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07-15-2014, 04:22 AM   #1
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Apeture iris behind lens

So, I asked a few months back if it were possible to build a AF 645 to K mount adapter (theoretically). The overall answer to this was that is was possible with some fancy engineering, to create an adapter with screw drove 645 FA lens. I've been giving this idea some thought, and I have come to the conclusion that I someone were to build this adapter, they could save themselves significant amounts of time if they didn't have to deal with find a way to convert the information from a 645 mount lens into a form that is readable by a K-mount camera, such as the Pentax K-5.This leads me to ask the following question.


Can the aperture be place in any place within the optical layout of the lens, would it be possible to build an adapter with an artificial iris in built (aka, one that can easily be controlled by a K-Mount camera). This would alleviate figuring out how the aperture is controlled in a 645 lens, and make building the adapter considerably easier. Just a theoretical question, but I would love to take on a challenge like this if the star align and I find myself with some spare cash and some genuine inspiration someday!

07-15-2014, 04:36 AM   #2
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You wouldn't be likely to pull off putting the aperture behind the lens without it shadowing when stopped down.
07-15-2014, 04:37 AM   #3
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QuoteOriginally posted by Joshua A Quote
Can the aperture be place in any place within the optical layout of the lens, would it be possible to build an adapter with an artificial iris in built (aka, one that can easily be controlled by a K-Mount camera). This would alleviate figuring out how the aperture is controlled in a 645 lens, and make building the adapter considerably easier. Just a theoretical question, but I would love to take on a challenge like this if the star align and I find myself with some spare cash and some genuine inspiration someday!
It might work- that's how the K to Q adapter is designed and it's quite effective. Some slight (probably via software) modifications would be needed to allow the camera to control the adapter's diaphragm electronically, since mechanical couplings are so last-century.

With that said, I don't really know if anyone would want such an accessory, hehe.

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07-15-2014, 04:47 AM   #4
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QuoteOriginally posted by Adam Quote
It might work- that's how the K to Q adapter is designed and it's quite effective. Some slight (probably via software) modifications would be needed to allow the camera to control the adapter's diaphragm electronically, since mechanical couplings are so last-century.

With that said, I don't really know if anyone would want such an accessory, hehe.


I'm honestly not worried about the marketability of such an accessory, it's more of a personal challenge sort of thing. But, hopefully in the future, if Parallax (I think that's his user name) figure how to load custom firmware in camera, couldn't you program the camera yourself to be able to read the adapter.


But, in the end, I would like to build an AF adapter. And who honestly wouldn't want to be able to use 645 lenses with AF capability.

07-15-2014, 05:28 AM   #5
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I suspect that 645 lenses are much rarer than K mount AF screw drive lenses, and an auto aperture function plus screw drive for KAF lenses would be likely easier, since part of the mechanism is already built, and marketable.

The other thing I have been toying with is why not an integrated optic like the 1.7x AF converter. Sure it adds focal length etc, but the AF converter has a reasonably low power need requirement for its moveable elements
07-15-2014, 06:09 AM   #6
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QuoteOriginally posted by Joshua A Quote
Can the aperture be place in any place within the optical layout of the lens,
Yes and no.

Yes, to some extend, if the lens is much larger than the sensor you are trying to adapt it to (K to Q comes to mind).

No, otherwise, most of the time.

You want to place the iris at the aperture stop (the place where the light beams occupy the smallest surface as seen from the side, generally speaking). Because otherwise, you would need to create a very large iris and that would become impractical and sometimes impossible to do.
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