Focal Length discrepancy

Discussion in 'Mirrorless Digital Cameras' started by shay_ohayon, Aug 30, 2005.

  1. Ok, so I'm burning with fever because I caught the cold, and I am
    quite bored at home - so I decided to do a comparison test between
    my two favorite lenses - Canon 70-200 F4L, and Sigma 24-70 EX.
    I mounted the two on a 300D + tripod, and set both to 70mm.
    Much to my surprise - the images I got were not the same. It seems
    that the image with the 70-200, set on 70mm has a larger focal
    length than the image I got with the 24-70 set on 70mm.
    I did not move the camera orientation or tripod position between

    Any ideas ?!?!?!?!

    And please don't tell me to throw away the Simga piece of junk, and
    buy the 24-70L, since I am short on cash... :)

  2. Hi Ido,

    Yep. That was the problem. I was focusing at a distance of 1.5-2m.
    BTW - they are both quite sharp at the middle of the frame, but the 70-200 defenitly win when I compare conrners. Both have nice results at F8. F22-32 turned bit blury, but I guess it's because the 300D doesn't have a mirror lock, and expsure was almost 3 sec.

  3. Your blurriness at small apertures is probably due to diffraction rather than not using MLU. I would avoid stopping down any more than f11 if it can be at all avoided. Every lens will degrade at f16-f22 due to this phenomenon.
  4. See this article for examples of how stopping down too far results in decreased image sharpness due to diffraction. I'd recommend not stopping down past f16 with a 20D/350D if maximum sharpness is your goal.

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