My first mount conversion

sorted78 · 2011-04-02T22:15:31+0100

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.

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sorted78 sorted78

519 posts
15 years
West Country

Posted 21/09/2012 - 20:44 Link

Quote:
How did you move the lens element forward?

Do you mean the Meyer 100/2.8? If so, the optical block actually needs to move backwards in the mount to accommodate the difference in registration distance. I did this by removing the name ring around the front element using a rubber sink plug and:

Quote:
...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 haven't plucked up the courage to have a go at a Canon 85/1.2 conversion yet. I've been temporarily waylaid by an FA77 and Pancolar 80/1.8... I have to say that in view of the amount of engineering likely to be needed to adapt an FD mount lens, I'd be tempted to get a Sony NEX mirrorless body or similar and an FD/FL adapter if I ever got into using old Canon glass.