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05-25-2011, 07:40 AM   #1
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Minolta lens on Pentax body - not your normal question!

Okay, so I'm aware the Minolta AF register difference is 1.5mm less than the Pentax register difference... but what about the Milota 500mm autofocus reflex mirror lens? Every mirror lens out there, as I understand it, will focus *PAST* infinity due to the nature of the design. The screw drive for AF looks identical... could a simple custom bayonet do the trick for this particular lens?

I must admit, I would pay through the nose for an AF mirror lens....

Charles.

05-25-2011, 07:56 AM   #2
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I have no answers, but a resource - there is a techie/guru you can find over on dyxum forum (dyxum.com) or pbase, Pete Ganzel, he would probably be able to give you some insight on this conversion.

Pete Ganzel's Photo Galleries at pbase.com

Good luck!
05-25-2011, 08:20 AM   #3
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I don't have a Minolta AF lens mount around to play with but since no mechanical or electrical contacts would apply to a fixed aperture mirror lens a conversion might be possible if you didn't mind permanently modifying the lens.

Consider grafting a PK bayonet mount plate from a sacrificial lens (or extension ring) to the mirror lens. The only radial registration issues would be optical centering and orienting the latching notch relative to the hole for the AF screw. Also making sure the AF screw length was sufficient to span the mount.

Given the light weight of most mirror lenses epoxy glue should suffice and would be much easier than drilling and tapping for screws. I've modified an Oly OM mount to PK function on a Tamron 300/5.6 this way.

That's an interesting project and it seems feasible to do a trial run with a PK extension ring to proof fit and function before committing to altering the lens.

H2
05-25-2011, 08:29 AM   #4
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in looking at a minolta mount, if you get a cheap pentax AF lens with a metal mount yes you may be able to graft the pentax mount onto the minolta lens. I would suggest a combined screw and adhesive attachment.

05-25-2011, 08:49 AM   #5
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QuoteOriginally posted by Lowell Goudge Quote
. . . pentax AF lens with a metal mount . . .
Using a PK/A - AF bayonet mount plate (and it will be PK/A if it's AF) will save you the trouble of locating the AF screw hole relative to the latch notch if you have one.

But if the only bayonet plate you have is the PK version you can easily transfer the AF screw hole location using a piece of frosted cellophane tape as a template to mark the holes' locations. I double the tape to provide a less "delicate" transparent template.

You can also transfer the location by carefully swinging arcs with drafting dividers.

H2
05-25-2011, 08:50 AM   #6
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Without electrical contacts...would a Pentax body know to engage the AF drive?
05-25-2011, 09:34 AM   #7
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QuoteOriginally posted by jgmankos Quote
Without electrical contacts...would a Pentax body know to engage the AF drive?
That's my thought. Even a Pentax-F lens can communicate a lot of data to the camera. I don't think anyone knows how much of this the camera depends on. It might be something you can fake with a chip from a broken AF lens.
05-25-2011, 09:42 AM   #8
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Well, first step would be to get the lens, remove the bayonet, install a bushing spacer over the lens the same thickness as the bayonet base, hold it in front of my pentax, and see if I can focus to infinity. If that doesn't work, no sense moving forward.

Charles.

05-25-2011, 12:11 PM   #9
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QuoteOriginally posted by ChopperCharles Quote
Okay, so I'm aware the Minolta AF register difference is 1.5mm less than the Pentax register difference... but what about the Milota 500mm autofocus reflex mirror lens? Every mirror lens out there, as I understand it, will focus *PAST* infinity due to the nature of the design. The screw drive for AF looks identical... could a simple custom bayonet do the trick for this particular lens?

I must admit, I would pay through the nose for an AF mirror lens....

Charles.
I think your chances of this working are exactly zero - that is an f/8 lens, and even if you could overcome the crazy hurdles of lens mount, register distance, and Pentax-compatible electronics to talk to the camera and communicate focal length, there's nothing you can do to overcome AF issues at f/8. I do not know how Minolta developed an AF system that works with that lens at f/8, but nobody else has ever done it, and no other lenses in the Minolta/Sony mount will AF at f/8: "In terms of the Minolta AF and subsequent Sony α SLR systems, this lens is an anomaly, being the only lens able to auto focus at f/8 (with the exception of this lens, all other AF lenses must be f/6.3 or larger for AF to function)"
05-26-2011, 05:56 AM   #10
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I just don't understand why someone won't build an autofocus mirror like Minolta did. It's obviously possible. There are f6.3 mirrors out there right now by Opteka/Vivitar/Bower.

I hate MF, I just really suck at it. Same reason I haven't bought the 85mm Rokinon, even though I'm in love with the images it produces. I just suck at MF, and especially MF with a razor thin DOF. Even with a dual split focusing screen installed.

Charles.
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