Backfocus - no error with one lens; error with another lens?

Discussion in 'Digital Photography' started by Alan Browne, Mar 14, 2005.

  1. Alan Browne

    Alan Browne Guest

    I tested the backfocus of the Maxxum 7D. Links below.

    For the 100mm lens, all seems well.

    For the 50mm lens, it seems to focus camera side of the target.

    Any ideas?

    Cheers,
    Alan



    Tested:

    100 f/2.8 macro against two targets on an oblique

    50 f/1.7 against one target on an oblique.

    --Max7D + lenses above
    --Tripiod
    --cable release
    --mirror lockup
    --angle finder at 2X for manual focus
    --A/S off
    --ambient light

    100mm results: perfect

    50mm results: appears to focus slightly close to the camera v. target
    (2 samples of each from bunch follow).

    100mm f/2.8:
    http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
    http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg

    50mm f/1.7:
    http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
    http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg

    note: on the paper target, the focus point is marked with bars with
    ambient light from window.

    ruler target, the focus point is the 15 cm line with
    ambient light from softbox on farside of ruler.

    Comments on why the 100mm shows in-focus at the selected focus point and
    the 50mm does not?

    Cheers,
    Alan
     
    Alan Browne, Mar 14, 2005
    #1
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  2. I read an article that showed that the cover glass on the
    sensor made that some lenses did not focus correctly.
    Non digital lenses are corrected for having air between the
    lens and the film.


    /Roland
     
    Roland Karlsson, Mar 14, 2005
    #2
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  3. Alan Browne

    Alan Browne Guest

    Very odd. Do you have this article online? Or what magazine was it?

    In any case, none of my lenses are "digital".

    Cheers,
    Alan.
     
    Alan Browne, Mar 14, 2005
    #3
  4. Alan Browne

    Jim Guest

    How did you get the non-macro lens to focus to 15 cm? Ordinary lenses don't
    perform well at such close distances.
    Jim
     
    Jim, Mar 14, 2005
    #4
  5. Pity you can't focus through the taking sensor!

    David
     
    David J Taylor, Mar 15, 2005
    #5
  6. Roland Karlsson, Mar 15, 2005
    #6
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