Registry distance quandary Canon/Minolta?

[motleypixel]

Hey gang,

I’m in the midst of planning a fun project. So a little background first…

I acquired a Minolta MC Rokkor-PF 58mm f/1.4 awhile back and decided to buy a fancy custom EOS adapter. Well, after adapting it and putting it on my 5DMK2 I quickly found out that the mirror would hit the rear element as I focused towards infinity; in fact, at only about 15ft it hit. I’ve since have left it on my 30D and haven’t had any issues.

I have found the some have grinded the rear element housing ring down and/or grinded their 5D mirrors to get infinity focus to clear. Since I purchased a used mini-lathe: http://mini-lathe.com and will attempt turning the rear element group and cutting down this area of the lens group in hopes to get closer to infinity. Even if I can get 25ft I will be happy and then start using it more on my 5DMK2. I will not grind my mirror. I would only do so if it were easy to remove the mirror completely from the camera.

Now that was the summarized background, now to my project. I have an MD Rokkor-X 50/1.4 (55mm thread version) and I’ve got a game plan. Seems like I can remove the aperture base plate and face about 2mm off (slight raised area) and then use a M42-EOS adapter to turn it down about 2mm (on the back side) and turn the inside threads to open it up to place over the original MD mount. Then using my small drill press and using the old MD mount as a template drill the four screw holes and then use longer screws to re-mount everything. This is a rough explanation, but I fully intend to document this effort and even make a short video of the process. I even found the M1.6 X 0.35 X longer (5mm) screws I need.

Anyway, I’m trying to get a firmer grip on registry distance (distance from the lens to the focal plane right?). Here’s what I know. Canon’s registry distance is 44mm and Minolta’s is 43.5mm. So is this the distance from the rear of the lens to the sensor? What’s confusing me is looking at several of my Canon EF lenses (70-200 f/4L, 100/2.8/macro, 17-40 f/4L) all at infinity seem to be varying in lens position relative to the EOS mount.

I want to understand this registry distance better so I can target a final position of the end of the lens relative to the new EOS mount of this Rokkor-X 50/1.4. Without knowing this aspect well will result in machining assumptions which will result in too little or too much material machined off the stock. I do have a good amount of infinity focus adjustment to fine tune, but I’m patient and want to understand this aspect first.

Thanks,
Roy

[sichko]

It's the distance from the film/sensor plane to the front of the camera flange - the circular ring that makes contact with the lens. The position of the back of the optical unit relative to the film/sensor plane will vary according to lens design including focal length.

EDIT Imagine that you have a Minolta lens on a Minolta camera with a register of 43.5 mm. The lens is focused to infinity. Now replace the Minolta sensor with a Canon CMOS device (+ associated electronics) and put a Canon badge on the front. The position of the lens relative to the film sensor hasn't changed so the lens is still focused at infinity. And you now have a Canon camera ? Not quite. Canon has a register of 44 mm. So you thicken the flange by 0.5 mm. But this pushes the lens 0.5 mm away from the sensor and you are no longer focused at infinity. So to get back to infinity you need to shorten the lens by 0.5 mm.

[visualopsins]

...placing the rear element exactly where it was before and it hits the mirror.

Use the lens on a short extension tube with your fancy adapter rather than tear it up, please. On extension tube; closer focus; mirror no hit.

[ZoneV]

My Minolta MC Rokkor 58/1.2 works on my EOS 5D (I) without grinding the camera mirror on infinity.
I had only to file the housing of the last lens down. That was easy with a hand held file - after screwing that tube away from the lens.
Some images from that could be seen on my Rokkor 58 mm /1.2 conversion page:
http://www.4photos.de/camera-diy/minolta-rokkor-58-to-canon-eos-conversion.html

[motleypixel]