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Back focus conundrum - how to fix

Discussion in 'Adapted Lenses' started by RobWatson, Jan 5, 2013.

  1. RobWatson

    RobWatson Mu-43 Hall of Famer

    So I have a lens that should close focus at 7" but it instead will focus at 3" ... clearly the back focus is off (also cannot focus at infinity).

    For those technically minded folk a question:

    Question is how can I determine the correct amount of material to remove to reach correct back focus distance? Trial and error is not a viable option.
  2. RobWatson

    RobWatson Mu-43 Hall of Famer

    Too easy?

    Lensmakers equation?

    1/s=1/f1+1/f2 so set the lens to minimum focus distance (f1) and it will focus at the correct back focus distance (f2). 1/s is now fixed.

    In the case where the back focus is 'wrong' still have 1/s=1/f3+1/f4 but here f3 is the back focus distance and 1/f4 is the actual location of focus (3 inches in my case) - since the lens settings arethe samee 1/s is the same!

    So 1/f1+1/f2=1/s=1/f3+1/f4 and since I know f1, f2 and f4 I simply calc f3.

    Difference between f3 and f2 is the amount to shave off for correct back focus.
  3. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    Rob, Is this an adapted lens? If so are you sure it's the lens and not the adaptor that's out?

    If it's the adaptor (which it probably is) then you'd just find the flange distance of the original lens/camera and subtract the flange difference of the adapthed camera (m4/3?) and that should be what your lens adaptor is. If it is the lens then wouldn't a lens tech adjust the lens internally rather than try to shave the mount?

  4. RobWatson

    RobWatson Mu-43 Hall of Famer

    Clumsy wording on my part. The lens is fine. My camera/adapter/lens_mounting_hardware is slightly out of kilter and needs some fine tuning.
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