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03-21-2008, 07:29 AM   #1
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Autofocus for manual lenes *could* be possible, read and tell me what you think

I direct your attention to the Contax AX

Contax AX - photo.net

This thing focused by moving the film plane!

Surely today this would be *much* easier. . with shake reduction too!

Set the lens to some mid focal length, and let the sensor move back and forth to focus the image. I bet you could make it dead silent and *extremely* fast too. At least as fast as shake reduction -- probably real time subject tracking.

Future lenses wouldn't even need the complexity of a focus ring -- much less expensive internal motors.

Alas, I'm sure someone has thought of this and discredited it for some reason. But it sounds like a hell of an idea IMHO.

03-21-2008, 09:21 AM   #2
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This is not the first design, but it was the first camera with that system to hit production...

The system was already obsolete when it came out:
if you look closely, you will see that not only the sensor should move, but also the prism of the viewfinder (if not then you would never cinfirm in the vf if the thing is in focus or not), and therefor the thing is dead slow...
03-21-2008, 09:21 AM   #3
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Huh. What'll they think of next. And by "they" I mean people 10 years ago.


But anyway, something I hope Pentax does implement is the "electronic rangefinder" in Nikon's D60 a little scale which shows you which way to turn the lens to get the selected point into focus.
03-21-2008, 12:09 PM   #4
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That's the basis of the Sensor Tilt & Shift idea, a much more feasible and interesting feature :]

You could do the same by pushing/pulling the lens mount in and out, that way you don't need to move the whole innards of the camera. But I think image quality would not be the same, lenses are designed for a specific focal lenght.

03-21-2008, 12:53 PM   #5
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Moving the mount in and out would change the registration distance and alter focal length. Not good.

On the other hand, sensor T&S would be fab.
03-21-2008, 03:24 PM   #6
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No, not really. Moving the lens mount (or the sensor) in and out will change the focusing distance (of the mounted lens), and this is what we want The only difference is that, instead of moving one or more groups in the lens you move the whole lens.
Of course, a design like this will have serious limitations: speed, design complexity, or the impossibility to use "advanced" lens designs like inner focus (really useful for a huge telephoto).
03-21-2008, 06:58 PM   #7
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I think the Pentax way (motor in body, lens has a screw only) was as close as was realistically meaningful. Yet, it was abandoned in favor of in-lens motors (SDM) ...
03-21-2008, 10:31 PM   #8
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QuoteOriginally posted by mattdm Quote
Huh. What'll they think of next. And by "they" I mean people 10 years ago.


But anyway, something I hope Pentax does implement is the "electronic rangefinder" in Nikon's D60 a little scale which shows you which way to turn the lens to get the selected point into focus.
Gee, my SF-1 tells me which way to turn the focusing ring. My MZ-S doesn't.

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