Re: Are many autofocus points nessesary??

On Sun, 29 Apr 2007 21:14:08 +0000 (UTC), davem@xxxxxxxxx (Dave
Martindale) wrote:

nospam <nospam@xxxxxxxxxxxxxx> writes:

it basically said that by focusing and reframing, you are in effect
measuring a longer distance to the subject. it used a right triangle
to show how aiming at the person's eyes would be the hypotenuse, but
the actual distance to the subject is the base of the right triangle.
therefore, the focus would be incorrect.

That is actually correct if you re-aim the camera, while what I
previously wrote is only strictly correct if you shift the camera while
keeping it aimed in the same direction.

When you focus on a point, you actually establish a plane of stuff that
is in focus. The plane is perpendicular to the lens axis, and passes
through the one point you focused on. If you shift the camera sideways
to recompose, everything in that plane remains in focus. If you aim the
camera in a different direction, the orientation of the plane of best
focus changes, and the original focus point will generally not be in the
changed plane. So the eyes will no longer be in the best focus.

As I understand it, the 'plane' of focus isn't flat, it's a surface of
a sphere (except for macro lenses).
So, if you focus on the eye, then rotate the camera to re-compose, the
eye should still be on the 'plane' of focus.
No? Or do I have that backwards?

On the other hand, the size of the error depends on the angle through
which you rotate the camera. If you move the eyes only slightly off
centre, the distance changes only slightly. You can see how large the
change is using the cosine function on a calculator. For example, a 5
degree change in camera aim gives a 0.4% change in distance to the focus
point, while a 20 degree aim change is a 6% change in distance.

i'm not sure how much that actually matters in the real world, since
the typical kit lens starts around f/3.5-4, and probably has enough
depth of field to cover the difference. on the other hand, i suppose
if one had the canon 50mm f/1 and shot with it wide-open, it would be a
problem...

Agreed - depending on the angle change.

Dave

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