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Thread: About Lens: Sharp Copy and Focus Issue Difference

  1. #1

    Default About Lens: Sharp Copy and Focus Issue Difference

    I am a bit confusing about the term of Sharp Copy and Front/Back Focusing.

    1)Is a non-sharp copy of a lens caused by front/back Focusing(inaccurate focus)?
    OR
    caused by the internal optics of the lens not built precisely?

    2)What make the lens lock focus? The Phase-detection on the body tells the lens that the subject is focused? If so, why does some lenses still have front/back focusing?

  2. #2
    Moderator Octarine's Avatar
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    Default Re: About Lens: Sharp Copy and Focus Issue Difference

    Quote Originally Posted by Learnphotography View Post
    1)Is a non-sharp copy of a lens caused by front/back Focusing(inaccurate focus)?
    OR
    caused by the internal optics of the lens not built precisely?
    Both.

    2)What make the lens lock focus? The Phase-detection on the body tells the lens that the subject is focused? If so, why does some lenses still have front/back focusing?
    In Single AF mode (please check your manual for exact term for your camera) the AF system gives a single command to the lens to focus. Something like "Turn by 2 degree". That's it. There is no verification whether this was sufficient. Three possible causes for errors:
    a) AF sensors not precise, giving wrong instructions. This can happen with older cameras.
    b) AF sensors getting distracted by other details (it's pattern recognition!)
    c) Lens does not perform the focus command precisely. Here, Front / Back Focus issues can be the reason, but also problems with 3rd party lenses. Those manufacturers need to reverse-engineer the body-lens communication (Canon, Nikon & Co aren't very talkative about it) and errors can occur. But many people use 3rd party lenses without issues.
    Front / Back focus issues can be detected using focus test charts. Google, print, follow the instructions. The service center of your lens will be your next contact point then.
    EOS

  3. #3

    Default Re: About Lens: Sharp Copy and Focus Issue Difference

    Also consider that in most DSLR cameras, the AF sensor is NOT the camera sensor. Any mistake in the alignment will mean that the AF sensor thinks it's sharp, but then when you take the picture, the actual plane of focus is not in line with the sensor. In this case though, almost all lenses will have a front/back focus issue; it's not a lens issue then.
    Alpha

  4. #4

    Default Re: About Lens: Sharp Copy and Focus Issue Difference

    i am new, i dont understand a word of what you guys are saying. it is all alien to me..

  5. #5
    Moderator Octarine's Avatar
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    Default Re: About Lens: Sharp Copy and Focus Issue Difference

    Quote Originally Posted by rickymartin View Post
    i am new, i dont understand a word of what you guys are saying. it is all alien to me..
    Then I suggest staying in "Read Only" mode, also called 'lurking'. Read, read, read .. and if you feel it's not relevant for you at the moment just skip it. No point getting overloaded with details. Focus on the newbies guides and the very basics then.
    No need to comment on any topic with the same line
    EOS

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