Focus Accuracy: When is enough enough?

Discussion in 'Digital Cameras' started by Eric Stevens, Nov 24, 2011.

  1. Eric Stevens

    Eric Stevens Guest

    I've just carried out simple focus tests on two Nikon lenses mounted
    on my Nikon D300: an 'AF-S Nikkor 16-85 f3.5-5.6 G ED' and a
    'AF-S VR-Nikkor 70-200mm 1:2.8 G'. The camera was mounted on a tripod,
    the lenses were wide open and spot focus was used for all tests.

    Both lenses focus slightly short.

    At 1 metre the 16-85 focusses at about 99cm when set to 85mm focal

    At 2 metres the 70-200 focusses at about 199cm when set to 200 mm
    focal length.

    I know I can 'fine tune' the focussing of each lens through the menu
    system on my camera but the manual says "AF Tuning is not recommended
    in most situations ... ". The question is, should I bother?


    Eric Stevens
    Eric Stevens, Nov 24, 2011
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  2. Eric Stevens

    Alan Browne Guest

    Do you do a lot of wide open, long focal length close ups?
    Alan Browne, Nov 24, 2011
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  3. Eric Stevens

    Eric Stevens Guest

    That question was one of the points which led me to post the original

    What does 1 cm at 2 meters translate into at 50 or 100 metres. What is
    the source of the focussing error? Is it a constant percentage (i.e. 1
    meter at 200 meters) or is it a 'bent needle' - a constant 1cm at all
    distances? Or is it something else altogether?

    Does this small error matter unless I'm going to crop hell out of an
    image to show a gnats eye at 500 metres?

    I'm inclined to ignore the errors but I would like to hear more
    experienced second opinions.


    Eric Stevens
    Eric Stevens, Nov 24, 2011
  4. Eric Stevens

    Alan Browne Guest

    You could use any one of a number of DOF calculators to answer most of
    your question.

    As to proportionality v. offset, that would depend on the mechanics,
    sensor and algorithms as much as optics. With the DOF calculator you
    will see it's mostly proportional/exponential and the offset may get
    buried therein.

    And you can experiment as well.

    And of course you're dead right: DOF is entirely dependent on
    enlargement factor which I neglected to include in the list above.
    Alan Browne, Nov 24, 2011
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