Custom Fn IV, * , half shutter press and recomposition

[trailblazer]

After reading the many threads here about recommending the splitting of AE/AF lock up by putting AF lock on * and AE lock on the shutter half press, I changed my custom functions to do the same. I now have a question about recomposition.

If for example, I am shooting moving subjects wide open with a small DOF whether it be people, animals or objects and I press the * to AF lock, press the shutter halfway to get AE lock, and then recompose and take the shot, it is sometimes lost because the subject's trajectory angle or mine has changed.

I read this:
http://visual-vacations.com/Photography/focus-recompose_sucks.htm

which explains why focus/recompose has issues.

How do you overcome them?

[Wilt]

Trigonometry is your friend. It is very easy to calculate (Cosine) the effect of focus plane shift when recomposing. Generally speaking, if you keep the angle of shift to less than 20 degrees, the inherent DOF of a lens, even at f/2.8, will accomodate and compensate for DOF plane shift and the subject's new distance from that focus plane!

[vkalia]

Actually, a Chuck Westfall paper recommends that for subjects closer than 15m, it is better to use another focusing point rather than lock/recompose.

In such cases, I use the nearest focus point (as opposed to the central one), and then recompose/shoot. As Wilt says, the inherent DoF is usually enough to cover for that. In extreme cases, simply use manual focus (which is what I do for macro).

Vandit

[Wilt]

yes, if you have time to manually shift focus points, that is always better than center focus and recompose, but I find circumstances where it is faster to center focus and shift, so it is important to understand the limitations. Indeed, farther with long fast telephoto (e.g. 200m with 200mm f/2.8) is more forgiving than closer with a fast short portrait lens (8' with 80mm) And, with the narrow AOV of a telephoto, on is not likely to focus at a point OUTSIDE the frame of the telephoto and then shift 10-15 degrees!

[trailblazer]

Thanks guys.
More practice required!