Question Correcting AF focus issues

Discussion in 'Photography Forums' started by RobinBanks, Jun 23, 2015.

  1. RobinBanks

    RobinBanks
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    So I've been reading through some threads on here and it has become apparent that my Sigma 70-200mm is suffering from back focus (I think). More often than not, my focus object is not in focus, but objects behind it are. For example, I have noticed it when taking a shot of a bird - the bird is blurred but the feeder it is sitting on (which is behind it) is not.

    Initially, I thought it was me just being a noob and I'd tried different things such as backing off the focal length and closing the aperture, but nothing really seemed to make a difference. It is more noticeable on smaller objects. I was beginning to think the lens was dodgy, I hope not.

    I have read about calibration, but my Nikon D5200 doesn't have an AF adjustment option. So, how do I correct this??

    Thanks.
     
  2. shotokan101

    shotokan101
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  3. shotokan101

    shotokan101
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    also test with another lens of course I case it's the lens - if it's the body then you're pretty much stuffed - if you are the original owner you could try a repair/replacement from the seller ?
     
  4. twist

    twist
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    Failing that send the camera and lens to sigma. It's about 40 quid for a calibration unless it's under warranty.
     
  5. RobinBanks

    RobinBanks
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  6. RobinBanks

    RobinBanks
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    My other Nikon lens seems to be fine.
    Both camera and lens are second hand.
     
  7. shotokan101

    shotokan101
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    sounds like you're only left with @twist suggestion then...
     
  8. RobinBanks

    RobinBanks
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    I'm assuming there's no way to correct the issue myself as I'm lacking AF adjustment on the body?
     
    Last edited: Jun 24, 2015
  9. snerkler

    snerkler
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    Not as far as I'm aware, only option is Twist's suggestion. Be sure to check it first on a tripod as already suggested. I personally don't find the battery test 'sensitive' enough, but good for a general quick assessment. I personally use this method, although I'm sure Jim will tell you this is just being anal ;)

    http://bobatkins.com/photography/technical/focus_testing.html

    Can you post pics so we can see the issue. Don't forget your AF point doesn't fill the AF box and so if the bird is small in the frame and the AF point is overlapping the background it could be focussing on that instead.
     
  10. biccies

    biccies
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    I'd do a few tests with stationary objects to double check it is in fact a focusing problem. It's a possibility that the bird's movement vs the feeders lack of movement could be giving you the effects you describe.
     
  11. RobinBanks

    RobinBanks
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    I'll give this a go, although I'm not into anal :D

    I will certainly post some pics up too.
     
  12. RobinBanks

    RobinBanks
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    Thanks for your suggestions guys. I will update accordingly...
     
    Last edited: Jun 24, 2015
  13. RobinBanks

    RobinBanks
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    OK. So this is an example.
     

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  14. RobinBanks

    RobinBanks
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    I also have a slight dilemma - my trial version of Light Room has ended :(
    So, what do I do? Do I buy a copy of LR or PS? I'm torn. Am I correct in saying that LR doesn't do layering?
     
  15. snerkler

    snerkler
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    Subscribe to Adobe cc for circa £8/month and hope the fees don't go up ;)
     
  16. snerkler

    snerkler
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    The little fella is quite small in the frame so could be that the AF point picked up the feeder. Tripod tests are the only way to know for sure.
     
  17. shotokan101

    shotokan101
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    yes LR doesn't yet include layers or masks PS Elements does but not as comprehensively as PS...
     
  18. AMc

    AMc
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  19. RobinBanks

    RobinBanks
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    OK. So I've completed a quick test and I think this proves my theory, please correct me if I'm wrong.

    Shooting at f/2.8, AF-C and single point.

    First photo. Batteries at approximately 45 degrees to the lens. I am focussed on the Duracell battery (other batteries are available!). As you can see, it is not in focus. Slightly behind, the table is.
    [​IMG]

    Second photo. I thought, OK, well let's put something behind in the area where I think it's focussing - took me a few attempts to get it right, but here it is. As you can see the battery second from the left is in focus more than the rest.
    [​IMG]

    Third photo. Just to prove the lens wasn't passing through the gap between the group of 5 batteries, I thought I would bunch them together leaving the battery which was in focus in place. Here is the result.
    [​IMG]

    So, what are everybody's thoughts. I think this proves that the lens is suffering from back focus. Do you?

    Thanks in advance.
     
  20. snerkler

    snerkler
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    Struggling to see what you've done there, doesn't look like your 'standard' battery test tbh. Does look off, but without getting the batteries set up right it's difficult to say for sure. Ideally you could do with filling the frame more too. Definitely not focussing on the Duracell though. Was this on a tripod?

    Get the camera sensor parallel to the edge of the table and then arrange the batteries so they are 45 degrees to the edge of the table about 1" apart front to back and see what you get then.
     
  21. johnmc

    johnmc
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    Heres one I took on Sunday . I still think the lens isn't as sharp as it used to be .
    [​IMG]
     
  22. RobinBanks

    RobinBanks
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    Yes, it was on a tripod.

    Thanks for the advice. I'll conduct some more tests...
     
  23. RobinBanks

    RobinBanks
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    OK. So I've had chance to take some more shots. This has completely confused me. It looks OK(?) Thoughts?

    [​IMG]
     
  24. snerkler

    snerkler
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    Looks like it's front focussing a bit, assuming it was all set up ok. I'm guessing it's not set up perfectly or the scale's not flat as the front focus is more obvious on the bottom scale than the top.
     
  25. RobinBanks

    RobinBanks
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    Hmmm. I dunno. The scale was flat as I had it pinned to an outside wall - assuming my garage isn't crooked! I think I need to experiment some more with real life photos. Maybe it's just me being a noob :)
     
  26. snerkler

    snerkler
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    Just refreshed my mind re the rest of the post. You think the lens is back focussing don't you, but that test shows its possibly front focussing :confused: I assume with the scale test the right side of the frame is the closest to the camera?
     
  27. RobinBanks

    RobinBanks
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    Yes, my other shots suggest that it is back focussing. And yes, the right side of the photo is closest to the camera. This is why I'm confused. I took a couple of photos of my cat after and they suggested it was back focussing. I don't get it.
     
  28. snerkler

    snerkler
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    Are you using the same aperture for all of these shots?
     
  29. snerkler

    snerkler
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    Are you using the same aperture for all of these shots?
     
  30. RobinBanks

    RobinBanks
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    Yes, same aperture. f/2.8
     

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