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08-13-2010, 12:23 PM   #1
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Minolta MD/MC to Pentax K Mount

Last week I picked up a Rexatar 300mm f/4 lens at a thrift and due to my lack of knowledge it turns out that this lens is a Minolta bayonet mount, rather than Pentax K mount.

Oops.

I've now plans on modifying this lens. I've ordered up two m42-to-K adapters from eBay for $6 total--one for modding, another for correctly modding !

I've already had the back of the lens in pieces a few times to understand the task I'm taking on as well as have been reading up on the registering distances for both specifications.

So far I know this;
1. I will need to remove the back of the current lens (a rather thick piece of metal will come off with it), then drill the screw holes
2. I will then need to build-up the back of the adapter with some spacer to get infinity to focus correctly
3. Then I will need rebuild the activator arm assembly for the aperture so that it will contact inside the Pentax body correctly

Easy peasy.

Here's the lens in question (which I think looks a lot like the SMC Pentax 300mm f/4);



Stay tuned!

08-13-2010, 01:32 PM   #2
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OK

I modified a MD mount kiron 28mm F2 and here's what I found.

THe extra little tabs etc will need to be removed from the aperture ring

the difference in thickness of the origonal mount is almost enough to compensate for infinity focus, but on the kiron that was also adjustable.

there is a thick base that provides the aperture stops detents, for the lens and also acts as the place to mount the lens mount THis might need to be reduced in thickness, I was able to surface grind my copy.

I don't recall but I think the aperture activation arm movement is the wrong way, so I deleted it totally and use the lens as a preset
08-13-2010, 03:48 PM   #3
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The apertures of the Rexatar I've got and the Pentax 50mm are controlled by the same motion; a clockwise direction, looking at the mount.

One thing to note is that the aperture actuator is almost in-line (180~ish offset) with the 'front' of the lens on the Rexatar, while the Pentax's actuator is at a 90~offset. (angles measured to the 'back' of the actuator lever arm)

Pentax Lens


Rexatar Lens


Front-vs-actuator arm offset illustration (bottom arrows depict 'front' of lens)
08-13-2010, 04:01 PM   #4
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Addendum;

Demonstration of the thickness of mount, measured at 4.70mm


Camera is centered where the new location of the aperture actuator (tab) should live, 90 from current position. Also, note that the thick silver ring is permanent part of the mount.


Worth noting from a post in another thread;

QuoteOriginally posted by vietlh81 Quote
The Minolta MC/MD register distance is about 43.72mm, whereas the Pentax one is 45.46mm. The difference between them is about 1.74mm.
(Wikipedia states 43.50mm and 45.50mm respectively)



Ergo, using vietlh81's numbers) when I remove the old mount I need to add;

(43.72 + 4.70 - x) - 45.46 = 2.96 - x, where 'x' is the thickness of the flange on the m42-to-K adapter plate. (I think my math is right on this one... I admit that I don't completely understand the whole flange-mount-distance measurement.)


Last edited by thefragger; 08-13-2010 at 04:16 PM.
08-19-2010, 06:46 PM   #5
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Bump!

Can anyone who know about this kind of thing check that I'm doing my math correctly?


pretty please!
08-19-2010, 07:09 PM   #6
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QuoteOriginally posted by thefragger Quote
Ergo, using vietlh81's numbers) when I remove the old mount I need to add;

(43.72 + 4.70 - x) - 45.46 = 2.96 - x, where 'x' is the thickness of the flange on the m42-to-K adapter plate. (I think my math is right on this one... I admit that I don't completely understand the whole flange-mount-distance measurement.)
I'm not sure where your 4.70mm came from. The Camera Mounts Sorted by Register page agrees on the Minolta MC/MD and Pentax K mount registers. I just measured the thickness of an adapter flange ('x') as 1mm. So I would *think* (don't hold me to this!) that you would want to remove (45.46-43.72)-1 = 2.74mm from the Minolta base. If you removed 3mm, the Minolta would focus past infinity, and would lose a bit of close focus. If you only removed 2.5mm, you wouldn't reach infinity focus, but you'd have a chance to shave off another 0.25mm. Good luck.
08-19-2010, 07:58 PM   #7
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QuoteOriginally posted by RioRico Quote
I'm not sure where your 4.70mm came from. The Camera Mounts Sorted by Register page agrees on the Minolta MC/MD and Pentax K mount registers. I just measured the thickness of an adapter flange ('x') as 1mm. So I would *think* (don't hold me to this!) that you would want to remove (45.46-43.72)-1 = 2.74mm from the Minolta base. If you removed 3mm, the Minolta would focus past infinity, and would lose a bit of close focus. If you only removed 2.5mm, you wouldn't reach infinity focus, but you'd have a chance to shave off another 0.25mm. Good luck.
Wonderful! Thank you very much for the confirmation.

The 4.70mm is the thickness of the shoulder that's on the old mount--it's this big hunk of metal that I plan on replacing with the K-mount+spacer. The black tube is held into the mount by a few 1.2mm screws around its diameter.

So what I calculated is that when I remove the old MC/MD bayonet piece, that 4.70mm shoulder comes with it. But then I'll have to build up a spacer, while taking into consideration the K-mount thickness, to get the right focal register distance.

This is the assembly in question showing the thick shoulder;

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