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Shimming a Helios-103
Old 03-21-2016   #1
alexandru_voicu
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Shimming a Helios-103

Hi everyone,

I am the proud owner of a Amedeo Contax to Leica M adapter and I just gave it a first try with a 1982 Helios-103 lens. Everything went smooth on my Zeiss Ikon body, but, after I developed the first roll, I noticed a slight front focus wide open (3cm at 1m). Right now I'm contemplating shimming the Helios-103 to correct this. At this point, I can see it done in two ways:

1. Reduce the main shim that sets the distance between the lens mount and the optical block; or
2. Increase the internal shim between the rear optical block and the front block, thus moving the rear block closer to the film (and, in effect, increasing the focal length of the lens).

I haven't disassembled the lens, so everything I'm writing here is theoretical. The main disadvantage of method 1 is that I'll have to drill new holes for the grubs, given that the entire optical block will find itself in a different position in relation to the lens mount. Therefore, my question is, what do you think of the second method? Will it accomplish what I am after? As a plus, the second method will be easily reversible.

Any thoughts will be greatly appreciated.

Best,
Alex
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Old 03-25-2016   #2
Valkir1987
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Option 1 can only be done with option 2 number two.

Check the flange to film distance and infinity focus first. If these parameters are correct and the focus at 1m is off it comes to shimming the optical part.

I always forget wether the actual dept of field is larger in the front or the back of the focus. But it its something which comes into consideration when de lens is set to infinity.

Don't forget to check the accuracy of your rangefinder first. If your rangefinder is off, you are messing up your lens. This is not very likely in the case of the gear you have, but better start from there.
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Old 03-25-2016   #3
alexandru_voicu
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Quote:
Originally Posted by Valkir1987 View Post
Option 1 can only be done with option 2 number two.

Check the flange to film distance and infinity focus first. If these parameters are correct and the focus at 1m is off it comes to shimming the optical part.

I always forget wether the actual dept of field is larger in the front or the back of the focus. But it its something which comes into consideration when de lens is set to infinity.

Don't forget to check the accuracy of your rangefinder first. If your rangefinder is off, you are messing up your lens. This is not very likely in the case of the gear you have, but better start from there.
Thank you for sharing your thoughts. The ZI is properly adjusted, as all my ZM lenses (35/2, 50/2, 50/1.5) focus accurately (the only one that misbehaves is the Sonnar, due to its inherent focus shift).

Since my first post, I actually put in practice option no. 2 and it works fine. I still have to run a few tests, but, at least at minimum distance (90cm), the focus is now spot on (or perhaps 5 mm backwards). Regarding your question about the dof, the ratio between front and back varies according to distance. At very close distance, it's usually 49/51. At a distance of several meters, it is roughly 33/67.
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