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Thread: question on focusing

  1. #1

    Default question on focusing

    Just bought a lens from a fellow CSer, happy with its sharpness, but got one problem:

    the actual focusing point is always behind where my camera is aimed at, in simple words - misfocus.

    I want to know whether it's my camera's problem or the lens'. Initially I thought it's the lens' fault, because I have never had this misfocusing problem before with other lenses.

    However, after some study, I begin to doubt theoratically the lens itself is able to "misfocus" at all! I mean, the lens is always doing what the body tells it to do, the lens is always focusing and giving the body the perfect point, just that the body gives wrong info on where to focus accurately. Am I right?

    Really hope someone here share their explanations. Thanks in advance!
    EOS 5D Mark II / 17-40 F/4 L / 50mm F/1.4 / 135mm F/2 L / 430EX Mark II + LSII

  2. #2
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    Default Re: question on focusing

    What you're experincing is called "backfocus". It's usually due to the lens but can also be due to the camera body. Do you have this problem with other lenses? If not, then it's probably due to the lens. Bring it down to Sigma (I assume the lens you bought is a Sigma? If it's a Canon, bring it to Canon, etc.) to adjust it.

  3. #3

    Default Re: question on focusing

    it's a canon lens, and how much will they charge for it?
    EOS 5D Mark II / 17-40 F/4 L / 50mm F/1.4 / 135mm F/2 L / 430EX Mark II + LSII

  4. #4
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    Default Re: question on focusing

    I have no idea how much it costs. Perhaps you can call Canon. They may be able to tell you the rough cost.

  5. #5

    Default Re: question on focusing

    I am not familiar with the Canon AF but before sending in the camera and/or the lens I would do some more tests. First, set the AF to spot focus in the center only, if possible. Take a brick wall (or something similar) outdoors or a book shelf full of books indoors. Important that the whole picture area is covered. Do not select a fence since the AF may be disturbed by the background and the fence pattern. Use tripod for 100% stability. Take a few shots from different distances. If they turn out to be good then nothing is wrong with the lens or the camera. Maybe the way you use the camera or the lens is not optimal for the lens. It is important to test on stationary objects. Testing on moving target is a bit more difficult.

  6. #6

    Default Re: question on focusing

    Hi bro, u may try this out
    http://focustestchart.com/chart.html
    头可断,血可流,倩女不可不追求 carpe diem,when in doubt, hoot first,apologise later:p GALLERY

  7. #7

    Default Re: question on focusing

    Quote Originally Posted by eow View Post
    Hi bro, u may try this out
    http://focustestchart.com/chart.html
    Yes, it is a well known test sheet by now, but the wall test answers the back focus problem more easily and clearly. If the camera / lens focuses on the wall than there are other problems. If not than it is time to visit service and let them sort it out.

  8. #8

    Default Re: question on focusing

    called Canon, recaliberate charge is $80.
    EOS 5D Mark II / 17-40 F/4 L / 50mm F/1.4 / 135mm F/2 L / 430EX Mark II + LSII

  9. #9

    Default Re: question on focusing

    Quote Originally Posted by xavier_xw View Post
    called Canon, recaliberate charge is $80.
    Not much but I would still test the way I described before paying $80, just to make sure it is definitely the lens.

  10. #10

    Default Re: question on focusing

    yes, I tested against my bookshelf, on my camera body and another one from a friend, confirm it's the lens. sigh...
    EOS 5D Mark II / 17-40 F/4 L / 50mm F/1.4 / 135mm F/2 L / 430EX Mark II + LSII

  11. #11

    Default Re: question on focusing

    Quote Originally Posted by xavier_xw View Post
    yes, I tested against my bookshelf, on my camera body and another one from a friend, confirm it's the lens. sigh...
    Well, look at it from the bright side. At least you don't have to part from your camera, you still have something to use while the lens is in the Canon Hospital and once you get it back it will be like new. Hopefully they do a complete cleaning of the inside also.

  12. #12

    Default Re: question on focusing

    Probably very obvious, but I'll ask anyway. Have you checked which AF point is selected?

  13. #13

    Default Re: question on focusing

    Quote Originally Posted by jmmtn4aj View Post
    Probably very obvious, but I'll ask anyway. Have you checked which AF point is selected?
    yes, I know that.
    EOS 5D Mark II / 17-40 F/4 L / 50mm F/1.4 / 135mm F/2 L / 430EX Mark II + LSII

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