Converting Canon 250mm f4 Mirror-Reflex to Micro Four Thirds

[xaprb]

I have an absolutely like-new-in-box copy of this lens but haven't really been able to use it much. I'd inquired with some well-known places about machining a replacement mount for this lens, but that wasn't successful. I tried some clamp-on adapters but couldn't get the lens close enough.

Today I rifled through my box of miscellaneous adapters and found one that has a rear I can use as a donor mount: the Kipon Leica M to Micro Four Thirds adapter. The Kipon adapters are underrated in my opinion. They consistently measure within much better tolerances than most of the popular brands that "feel more premium." Part of their non-premium-feeling is because they're made of aluminum, which is perfect for a project like this, because it makes it easy to drill and file and work with a Dremel tool.



The VL mount removes easily from the back of the lens, provided you have the CORRECT JIS screwdrivers of high quality! I would not attempt this with make-do tools or there's a lot of risk of damaging the lens, because the screws were very tight and required a lot of force to break free. The mount has contacts and a chip, but there's no engagement with the body of the lens; it's just screwed in and the electronics are contained entirely within the mount.



The VL mount fits within a small protruding flange on the back of the lens, and the Kipon donor mount is larger than that. However I decided not to try to reduce the Kipon mount in size to fit inside that flange like the native mount. I mounted the Kipon part on the camera and held the lens, sans the native mount, against it; and it focused well past infinity.

I did some calculations to see where the flange-focal-distance might theoretically need to be adjusted, but a) the information I've found online stating that the VL mount has a FFD of 20mm is apocryphal, not from official sources and b) since it focused past infinity, I decided it was good enough to just mount it without further thought.

I made a paper template and drilled holes, then adjusted them with a dremel diamond burr attachment until all the screws fit well. This is another area I would caution anyone else trying this. You don't want to force the screws if they're misaligned, or you'll end up with mangled threads in your lens's screw holes. As it was, I needed to mount and remove the Kipon part about 20 times, so I worried a lot about that much wear on the screws. But it all ended well as far as I can tell.

I countersunk the holes with a larger drill bit. Not ideal, but it seemed to work well. Also perhaps not ideal: the Kipon part is a bit thin, and there's not a lot of aluminum left for them to grip after the holes are countersunk. But they tightened firmly into that material, so although I worry it might not be strong enough to survive anything forceful, it does seem strong enough as long as I don't abuse the lens.

There are five screw holes in the VL mount, unevenly spaced. This is to allow the mount retaining pin to slide over smooth metal as you mount the lens, without it sliding over a screw hole. However I would note that if you look carefully, it isn't the case that all but one falls on the points of a regular pentagon. I don't know why, but they are more irregular than that -- just barely. You can verify this by rotating the VL mount around and seeing if different alignments will make several of the screw holes line up perfectly, and they won't.

I got confused and drilled the screw holes in the Kipon part such that one of them had to land inside the hole where the Micro Four Thirds retaining pin goes. I would not recommend this. I'd recommend carefully analyzing how to drill them so you have a large screwless stretch of metal leading to the retaining pin hole, plus five screw holes. But in the end, this too turned out to be a non-fatal error. The lens doesn't end up mounted with its infinity-mark straight up, but that doesn't matter to me.



I immediately took the lens out for a walk on a cloudy gray day. This is a really, really nice lens compared to most mirror lenses. If you are attracted by the idea of owning one and mount-converting it, I'd say go for it. The lens isn't designed to cast such a big image circle, so the edges and corners get a little wild, and there's high distortion, but it's great for center-weighted things, and it's got nice character for portraiture and so on (I won't share photos of my family, but I took some and they look gorgeous). Sharpness, contrast, and colors are all very, very good for a mirror lens, plus it's f4, which is great when the light isn't as strong. If you've wanted a Minolta Rokkor RF 250mm f5.6 but don't want to pay that much, and you can tolerate interesting edges on a cropped sensor, this is a good alternative in my opinion. (I doubt this lens is up to the task on full-frame, based on sample photos I've seen, unless you want a LOT of character at the edges). The build and handling of mine are absolutely wonderful! Also, at least as I've converted it, it focuses quite close and makes a very good pseudo-macro, albeit with an incredibly thin depth of field: literally a single antenna on a bug I photographed. And there's the built-in neutral density filters, which I used to good effect to blur moving water. Very, very nice lens!

I'm actually ordering a VL-to-EOS adapter so I can measure it and see if I can determine the true flange-focal-distance of the VL mount. I have a feeling that if this lens is spaced correctly, some of the issues like the distortion might be better. I can't prove it, it's just a gut feeling. I think it's got a lot of focal-plane curvature as I have converted it, but it's also hard to tell, because the edges get so blurry.

Here's some samples, all unprocessed; JPEGs straight from the Olympus E-M1 MkII. These are not the nicest or even best focused samples I took today, they're just the ones without family members' faces!

[xaprb]

I've measured the "Canon EOS Adapter VL" and the flange-to-flange distance is 23.96mm. Canon EF (EOS) mount has a flange-focal distance of 44mm, so this confirms to me that VL mount's FFD is 20mm. The internet was right!

[Knudsen]

[Raxar]