My first mount conversion


sorted78

Link Posted 02/04/2011 - 22:15
I've been after a Meyer Optik Orestor 100/2.8 for ages but had lost out a quite a few to the eBay snipers. A coupe of weeks ago, I found one in a lot with some other, random, fungus infested lenses and won it from a reasonable price.

What wasn't mentioned in the listing and I didn't spot from the poor photo was that it was an Exakta mount, not M42 (caveat emptor). On Exakta cameras, the lens mount is closer to the film/sensor plane than on M42 or PK mounts. This means that the lens would need to be mounted closer to the sensor than the PK mount to work on a Pentax DSLR without additional optical elements. However, I had a quick Google and it looked as if it should be possible to adapt it to an M42 mount without doing anything too drastic.

I thought it worth a quick write up here, as it turned out to be quite easy, non-destructive to the lens and well worth the effort.

From lurking over at mflenses.com, I know that Meyer Optik lenses, like Pentax SMC lenses are usually salvageable from fungus. It was pretty filthy when it arrived, but the optics cleaned up well, as did the outside:




The diaphragm was also stuck and needed cleaning up, which was a bit easier than the Takumars I have worked on (due to the lower build quality of the Meyer), but quite quite fiddly. The Oreston in Exakta mount has an auto/manual switch, meaning that the diaphragm can be used in stop down mode, ideal for use with an M42 adapter.

I bought a cheap m39 to m42 adapter (the type used to mount m39 lenses on m42 mount cameras), which is a snug fit over the Exakta bayonet mount:




After trial mounting it on a broken Spotmatic body I had lying around, to get the axial alignment right (so that the lens is the right way up when mounted on the camera) I glued it in position. Depending on what sort of glue you use, it can always be removed later to reverse the conversion.

The most complex part of the process was accommodating the difference of approx 0.75mm in register distance (lens mount to sensor plane) between the Exakta and M42 mounts. The glass needs to be moved nearer to the sensor in order to get infinity focus on an M42 mount.

There is provision for adjustment of the focus distance by slackening the screws holding the focus ring to the focussing helix and moving the ring to a new position. By doing this, calibrating for infinity focus by focussing on a far away object (I used a telegraph pole approx 100 meters away) with it mounted on the Spotmatic and then re-tightening the screws, I managed to adjust it enough to get infinity focus on the M42 mount.

I took a few shots with it this morning and am quite pleased with the initial results:

Shot @ f/2.8 (I think), JPEG striaght out of my K-m:



100% crop from the above:



It seems pretty sharp wide open with colours a bit more muted than Pentax glass, but not unpleasant.

I hadn't really intended to get in to doing mount conversions as I'm not fortunate enough to have access to machine tools, but it was a lot more straightforward than I thought it would be; the only tools I used were a screwdriver, a rubber sink plug (to remove the trim ring around the front element), a pen and some glue. No drilling or machining to the original lens in needed, so the conversion can be reversed to allow the lens to be used on an Exakta again should anyone want.

testdasi

Link Posted 02/04/2011 - 22:48
Nice! Do you know how to convert Canon FD to Pentax K? I love my FD 50mm f1.4 and feels very bad to sell it.

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hefty1

Link Posted 02/04/2011 - 23:43
Very nice results. I was put off Meyer a little after exposure to a couple of Domiplans, but this design looks to be altogether superior. Good job!
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Mannesty

Link Posted 03/04/2011 - 07:53
testdasi wrote:
Do you know how to convert Canon FD to Pentax K?

I'm 99.999% certain it can't be done. The principal reason being that the Canon mount has a larger diameter than the Pentax. You can fit Pentax lenses to Canon bodies, but not Canon lenses onto Pentax bodies.
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sorted78

Link Posted 03/04/2011 - 08:07
Quote:
Do you know how to convert Canon FD to Pentax K?

I've looked in to this, as I really fancy having a go with one of the early FD 85/1.2 SSCs on a Pentax but I don't think it's straightforward. Again the register distance is smaller, so you either need to move the glass back in the lens or take some metal off the lens mount.

Some of them also have 'floating' lens elements near the original mount which need to be moved if you do this. It's not impossible, but probably involves metalwork and optics beyond my skills at the moment - I think that the chances of unintentionally converting the FD lens into a paperweight are higher!

Quote:
I was put off Meyer a little after exposure to a couple of Domiplans

I've never had a Domiplan, but have been put off by their reputation. However, some of the other Meyer lenses are quite nice. I also have an Oreston 50/1.8, which is sharp from wide open and has rendering that I quite like. I've tried the Oreston 135/2.8 as well, which was OK, but nothing special (may have been a duff copy, as others seem to rave about them).

karma mechanic

Link Posted 03/04/2011 - 10:32
sorted78 wrote:
I think that the chances of unintentionally converting the FD lens into a paperweight are higher!

Putting it back together in some sort of random element order might work quite well, after all look how much people pay for Lensbabies
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Algernon

Link Posted 03/04/2011 - 10:37
Very interesting conversion. I can't imagine anyone
wanting to reverse it.
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sorted78

Link Posted 03/04/2011 - 14:34
Quote:
Putting it back together in some sort of random element order might work quite well

I hadn't thought of that - sounds like a good option for plan B.

Stuey

Link Posted 03/04/2011 - 17:37
I like your thinking with regard to the conversion

I have a Prakticar 50mm f1.8 I bought a few weeks ago having made an error of bidding on the wrong one (one had pk and the one I got pb mount) of course when you make an error in a bid you end up winning

I have mounted the pk mount to the lens but to achieve infinity focus I need to lose 1.1mm which I reckon can be done but in a destructive way

I haven't yet tried for inififnity focus as I have been busy but with your mention of adjusting the focus internally this is an option I may look into before getting the grinder out

Nice photo's from your conversion - well done
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sorted78

Link Posted 03/04/2011 - 20:24
Quote:
I have a Prakticar 50mm f1.8 I bought a few weeks ago having made an error of bidding on the wrong one

I've got one of those too, badged Carl Zeiss Jena. I haven't got around to taking mine apart yet, but I believe it is optically identical to the Pentacon 50 1.8 which is itself a rebadged Meyer Oreston so it's probably worth a go with the grinder. I'd be interested to hear how you get on. I bought mine thinking it was a rebadged CZJ Pancolar and couldn't understand why it was so cheap.

My next project is a Komine 90mm 1:1 macro lens in C/Y mount. I've had one previously in M42 mount and was pleased with it and recently came across a really good deal for one in C/Y mount. I thought it would be worth a go since the register distance is similar to the M42/PK mount, but the bayonet is integral to the rear part of the lens body, so it will involve some metalwork and possibly a donor lens for an M42 mount.

Stuey

Link Posted 04/04/2011 - 12:26
[quote:3496ace15f="sorted78"]
Quote:
I have a Prakticar 50mm f1.8 I bought a few weeks ago having made an error of bidding on the wrong one

I've got one of those too, badged Carl Zeiss Jena. I haven't got around to taking mine apart yet, but I believe it is optically identical to the Pentacon 50 1.8 which is itself a rebadged Meyer Oreston so it's probably worth a go with the grinder. I'd be interested to hear how you get on. I bought mine thinking it was a rebadged CZJ Pancolar and couldn't understand why it was so cheap.

Hello, so far I have removed the PB mount, removed the electrical contacts and mounted a PKM mount to the lens - my next attempt would have been to lose the 1.1mm from the lens or more likely the mount - I like the idea of adjusting the focus ring/ mechanism if this turns out to be possible - I will take a look at it when I get a chance.

I will let you know either way, success or disaster
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gillbod

Link Posted 07/04/2011 - 08:23
good work. i always thought exakta would be difficult to convert due to the big piece they use at the back, but never thought of glueing the m42 over the top. the m39 to m42 adapter is a nice bit of thinking.

are there no worries with the exakta mount hitting the mirror? if not, then i'm picking up some exakta zeiss stuff to play with...
k7, 15/4, 31/1.8, 70/2.4, 200/2.8, Metz 58

sorted78

Link Posted 07/04/2011 - 08:53
Quote:
are there no worries with the exakta mount hitting the mirror?

I don't think so, certainly not an a crop sensor DSLR anyway - I checked the Meyer above by measuring how far the rear of the Exakta mount protruded from the flange and it is similar to the protrusion of the rear element of my Chinon 55mm f/1.4 at infinity.

You're right about the Zeiss glass - it might be worth a look, as the Exakta mount versions seem cheaper than M42. I don't know whether it would be possible to adjust the focus enough on Zeiss lenses, or how you would do it. I've taken apart an M42 Tessar before and that had a Helix that could be started in a range of positions, each one leaving the optical block at a different distance from the mount flange, so it may be possible to accommodate the difference that way.

Alternatively I think that the focus calibration is done by loosening a ring that can be unscrewed using a lens spanner, but I was so glad to get the helix back in the correct orientation, I didn't touch this!

SpottyDog

Link Posted 11/04/2011 - 21:50
I think that's right about the Tessar. The ring can also loosen itself in use, which uncouples the focusing ring from the helix so you have no focusing action at all

I managed to retighten mine (with two screwdrivers!) but it's fiddly to get both this ring and the helix correctly positioned at the same time!

fishin

Link Posted 21/09/2012 - 07:07
How did you move the lens element forward? I have the same lens and would like to convert it for my a77.
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