Re: EF 50/1.8 AF Experiment?

Alan Browne wrote:
Wilba wrote:
Alan Browne wrote:

That softness may fall outside the AF hysteresis band.

More information please.

In order to avoid hunting near focus, the system is designed with an error
band such that once the focus lies within that band the focus action
(control loop) can stop. Otherwise there could be hunting. For most
lenses, the sharpest focus falls somewhere inside the hysteresis band.
With your 50, the AF might determine the focus as in there or slightly to
either side making the focus error range larger. This due to the softness
wide open. The control loop must have several stop (exit) conditions
including 'best case' (to cover this case as well as dim light).

Thanks, no worries with all that, I just hadn't heard the term before.

What you say fits with my thinking, the problem is finding a way to test the
theories.

There may also be more mechanical slop built in to the lens to allow for a
smaller motor to drive it. That's harder to pinpoint.

It sure feels and sounds like a crude sloppy mechanism with the 50/1.8, and
I'm sure that has a lot to do with people inferring that that causes the
focus performance they observe.


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