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04-23-2013, 03:02 PM   #1
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Pentax closed loop AF

LensRentals.com - Autofocus Reality Part 3B: Canon Cameras

There is a interesting post about closed loop motor inside lens (Canon & Nikon have it). I just wonder if Pentax has anything similar.

04-23-2013, 05:00 PM   #2
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If I understand half of what was said, yes Pentax has had something along that line. Maybe not the exact idea but its focusing is based on bidirectional hunting for better precision. I've heard it stated for many years that although Pentax is slower than other brands, it is often more accurate because of its ability to hunt for focus in both directions until it gets it as close as possible..
I personally have not found that to be an outstanding truth. Maybe in some cases, but obviously Canon and Nikon have been able to focus accurately enough to satisfy their users.

But the other brands have of course made their focus system better and faster, where Pentax has basically sat on essentially the same system for many years while their users cry year after year for faster focusing.
04-23-2013, 07:00 PM   #3
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I'm thinking it's closed loop because of the behavior I've seen using manual focus lenses in AF mode.

If it was open-loop, the camera would drive the screw for some time (a known amount to get it into position) and then assume it was focused. That's not what happens.

Instead, the camera detects when the focus is correct. I don't have a good way to monitor it, but I think it's probably still driving that screw and thinking that it's moving the lens, at least until some maximum timeout is reached. When focus is achieved the camera would think it finally did the job and turn off the screw.

Thoughts?
04-23-2013, 07:05 PM   #4
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Pentax indeed has "something along that line", but this is not a continuous feedback.

The AF mechanism must stop at the position the first measurement indicated, to make the next, correcting measurement.
Without this "interrupted" closed loop, AF with third party lenses would only work if their contruction was a Pentax dedicated one.

One of the culprits is the precision matching of the screwdrive inside the lens to the specification of the Pentax AF system.
The feedback should correct matching errors (resulting in "hunting").
The sensor/cross sensor measurement provides exact information about how far the lens actually is out of focus. The firmware of the body decides the travel and direction (turnings of the screwdrive) which will be needed to get in focus. If the focus difference as a function of the screwdrive turnings does not exactly match the Pentax AF specification, the result will be a lot of hunting. But without this hunting, most or all of the pictures would be out of focus.

I can't believe any of the competition is still working without additional correcting measurement, at least with FF or APS-C. For smaller sensor sizes, it may not be needed.

04-23-2013, 08:12 PM   #5
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Pentax AF control is most definitely closed loop.

Check for yourself:

- mount a screw-drive AF lens
- hold down the lens release button
- half-press shutter
- play with the focus ring to bring focus point in and out of focus and see what the camera does.

QuoteOriginally posted by RKKS08 Quote
Pentax indeed has "something along that line", but this is not a continuous feedback.

The AF mechanism must stop at the position the first measurement indicated, to make the next, correcting measurement.
Without this "interrupted" closed loop, AF with third party lenses would only work if their contruction was a Pentax dedicated one.

One of the culprits is the precision matching of the screwdrive inside the lens to the specification of the Pentax AF system.
The feedback should correct matching errors (resulting in "hunting").
The sensor/cross sensor measurement provides exact information about how far the lens actually is out of focus. The firmware of the body decides the travel and direction (turnings of the screwdrive) which will be needed to get in focus. If the focus difference as a function of the screwdrive turnings does not exactly match the Pentax AF specification, the result will be a lot of hunting. But without this hunting, most or all of the pictures would be out of focus.

I can't believe any of the competition is still working without additional correcting measurement, at least with FF or APS-C. For smaller sensor sizes, it may not be needed.
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