Re: Shimmed Mounts and Extension Tubes

Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxx> wrote:
In article <30p1c6-udb.ln1@xxxxxxxxxxxxxxxxxxxxxxx>, Wolfgang
Weisselberg <ozcvgtt02@xxxxxxxxxxxxxx> writes

The focus motor is driven closed loop, even in one-shot mode.
It's the decision where to drive it to so that focus is archived that's
open loop.

No it isn't. Stop reading literature and try some open/closed loop
testing of the system at hand. It's closed loop control from the PD AF
sensor right through to the lens and back.

It's open looped, and it's proven by your test.

Use the Canon 100mm f/2.8 macro lens - slow, precise focus drive.

Too long, too much travel to macro mode without the limiter
switch.

Use a 50mm f/1.4, a target at arms length and one larger one,
directly behind, at 3 meters to infinity distance. Use a target
that's easy on the AF. Make sure the AF can snap from infinity
to the close target and vice versa and use enough light ---
we are not testing AF corner cases or low light performance.

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.
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.

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. Repeated,
yes, but open loop. 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.
n. The AF marker does NOT blink a second time at all.

There is no way that the lens can rack back and forth
based on merely the subject changing distance UNLESS the AF is closed
loop.

No, there is no way. There cannot be a way, since you decree
that. Just like fetching email. You have to trigger each
fetch manually, there is no way a computer could be told to
do so every 5 minutes. There is no way the camera could be
programmed to repeat the AF measuring after focus has been,
supposedly, reached. No way at all. Inconceivable.[1]

PD AF is closed loop.

PD AF is by design open to both open and closed loop
implementations.

However,
the control loop is closed.

As proven by your experiment, it is open in Canon's one-shot
implementation.

Since the PD sensor can be misaligned with
the image sensor, Canon erroneously distinguish between this and CF,
which uses the image sensor and thus a direct measure of focus, by
terming the former as open and the latter as closed.

Stop putting up that smoke screen, nobody believes that junk
anyway.


-Wolfgang

[1] Look it up in the Princess Bride. As in "You keep using
that word ..."
.

Relevant Pages

  • Re: close focus formula
    ... the lens and reverse it and put it back to the cell.. ... so when the target is focus closer, ... a russian 4" maksutov at distance of 1 meter. ... This means the exit pupil coming from the ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... Yes, that won't change, the lens still behaves as a positive f8 lens, ... so when the target is focus closer, ... a russian 4" maksutov at distance of 1 meter. ... This means the exit pupil coming from the ...
    (sci.astro.amateur)
  • Re: Shimmed Mounts and Extension Tubes
    ... The AF marker blinks in red --- focus distance aquired. ... That's the wrong interpretation of the first AF point blink. ... AF" with the lens cap on. ... The AF marker (now on the far target) blinks red --- focus ...
    (rec.photo.digital.slr-systems)
  • Re: IRT: A New Theory of Relativity
    ... >nothing to do with distance. ... It is much like moving in the x-axis direction ... >the speed of light is much faster than the speed of the target that means ... >The first photon will have to travel a full two light-second to reach the ...
    (sci.physics)
  • Re: IRT: A New Theory of Relativity
    ... >nothing to do with distance. ... It is much like moving in the x-axis direction ... >the speed of light is much faster than the speed of the target that means ... >The first photon will have to travel a full two light-second to reach the ...
    (sci.physics.relativity)
Loading