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Robert Feldman (Rafeldman)
Tinkerer
Username: Rafeldman
Post Number: 6
Registered: 11-2021
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 03, 2023 - 05:49 pm: | 
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I have been trying to adapt a CZJ 3.25cm f2.8 Tessar from a Robot camera for use on an Olympus Pen FT. The film-to-flange distance of the Pen is 28.95mm and that of the Robot is 31mm, so there is some room for an adapter. However, at 31mm, the rear of the lens sticks about 2mm too far into the Pen and would be hit by the mirror when taking pictures. Mounting the lens farther out on the Pen means it cannot be focused to infinity.
I am thinking of mounting the lens farther out anyway on the Pen and using a negative diopter lens on the CZJ to bring it back to an infinity focus. Using the information in Rick Oleson’s article on diopter, I calculate that a -2 diopter would give me about 2.25mm more film-to-flange distance, which should be enough. I can get a -2 diopter lens of the right diameter from a Nikon eyepiece correction attachment.
Am I correct in my thinking? Will what I want to do work optically?
Pre-2013 Username: r_a_feldman
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Peter Robinson (Monopix)
VIP
Username: Monopix
Post Number: 239
Registered: 12-1997
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 04, 2023 - 10:05 am: |
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In principle, yes. Whether -2 is the correct strength I don't know. And whether an eyepiece lens would be good enough I'm not sure. Sounds like something you would need to try in practice.
www.contax139.co.uk
www.contax-yashica-repairs.co.uk
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Rick_oleson (Rick_oleson)
Legend
Username: Rick_oleson
Post Number: 1370
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 04, 2023 - 10:40 am: |
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I can check your math on it if you like. Beware of optical quality in the result though, 2 diopters is a powerful lens. I did this once with a 400mm lens to convert it for medium format… went to a local eyeglass shop and got an uncut -1/4 dope eyeglass lens to stick on the front. It worked but the quality through the negative singlet wasn’t really acceptable. |
Robert Feldman (Rafeldman)
Tinkerer
Username: Rafeldman
Post Number: 7
Registered: 11-2021
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 04, 2023 - 02:14 pm: |
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Thanks for the replies.
I understand that what I want to do will take a bit of trial and error.
Two more questions:
1. Does the distance from the front element to the diopter lens matter?
2. Will using a minus diopter lens change the angle of view of the camera lens?
Pre-2013 Username: r_a_feldman
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Peter Robinson (Monopix)
VIP
Username: Monopix
Post Number: 240
Registered: 12-1997
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 05, 2023 - 06:43 am: |
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1. Yes in as much the diopter lens needs to cover the full field of view of the main lens. If you move it too far, it may not fully cover the field of view. I don't think it makes any difference to the effect though.
2. It won't, in itself, change the angle of view but moving the main lens further away from the film plain, which is what you are trying to do, will as it effectively increases the focal length of the lens.
I'm sure Rick will correct me if I'm wrong.
www.contax139.co.uk
www.contax-yashica-repairs.co.uk
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Rick_oleson (Rick_oleson)
Legend
Username: Rick_oleson
Post Number: 1371
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 05, 2023 - 07:44 am: |
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The negative diopter will slightly widen the view, but extending the lens forward slightly narrows it, so that’s about a wash. Mount the diopter as close to the lens as you can |
