target. If one has time to compose and the ability to track, choosing single, 9 or 21 points will allow you to be more selective on which part of the bird is in-focus. Like the face or eye.

The narrower the DOF, the more accurate your panning/zooming technique needs to be. A single AF point and f/2.8 and a high burst rate leaves a smaller margin of error in panning/zooming. 9 or 21 points increase the likelihood that your target will be in-focus. (As will a smaller aperture.)

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