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Bluelight
Tinkerer
Username: Bluelight
Post Number: 3
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 01, 2008 - 09:56 pm: | 
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Hello,
I have some LTM lenses that I had a hard time trying to focus on infinity and close range. ( Canon 50/1.2 and Nikkor 50/1.4)
I tried adjusting the LTM adapter thickness and I a thinking that I may need to shim the lens block to helicoid.
What's the general procedure to shim a lens and what material should I use as a shim?
Thakns.... |
Glenn
Tinkerer
Username: Glenn
Post Number: 338
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 02, 2008 - 03:42 am: |
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Thick 'shims' are generally a turned alloy ring. The thinner ones I have come across were made from brass or steel 'shim stock'. Russian manufactures even used paper or thin card.
However a word of warning. It would appear you are chasing a problem that has three variables! Camera flange to film distance, adapter face to face distance and lens optical cell to lens mount face distance.
Lenses were shimmed during manufacture for a number of reasons. The main one being to correct manufacturing tolerances. In quality LTM lenses there appears to be a very limited range of thicknesses for each lens type. This adjustment along with a similar procedure for the body flange to film plane, allows for the interchangeability between various bodies. You stand a chance of loosing this 'swap-ability' if you do not check that ALL dimensions are to specification.
The LTM adapter is the fly in the ointment. What camera are you trying to use the lenses on? More importantly; where does the adapter originate from? There are some very 'iffy' ones being sold.
If you have not already done so. Check the infinity focus of the lenses when mounted on a suitable LTM body. Have this done at a repair shop if you cannot do it. You should then check the flange/face to film plane dimension, when the adapter is mounted on your particular body. Only when all these variables have been checked, should you go down the shim route.
A quick method of carrying out a 'test shim' is to use small squares of card/paper located at 12, 3 , 6 and 9 o'clock on helicoid face. Retain these with a smear of grease, whilst you insert the lens block. Vary the thickness of the pieces, until you obtain correct infinity focus. Then measure the thickness of the test shim and make a suitable one out of shim stock.
For thin shims you can use aluminium foil. For thicker ones it is possible to use copper wire - from electrical cable - formed into a ring. The ring can be pressed between steel plates or gently filed, to the correct thickness. Aluminium wire can also be used in this way. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 569
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 03, 2008 - 03:19 pm: |
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What IS an "LTM adapter"? |
Glenn
Tinkerer
Username: Glenn
Post Number: 341
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 03, 2008 - 08:11 pm: |
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I read it as the adapter to mount Leica 39mm screw onto M bodies. The arrival of the Cosina manufactured Voigtlander optics in LTM, has led to the introduction of various M mount adapters. Some of these are of varying quality/dimensions, and usually sold via eBay direct from China. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 571
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 04, 2008 - 04:00 pm: |
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ah! i was only aware of the ones made by Leica, which seemed unlikely to be off, and the M39/M42 ring that the Russians made to adapt old Zenit lenses to later Zenit cameras... which would leave quite a bit to be desired in terms of infinity focus when used with an LTM lens.....
: ) = |
John_shriver
Tinkerer
Username: John_shriver
Post Number: 28
Registered: 12-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, June 02, 2008 - 07:16 pm: |
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It really takes a bunch of precise jigs to calibrate up Leica rangefinder lenses and cameras.
Cook and Perkins made a very nice ground glass that screws onto the rear of an LTM lens, and holds the ground glass the correct 28.80mm from the lens flange. Hard to find, and pricey if anyone else knows what it is.
Then there are reference dummy lenses, which have a rangefinder cam protruding a calibrated amount. Canon specified using one that was set at infinity, cam 7.5mm from the lens flange.
Measuring the flange to film distance on the camera is tricky. You can use a dial depth gauge. Or Canon specified inserting a 28.80mm thick block, and measuring that it it flat with the lens flange. Realize that the lens flange has to be collimated not only to be the right distance, but to be parallel to the film.
I'm perfectly happy to replace the shutter curtains in an LTM camera, and adjust the shutter speed. But I leave this sort of work to the pros with the right equipment.
Yes, Tomosy says how to use a Focaslide as a substitute for that Cook and Perkins device. But his method assumes you know the exact focal length of your lens, which you may or may not know. (This is because you're not collimating at infinity with the Focaslide.) |
