Need measurement of Pentax PK mount

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

I'm wondering if someone with Pentax PK mount lenses can help me with some measurements. I have just purchased a Vivitar Close Focus 135mm lens with PK mount. My desire is to convert the mount to Minolta/Sony A-mount. The registration distance is perfect to allow for an easy conversion if other factors cooperate. I need to know the inside diameter of the PK mount. If this lens allows me to remove the mount without also removing the rear element, I would like to place a M42 adapter in place of the PK mount. I need to know if it will fit.

More specifically, I will likely remove the bayonet portion of the PK mount, leaving only the flange which determines the registration distance. I would then mount the M42 adapter on top of the PK flange to accomplish the needed registration distance.

Unfortunately, some of these lenses have the center black surround of the rear element screwed to the original mount. If this is the case, it may be too much trouble if not impossibly to convert. In that event, I'll have a lens to sell.

Thanks much.

hk300 · Joined: 30 Oct 2008 Posts: 1041 Location: Hong Kong

please post some pics of the rear of this lens.

It is often possible to remove the rear of a non-OEM lens, because most are designed to be easily adapted by the lens manufacturer.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

Unfortunately the image is from the sale. I've tried to enhance.

The mount will easily remove, but my concern is that the center black element may remove with it. It will all come down to those three screws in the black center - whether or not they go into the chrome mount. I'm hoping not.

peterqd · Expire: 2014-01-04

The diameter across the black part on this Pentax-M lens is 43.1mm, sorry! Sad

hk300 · Joined: 30 Oct 2008 Posts: 1041 Location: Hong Kong

you probably can take off the black plastic rear surroundings and use a M42 rear or a m42-SonyAF adapter to convert the lens.

You will probably need a spacer to adjust for the difference in registration distance.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

Yes, if the black can be removed without the rear glass too, I can work around it. I can also remove the threads from the M42 adapter to increase the inner opening, but to 43.1mm, I don't know.

The registration distance should work out perfectly. I either cut the bayonet away and use the flange, or just make a spacer of equal thickness. The adapter I purchase is already at a thickness to correct for M42 registration distance, so with PK being the same, it will also correct for that.

At some point these conversions become more work and expense than they're worth. I'll have to make that assessment when I see the lens. Boomer is the beneficiary of one other such assessment by myself.

slick204 · Expire: 2012-04-22

I'll trade you the minolta mount from my 200/3 (minus the center tube with the rear element) for your pentax mount!

I don't suppose there's any way this would work, but it sounds interesting.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

I'd bet it would work if they were the same focal length. Maybe. Oh, wait, that wouldn't do me any good because I'd still need to convert to A-mount.

thePiRaTE!! · Joined: 31 Oct 2008 Posts: 416 Location: Canada

Heya Woodrim. I don't know specifically about your lens in question, but it's worth considering there may also be other unforeseen proprietary build anomalies that can impede the conversion. For example, you may recall the Topcor I modded in the way you described. It worked in that the mount (James Lao M42-A) was able to lay flat on the base of the lens once the mount was removed.

OTOH, I'm modding a Porst Color Reflex right now and realized that the aperture ring over-hangs the base, surrounding the smaller diameter of the original mount but preventing the wider diameter adapter from seating. I think in my case, I'll need to have the adapter scaled down before I can continue. Just something else to factor in when planning a walk into the wild, lol.

K.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

It's always something.

peterqd · Expire: 2014-01-04

francotirador · Joined: 17 Sep 2009 Posts: 894

Firts Problem
Minolta/Sony A 44.6mm
Pentax K 45.46mm
M 42 45.46mm

Second
If I understood correctly, you want to put a M42 mount a lens to work on a Sony Alpha?
If that is correct, it will not work.

You need the Camera Mounts Sorted by Register?
greetings.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

This seems to have gotten confusing for some trying to follow. The M42 terminology is probably doing it. The only way M42 plays into this is that the adapter is intended to convert a screw mount, M42 lens to Sony A-mount, but I use it instead as a source of Sony mount for the conversion. So M42 is only what the adapter is called, but not being used as intended.

I've found that different lenses' infinity point varies somewhat when using M42 converters even as intended, so I'm not worried about .01 of a mm. Within reasonable rounding, Sony registration distance is 44.5mm and PK is 45.5mm. Since the lens I wish to convert is PK at 45.5mm, and the adapter I'm using is roughly 1mm thick at the flange - Add the adapter to the existing flange level and voila! Correct distance. Maybe not quite that easy, but in theory, yes.

francotirador · Joined: 17 Sep 2009 Posts: 894

Not well understood, is the language difference.
I think you should have no problems. What might be done to remove the PK mount, is to put a washer separation measured for the 0.9 mm lens more.

Good luck

I love to do these things. Smile

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

We will see when the lens arrives. I have had success, I have had failure.

hk300 · Joined: 30 Oct 2008 Posts: 1041 Location: Hong Kong

As a DIY project it may be fun, but i guess it will cost more than to find a mint m42 135mm lens.

woodrim · Joined: 14 Jan 2010 Posts: 4060 Location: Charleston

If I factor in my time at going rate, maybe. But M42 commands a premium. If I use a plain adapter, it will cost under $10. If I use a chipped adapter, a little over $20. I have been buying chipped adapters, but recently tried a plain arrangement and shot in manual mode. It wasn't as bad as I thought it might be. No focus confirm, but honestly I didn't pay a lot of attention to that anyway. Manual mode brings me back to the old days.