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Thread: Accuracy of AF in APS-C and FF

  1. #1

    Default Accuracy of AF in APS-C and FF

    Just wonder.

    Say an APS-C DSLR fitted with a 200mm lens vs a FF DSLR with the same 200mm lens.

    Assuming that both camera's AF has similar performance and sensitivity.

    Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?

    Thanks

  2. #2
    Senior Member giantcanopy's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    if u r saying all else equal except sensor size, then it shld be the same. If i am not mistaken, the autofocusing sensor/strips in dslr does not reside in the optical sensor that captures the image.

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    Moderator Octarine's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    Quote Originally Posted by torak View Post
    Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?
    Who says that a 200mm lens can only be used at 'far targets'?
    The AF sensors are not sitting on the image sensor but are an independent component (unless using live view). During focusing the shutter is closed, the sensor is not in use. What impact can the sensor possibly have here?
    EOS

  4. #4

    Default Re: Accuracy of AF in APS-C and FF

    From my logic, AF depends on the optical signal falls on sensor. So it doesn't matter which focal length you are using.

    IMHO, bigger subject does not fasten your AF speed, it's based on phase or contrast detection... Imagine if you are zooming into a plain colour object, the object becomes bigger but AF system will have problem finding the right focal plane.
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    Senior Member Numnumball's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    Quote Originally Posted by torak View Post
    Just wonder.

    Say an APS-C DSLR fitted with a 200mm lens vs a FF DSLR with the same 200mm lens.

    Assuming that both camera's AF has similar performance and sensitivity.

    Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?

    Thanks
    It shd be the same regardless of sensor size..
    And why should the sensor have any impact on the AF since sensor is not in use (closed) at the point of AF before the shot is taken?
    Last edited by Numnumball; 28th April 2010 at 11:35 AM.
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    Default Re: Accuracy of AF in APS-C and FF

    As the good fellas above mentioned, the AF sensor does not reside at the same place as the image sensor. Typically, it lies under the mirror or above in the pentaprism/mirror assembly.

    If you are using a APS-C and full frame camera like the Canon 500D vs the 5D to shoot the same scene, I think the image circle from the lens hitting the sensor plane is the same size. It's only because the APS-C sensor captures only a smaller portion of this image circle, that's why it appears to be bigger.

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    Senior Member wildcat's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    Quote Originally Posted by Numnumball View Post
    It shd be the same regardless of sensor size..
    And why should the sensor have any impact on the AF since sensor is not in use (closed) at the point of AF before the shot is taken?
    But it is also unlikely that the focusing system will be the same for an entry level system vs a full frame which generally will have better focusing system, correct?

    Thus, even for the same sensor, you may get different AF from different cameras, right? e.g. focusing on D300s vs D5000.
    Last edited by wildcat; 28th April 2010 at 11:42 AM.

  8. #8

    Default Re: Accuracy of AF in APS-C and FF

    Thx for the responses.

    I wasnt too sure about the technical aspect of the af sensors thus i asked this question. Had the assumption that a larger object will be easier to focus thus this question pops up.

    Thx for the clarifications

  9. #9
    Senior Member Numnumball's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    Quote Originally Posted by wildcat View Post
    But it is also unlikely that the focusing system will be the same for an entry level system vs a full frame which generally will have better focusing system, correct?

    Thus, even for the same sensor, you may get different AF from different cameras, right? e.g. focusing on D300s vs D5000.
    No bro.. this is OT liao..
    We are addressing TS qs on the Sensor size with regards to AF which he raised at the very beginning of this thread.

    Of cos the Af system is different lar... U dun have to compare DX and FX..

    Just look at D500K and D300 will be enough to validate my point..

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    Default Re: Accuracy of AF in APS-C and FF

    well theres always a d300 and d700 to compare.
    not exactly the same, but the closest

  11. #11
    Senior Member Override2Zion's Avatar
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    Default Re: Accuracy of AF in APS-C and FF

    This is good reading material on what the auto-focus mechnism is about
    http://www.nikon.com/about/technolog.../caf/index.htm

    The auto-focus mechanism does not rely on the imaging sensor (CCD/CMOS). During autofocus, the reflex mirror does not move and the shutter does not open. This means, focusing accuracy and speed has nothing to do with imaging sensor size, it depends on focusing module built into the camera the lens used.
    Last edited by Override2Zion; 29th April 2010 at 12:09 PM.
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