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08-13-2010, 06:02 AM   #1
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Lens Registration and Compatibility

With all this talk about EVILs and registration distances I have a question. This may or may not unveil me to be incredibly ignorant about optical design but I don't care!

Would it be theoretically possible to design a lens for, say 20mm registration distance, which is perfectly compatible with the K-mount's ~45mm registration distance? i.e. a lens which can achieve infinity focus across two different reg distances?

I ask because I wanted to know if Pentax could make a full sized (diameter wise) K-mount EVIL with a short reg distance but make a lens which is compatible both with the EVIL and with the original K-mount as well. Obviously you'd have issues to consider with field of view etc, offset lenses and so on.

I've put this in lens discussion because I wanted to know if this was physically possible rather than discuss it's rumourish potential!

I am guessing it isn't. Or if it is it would be very complicated and not worthwhile.

08-13-2010, 07:10 AM   #2
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no, this is an optically impossible concept for a single focal length lens. It would be possible if some sort of additional optics were added to the exit pupil of the lens in order to shorten the back focus distance. However, there would be a high penalty in terms of optical quality, not to mention lens speed If such a thing was attempted.
08-13-2010, 07:27 AM   #3
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In theory, the only way to do this would be to have a mount adaptor that would fit inside the K mount,

this would require an electronic as opposed to mechanical aperture because the linkage would not work, and even if it was achieved, it would need the mirror locked up because the registration distance is largely to allow the mirror to move.

the other option would be an adaptor, so the 46mm registration of the K mount would be spaced out by the adaptor.

this has all sorts of interesting possibilities attached.

in 26mm there is enough space to put machanical linkage in place to have an M42 to EVIL adaptor with automatic aperture, and, for the SMC taks, and K mounts. maximum aperture encoding and aperture coupling for set aperture, so p-ttl could work.

It would be a really interesting concept.
08-13-2010, 03:03 PM   #4
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Thanks for the replies. Good to know.

08-13-2010, 05:00 PM   #5
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I am curious (not yellow). Teles contain rear elements called a TELEPHOTO GROUP, and wide-angles contain rear elements called a RETROFOCUS GROUP, a reversed teleohoto group, to change the assembly's optical length. Essentially, these are matched teleconverters. Why not include matched TC's with lenses, as register adapters? Other than the financial hit, of course.
08-13-2010, 05:13 PM   #6
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QuoteOriginally posted by RioRico Quote
I am curious (not yellow). Teles contain rear elements called a TELEPHOTO GROUP, and wide-angles contain rear elements called a RETROFOCUS GROUP, a reversed teleohoto group, to change the assembly's optical length. Essentially, these are matched teleconverters. Why not include matched TC's with lenses, as register adapters? Other than the financial hit, of course.
mainly because people want to use the focal lengths they have not give up something.

that is why the AF adaptor, while a fine TC never really took off, it changed the focal length and lens speed reducing the ability to use all lenses effectively

also, because there is a 20mm potential difference in regestration, why put a peice of glass in the adaptor, it does not need it.

also, above the reverse telephoto group on wide angle lenses would disappear with an EVIL lens because they are used only to lengthen the optical path. eliminating the mirror box would eliminate the need for these groups and reduce the optical complexity of a WA lens.
08-13-2010, 05:32 PM   #7
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QuoteOriginally posted by RioRico Quote
I am curious (not yellow). Teles contain rear elements called a TELEPHOTO GROUP, and wide-angles contain rear elements called a RETROFOCUS GROUP, a reversed teleohoto group, to change the assembly's optical length. Essentially, these are matched teleconverters. Why not include matched TC's with lenses, as register adapters? Other than the financial hit, of course.
Or simply design the lens with a long registration distance and make adapters of different thicknesses. Think Tamron Adaptall and T-mount.


Steve
08-13-2010, 09:11 PM   #8
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QuoteOriginally posted by Caat Quote
With all this talk about EVILs and registration distances I have a question. This may or may not unveil me to be incredibly ignorant about optical design but I don't care!

Would it be theoretically possible to design a lens for, say 20mm registration distance, which is perfectly compatible with the K-mount's ~45mm registration distance? i.e. a lens which can achieve infinity focus across two different reg distances?

I ask because I wanted to know if Pentax could make a full sized (diameter wise) K-mount EVIL with a short reg distance but make a lens which is compatible both with the EVIL and with the original K-mount as well. Obviously you'd have issues to consider with field of view etc, offset lenses and so on.

I've put this in lens discussion because I wanted to know if this was physically possible rather than discuss it's rumourish potential!

I am guessing it isn't. Or if it is it would be very complicated and not worthwhile.
Yes, it is entirely possible, if the lens is originally designed for the 46mm registration distance. All you need is a 25/26mm extension tube to mount the lens to the 20mm registration mount. If the 20mm mount was the PK-AF2 mount, a properly electronically equipped extension tube would neatly allow you to use the lens on the 20mm registration camera.

As to stopping the lens down, I have been stopping the lens down on extension tubes for years. All you need is an extension tube with the mechanical lever fitting. For an A exposure lens to work as if it were attached to the camera, you need the electronic contacts as well, of course. I don't have them on my 50mm tube, but I do have the aperture stop down lever, and it works just fine.

08-14-2010, 09:06 PM   #9
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Another tack: I've just run across a rare breed, the Soligor DualFocus lenses. The C/D 28+35/3.5 and the C/D 85+135/4 were primes with moveable elements, actuated by a slide on the former and a switch on the latter. Yes, they're TWO (smack) TWO (smack) TWO LENSES IN ONE! So why not build a lens that is register-switchable? Other than the money hit, of course.
08-15-2010, 12:27 AM   #10
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QuoteOriginally posted by RioRico Quote
Another tack: I've just run across a rare breed, the Soligor DualFocus lenses. The C/D 28+35/3.5 and the C/D 85+135/4 were primes with moveable elements, actuated by a slide on the former and a switch on the latter. Yes, they're TWO (smack) TWO (smack) TWO LENSES IN ONE! So why not build a lens that is register-switchable? Other than the money hit, of course.
Leica have had their Tri-elmar lenses for years now, which are based on a similar concept and they are reasonably good as far as Leica glass goes. However they are slow,(f/4) and optically can't hold a candle to the primes Leica makes.

I think having an extension tube system to cover the different back focus distance to defeat the purpose of small cameras of that require short back focus lenses. The Leica 35mm f/1.4 is half the size of a Canon 35mm f/1.4 so you're telling me you would be happy with putting a larger SLR designed lens PLUS an adapter in order to use it with a camera that would already have smaller lenses designed for it? It honestly doesn't make sense to me, if you want to use SLR lenses use an SLR. If you want to use RF lenses get a rangefinder!
08-15-2010, 05:02 AM   #11
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In re reading the OPs question I believe the short answer is NO. There would be too many obstacles to take a lens designed and optimized for a 20 mm registration to make it work on the 46 mm registration of pentaxs other camras. Going the other way is simple but nut cost effective
08-15-2010, 07:52 AM   #12
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QuoteOriginally posted by Lowell Goudge Quote
In re reading the OPs question I believe the short answer is NO. There would be too many obstacles to take a lens designed and optimized for a 20 mm registration to make it work on the 46 mm registration of pentaxs other camras. Going the other way is simple but nut cost effective
It would be as cost effective as using screw mount lenses on K mount SLR cameras. A small adaptor to use your existing lenses.
08-15-2010, 08:52 AM   #13
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In 2000, Pentax reworked the FA 43mm ltd into a Leica screwmount model called the L 43mm/1.9 Special. Pentax has always had a registration distance of 45.46mm including the Asahiflex M37 mount. The Leica screwmount is 28.8mm.

Samsung just came out with an APS-c EVIL. The interesting thing about it is its registration distance of 25.46mm. Samsung has a K-mount adapter available. It costs $199 through BH at the moment. So yes, its doable with a concurrently designed adapter.



Samsung ED-MA9NXK K-Mount Adapter for NX10 Digital ED-MA9NXK -

Last edited by Blue; 08-15-2010 at 09:20 AM.
08-15-2010, 09:05 AM   #14
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By maintaining backward compatibilty you don't utilize the benefit of EVIL, being smaller lenses, which results in lower cost to get higher margin (or just acceptable margin at larger market share or against stronger competition).
08-15-2010, 09:10 AM   #15
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QuoteOriginally posted by Digitalis Quote
I think having an extension tube system to cover the different back focus distance to defeat the purpose of small cameras of that require short back focus lenses. The Leica 35mm f/1.4 is half the size of a Canon 35mm f/1.4 so you're telling me you would be happy with putting a larger SLR designed lens PLUS an adapter in order to use it with a camera that would already have smaller lenses designed for it? It honestly doesn't make sense to me, if you want to use SLR lenses use an SLR. If you want to use RF lenses get a rangefinder!
I would not want to use my K/M42 lenses on a m4/3 camera/rangefinder. I would only buy an m4/3/rangefinder camera for its diminutive size and the higher specced lenses available for it. Putting a relatively giant and slow SLR lens on it defeats the purpose of having an m4/3 camera. Perhaps it would work in a pinch but it's not ideal.

M4/3/rangefinder lenses also hold better specs. You can get an f1/0.95 m4/3 lens for $300 and an amazing Angenieux 25mm f0.95 for under 1k.
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