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

Started May 13, 2012 | Discussions thread
2wheel
Regular MemberPosts: 389
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Re: 2 wheek answer
In reply to corneaboy, May 15, 2012

corneaboy wrote:

Thank you again for taking the time to answer.

So does what you say mean that if you had a tiny bird whose image in the viewer was the same size as the 9 point array then it would track better than when using the 21 point array, which would be incorporating unwanted information from around the bird. Otherwise, I don't see where the 9 point array would ever have an advantage over the 21 point array, unless maybe it is faster.

You've got this mostly correct.

If you were trying to track a single bird in a flock, then in your above scenario, the 9 point might be the better choice over a 21 point array. But, not necessarily either. If your DOF is great with an aperture above, say, f/8, (depending on the lens) the surrounding birds in the flock are likely to be in focus anyway. Now, if you add a low shutter speed and are attempting to blur the wing motion of the target bird, then you would not want the focus system to foil your composition by tracking an adjacent bird in the flock. Which it might do if you stray too far from the target bird and he ends up on the periphery of the 21 point array. (Due either to you over/under panning or his sudden change in direction)

Remember that your primary selected point is the one you use for tracking and also the only one you can see in the viewfinder. You should be sighting this point on the target bird. The extra points in the 9 or 21 are just assisting you in maintaining a lock on your target. The helper points are effectively enlarging your area of focus allowing for a larger margin of error. Conversely, in some circumstances, too large a focus area coupled with a sloppy panning technique may also cause the system to jump to an adjacent target. This all depends on the size of your target. A single point might be your best bet if the situation warrants.

Side note: I shoot with a single point for most bicycle races as my panning technique is pretty stable and I'm confident I can track my subject. If a situation arises that causes me to question my ability to track my target, then that is an indication to me to either change my position or to maybe consider a 9 or 21 point array.

Back to birding.. A simple way to think about helper point behavior... If the center point moves completely off the target bird, even though the helper points are still on that bird, and another bird is very near the same plane but now occupying the center point, the AF system may select that second bird. Actually, this is not only possible. It's likely. Follow?

With a 9 point system and the above scenario, the switch between targets might be more acute in that the focus system would more readily switch to the adjacent bird because the array is smaller. But, if your panning technique is solid and the bird does not pull a crazy Ivan, it will also be more likely to track the bird than the 21 point might because by using 21 points, you've told the AF system to consider more info in the form of a larger percentage of the frame. Still with me?

Now, change the scenario to a single bird in a clear blue sky and the 51 point dynamic system will find that bird and track it perfectly even if your panning is not so perfect. Make sense?

Every composition has it's elements and all those elements need to be taken into consideration for your exposure. One should take as much care in selecting the focus mode as they might the positioning, aperture, shutter speed, etc. And as you do with every other aspect of your shooting, you should experiment and take note on what works for each of your most oft experienced situations. And then be flexible.

If I understand you correctly, then if I align a small bird within the 21 point array but not the focus point, the camera won't know where to focus. That doesn't seem right.

Not correct. Any item within the array will be considered a target. (See above explanation.) As long as you don't stray too far from your target or the composition contains too many other options (Flock of birds) the system will track what you are pointing at. Bias a great amount of the array toward an unintended but very near target and it'll likely switch on you.

In the real world I have a hard enough time trying to pan a small bird in irregular flight to be concerned about focusing on his eye. I'm just happy to get him in the frame and focused. Your guidance is most helpful for those shots of big birds in straight flight or just taking off from a perch, where trying to get an eye focus and fill at least half the frame is the goal. At least now I can concentrate on testing the 9 and 21 point arrays.

I would suggest beginning with single point AF and practicing your tracking technique. If you experience a low number of keepers, try the 9 point and so on. But, also always be willing/ready to change the focus array should the scene change. Don't get locked into one system. As with everything in photography, one setting does not do it all.

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