Thread: Sigma 20mm T1.5 Collimation

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  1. #11  
    Senior Member Patrick Tresch's Avatar
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    Quote Originally Posted by Jon Dishler View Post
    Yes, this is what I found which at first was confusing, with an 85mm the marks were correct, but with a 20 mm they were way off. After adjusting the back focus, which is really an amazing capability on the DSMC2 camera, the 20 (and 14) were correct but interestingly the 85 remained correct as well. Even the 135 was perfect. The trick is to use a wide lens up close to adjust the back focus, then the others fall into line.

    Back focus reacts exaclty the reverse as "front" focus.

    You get critical focus with a long lens in "front" focus but it's less critical with shorter lens, and back focus is critical with short lenses and less critical with long lenses. It's due how the light rays are parallel or not in the entrance pupil to the subject or the exit pupil to the sensor.
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  2. #12  
    Member Mark A. Jaeger's Avatar
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    Well...after a bit of lab work (in the garage) I decided that I owe Michael T a donut or two or maybe some dollars.

    He was right that the back focus was off. After 3 adjustments I found the Zeiss CP.3 50mm and the Sigma 20mm were both reasonably well correlated/collimated at 36 and 60 inches and at infinity. At 96 inches the error was a bit larger but I don't expect that to be a problem. I do not know if the lock screw being in the storage location had anything to do with the need for adjustment but I put the screw into lock at the conclusion of my fiddling.

    I did not check my Nikon glass as I almost never focus them using the relatively coarse distance markings.

    Thanks to all.
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  3. #13  
    Senior Member Tommaso Alvisi's Avatar
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    Quote Originally Posted by Patrick Tresch View Post
    So there is no back focus made when they come out of the RED shop?
    They usually do since quite a few years, every time I got back my RED brain(s) were perfectly spot on, even with extreme wide angles and pl zooms...
    TOMMASO ALVISI | tommasoalvisi.com
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