Re: Shimmed Mounts and Extension Tubes

Wolfgang Weisselberg wrote:
Kennedy McEwen wrote:

Aim at close subject and half press the shutter.

You mean "activate the AF"

After lens starts moving, aim at distant subject. Then back to close
again.

What happens is:
1. The AF marker blinks in red --- focus distance aquired.

That's the wrong interpretation of the first AF point blink. "Activate the
AF" with the lens cap on. Your selected AF point will blink, then the system
will fail to focus. No focus distance can be acquired in that case. The
first AF point blink is just showing you which point is active. If all
points are active, none of them blinks when you "activate the AF", whether
you have the lens cap on or with a normal scene. A second AF point blink
happens on focus confirmation.

2. While you switch to the distant target, the lens moves to the
close distance and the near target comes in focus. The lens
finishes travelling.
3. The AF marker (now on the far target) blinks red --- focus
distance aquired.
4. While you switch to the near target, the lens moves to the
far distance and the distant target comes in focus. The lens
finishes travelling.
goto 1.

This is nothing like what happens with any of my lenses (none of which is
the 50/1.4).

With a bit of practice you can get two or three close/far cycles
before the AF system gives up. The lens motor CHANGES
direction several times, towards the subject on the AF sensor!

Which absolutely proves that the AF is open loop.

Sorry Wolfy, it would only be possible to say that from a position of
profound misunderstanding of what's going on.

Repeated, yes, but open loop.

The thing that makes it closed-loop is the fact that it acts on feedback
from the sensor. An open-loop system can not do that, by definition.

If it was closed loop, it would only stop when it actually was in focus:
1. The AF marker blinks red.
2. the lens moves towards the close distance.
3. as you switch to the distant target, the focussing motor
reverses, never reaching close distance.
4. the lens moves towards the far distance.
5. as you switch again to the close target, the focussing
motor reverses again.
...
n-1. as you don't switch the target, the focussing motor
finally stops perfectly in focus.

Yes, that's exactly what it does!

n. The AF marker does NOT blink a second time at all.

It sure does, just like it does when you are manually focussing and the
point is over a subject that is "in focus".

I think your beliefs about what's happening are stopping you from observing
and understanding what's happening. One such idea is that the AF system
simply measures the subject distance and tells the lens where to focus.
Another is that the first AF point blink signifies the acquisition of a
focus distance. The facts just don't fit with those ideas, and I suspect
that your beliefs are stopping you seeing the facts (e.g. via the squirrel
test).


.

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