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Canon AF micro adjust on a zoom lens.

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    cheddar-caveman
    22 Sep 2012 - 5:39 PM

    Here's an interesting one. Just bought a 15-85mm Canon lens and AF is slightly off at each end. On full zoom I get the sharpest result at -6 on the micro adjustment but it's +2 on the wide angle end. All the camera appears to accept is one setting for one lens. Am I right or can one "log" each end of the lens as a separate lens under the "adjust by lens" option, if so, how please?

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    lawbert
    lawbert  71701 forum posts England15 Constructive Critique Points
    22 Sep 2012 - 6:23 PM

    unfortunately you cant register both ends of the focal range.
    Personally I would register the setting at the 85mm end which would be more critical for sharpness than the 15mm end which you can tidy up with a light sharpen in PP.
    Have you tried a half way setting at say 50mm to see what results it then gives you at the near and far end of the focal range?

    cheddar-caveman
    22 Sep 2012 - 7:42 PM

    Thanks Lawbert, didn't think I could. No haven't tried the mid point but from what I see end to end it will probably be in focus at +/-0!

    LenShepherd
    LenShepherd e2 Member 62450 forum postsLenShepherd vcard United Kingdom
    23 Sep 2012 - 8:47 AM

    Although I shoot Nikon and not Canon I work alongside a number of Canon photographers - there is a problem common in both camps - assuming phase detect focuses 100% accurate on anything - it does not Sad
    One piece of Nikon web advice is “Cameras AF systems require a difference in contrast to achieve focus. Plain subjects or subjects without much detail or a pattern may mean that the camera is unable to focus accurately".
    Nikon's guidance is to be aware some types of target may not always be 100% reliable. These targets, and the work around to get good focus, are in all Nikon DSLR instruction books and at https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/4585/kw/Getting%20go...
    Canon uses similar phase detect AF - the guidance can be applied to either brand.
    You have not posted a link to your test target - but it is extremely rare to see images supporting comments like yours using a first class for calibration AF target.
    Usually when a first class target is introduced and photographed at different distances depending on the zoom setting (often 27 times focal length being tested) the focus is confirmed as accurate without fine tune at either end, and in the middle of the zoom range.

    cheddar-caveman
    25 Sep 2012 - 8:01 AM

    I started by using this target in doors and then used an A4 sheet of graph paper with a heavy black cross along its center axis, about 20ft away. Camera on tripod at same height etc. At 15mm it came out as +2 and at 85mm -6 to achieve a sharp image on the LCD review.

    As I bought the lens mainly for the WA aspect I'm quite happy to leave the micro adjust at +2 and then manually set the micro to -6 as and when I use the tele end.

    LenShepherd
    LenShepherd e2 Member 62450 forum postsLenShepherd vcard United Kingdom
    25 Sep 2012 - 7:29 PM

    Sorry - I would not rely on any of your targets to be accurate enough for caibration.
    Of the heavy black line the number one at Canon USA went on the record some years ago as saying
    Chuck Westfall “I recommend using a flat, detailed target parallel to the focal plane. After reading through the PDF linked from your message, it appears that the author has missed a major point, i.e., any individual focusing point in a digital SLR is much longer than the simple line he is using on his chart. The nature of the AF sensors used by EOS digital SLR’s as well as those from other manufacturers is that they perform most reliably when the entire length of the focusing area sees readable detail. This condition is not satisfied by a thin line on a piece of paper. It's OK to include an angled chart in a test photo."
    It can be important to vary focus distance with lens focal length (not a constant 20 feet as implied) so that the target size in the viewfinder remains constant.
    If you did not change focus distance between 15mm and 85mm the AF result may give two different but spurious focus indications.
    Some of Nikon's guidance is athttps://nikoneurope-en.custhelp.com/app/answers/detail/a_id/4585/kw/Getting%20... and https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/48334/kw/Auto%20focu... as well as in all Nikon DSLR instruction books.

    bainsybike
    26 Sep 2012 - 9:11 AM

    Tricky subject!

    However, as lawbert says, I would concentrate on the 85mm end. According to this site, at 15mm even with the lens wide open the hyperfocal distance is about 12ft, so that if you focus at 12ft everything between about 6ft and infinity is "acceptably" sharp. At 85mm the depth of field is much narrower, so focus accuracy is more critical.

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