AF hunting. Lens or body?

Discussion in 'Digital Photography' started by jwt, Apr 18, 2008.

  1. jwt macrumors 6502

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    Mar 28, 2007
    #1
    I'm noticing that my autofocus is hunting where it shouldn't. I have a Rebel XTi. Is this an issue with the lens or body?
     
  2. harcosparky macrumors 68020

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    Jan 14, 2008
    #2
    By 'hunting' do you mean it is having trouble obtaining focus?

    Does it do this with all lenses or just one?

    Are you trying to focus on something too close?

    I had a zoom that developed similar issues, oddly enough it was one particular 75-300 on one particular body.

    At the time I had an EOS 10D and my son had a film based Rebel.

    That 75-300 had trouble focusing at 300mm on the 10D but not on the Rebel.

    Made me think it was the body so I took it in and they put a new 75-300 on the body and it worked flawlessly.

    Before I took the 10D in to test with a new lens, on the phone they said it could be the body needed to be calibrated, but in the end it did not need to be done. They also said it was rare for the AF in the body needing to be re-calibrated.

    Best bet - find a Canon camera dealer ( camera shop-not big store ) and ask them to help diagnose it.
     
  3. compuwar macrumors 601

    compuwar

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    #3
    It depends, AF is mostly the body, but the lens and subjects both have an effect. There are two things that come into play with autofocus- the largest aperture the lens has available (faster lenses gather more light, which gives the body an edge in detecting focus points and the body's algorithm. The higher-contrast the target, the easier it is for the body's AF module. More expensive cameras will generally have better AF modules, but even the best AF module will have issues with low contrast subjects, low levels of light from the lens, and subject movement directly towards the camera (the last one having been mostly solved in high-end bodies.)

    If you have a fast 50mm lens, then try focusing in the same conditions, and that'll tell you if it's a lens/subject issue.
     
  4. jwt thread starter macrumors 6502

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    #4
    The XTi has TTL with phase detection, if that helps. AF in poor lighting conditions is mitigated with this type of system.

    I don't want to say what lenses I have just yet, because I feel that info may color people's judgement, and I don't think it's necessary info. But I'm not shooting outside the lens' focal range.
     
  5. Abstract macrumors Penryn

    Abstract

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    #5
    Widest aperture you shoot at?

    What are you photographing? Is it a plain wall? This matters a lot.
     
  6. ChrisA macrumors G4

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    #6
    AF systems will "hunt". It there is not enough contrast in the subject or if the lens is slower than f/5.6 or of there is not enough light. Might be good if you told everyone a few more details. It could be nothing is wrong at all and that is just the way it works
     
  7. harcosparky macrumors 68020

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    Jan 14, 2008
    #7
    One other idea ... I had a lens that loved to 'hunt'.

    Not sure why you don't want to give out lens info, but I will tell you the one I had that loved to 'hunt' has been sold. It was a non-Canon lens that zoomed from 50mm to 500mm. It was also 'slow' in focusing due to the mass of the lens.

    Because of the above lens, I no longer use non-Canon lenses.
     
  8. jwt thread starter macrumors 6502

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    Mar 28, 2007
    #8
    OK, so everyone wants to know what lenses I'm using. It's my Canon 70-200 f/4L IS USM. My 17-85 IS USM won't hunt under the same lighting at the same aperture.

    My 70-200 hunted badly to find a good focus taking the following shot (with no PP). This shot was taken 5 m away from the subject with focus eventually locked on his forehead. Unfortunately he jumped up as the shutter opened, so it's not that crisp. If the autofocus hadn't hunted, I would have gotten the shot I wanted.

    1/160 at f/4.0
    176 mm
    ISO 400
    no flash

    lens settings:
    1.2 m - infinity
    AF on
    stabilizer on
    stabilizer mode 1
     

    Attached Files:

  9. compuwar macrumors 601

    compuwar

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    #9
    With the distance difference to give the AF sensors the same view you can't really do a straight comparison. However, you should look at IR AF assist, either built-in, as a part of a flash, on independently if you're having constant errors. The picture looks low enough contrast that you're likely to have problems.
     
  10. TWLreal macrumors 6502

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    Jul 9, 2006
    #10
    Well for one, if your subject is 5m away, your limiter should have been at 2.5m-infinity.

    That alone would make it focus much faster and hunt less.
     
  11. robbieduncan Moderator emeritus

    robbieduncan

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    London
    #11
    The body does have a small IR AF assist beam built in, but it struggles at times. Adding a flash unit with more powerful assist helps a lot. With a 580 EXII attached I can get a 400D/XTi to autofocus more or less in the dark at 3-5m!
     
  12. harcosparky macrumors 68020

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    #12
    +1 on the lack of contrast.

    Sharp contrast is important for AF.

    Looking at the submitted photo, I do not see that the AF system has a lot to work with as far as contrast.

    Also something that might have helped would be turning the camera 45 degrees one way or the other, locking the exposure, recomposing and shooting.

    I have had instances where the AF would not lock with the camera oriented in one direction, but did so after I changed it.

    It's to the point where I instinctively do this on shots that I know will give the AF trouble.

    The lens I had most trouble with was a Sigma 50-500 .... in its defense it was a beast of a lens with a lot of weight to move when focusing.


    ALSO what TWLreal said is true .... limiter should have been set to 2.5-Infinity

    This is why we practice with new gear before going out into the field with it. Different lenses will have different performance characteristics and the 70-200 will function differently compared to a 17-85.
     
  13. jwt thread starter macrumors 6502

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    Mar 28, 2007
    #13
    OK, I'll try that, but I've had stuff hunt in the range of 1.2 - 3 m on the 1.2 setting. The interesting thing is that I don't remember any hunting when I first got the lens.

    Anyhow, I'm not sure what you guys mean by low contrast. Although not apparent in the photo, since the subject is dark, there is a lot of color variation in the dog's hair. Additionally, there is a lot of luminance variation, from the strong backlight coming through his hair. Also, the dog appears dark because since everything outside was clipped, I had to reduce the exposure. In all actuality, there was quite a lot of light falling on the dog. So my question is, does the AF system use the absolute amount of light falling on the subject, or is it dependent somehow on the exposure settings?
     
  14. harcosparky macrumors 68020

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    #14
    There may be a lot of color variation but the transition form one color to the next is very gradual. If it locked up on the dogs forhead, it is because of the white spot.

    Think of two pieces of paper, one black, one white, set the beside each other and there is high contrast.

    Now imagine one piece of paper as wide as those two pieces of paper where the color goes from white to black gradually.

    You AF will lock up easily on the Black/White paper, but not on the White to Black faded transition.

    Some camera AF systems have problems with orientation. I recall that some cameras would 'hunt' on a test target with horizontal black stripes on white paper, turn the test target 90 degrees so that the lines are vertical and it locks up fast. AF systems have evolved, but are not perfect.

    If I were shooting your dog, I would have focused manually using an aperture to give me the most depth of field and the shutter speed appropriate.

    If I was to use AF, I would set the camera to use one focus point and place that point over the dogs eye, or some other area with higher contrast.
     
  15. jwt thread starter macrumors 6502

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