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Thread: Af question

  1. #1
    Jamesmd
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    Af question

    just a stupid question perhaps , but I was wondering

    when you have AF on wide , does the camera use the DOF and focus on what will get the best DOF or simply focuses on what it thinks is the main object of the shot ?

    I don't know if I explained properly .

    james

  2. #2
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    Re: Af question

    Without the specific model camera, the info I'm about to provide may be wrong. It holds true for all of the Minolta/Sony cameras I have direct experience with, and even seems to hold true for the many P&S cameras I've used for 1 or 2 pics each(as in, "would you ming taking a picture of us with my camera?").

    In general, when all AF points are active, the AF point covering the subject area that is closest to the camera will be used.

    From there, it gets very fuzzy(heh). Some cameras can pick the "best" AF point to use based on some vaguely-explained algorithm. I believe there was at least one Canon model that would allow you to focus on one point(e.g. near) then focus on another point(e.g. far) and the camera would select an aperture and a focus setting that would render both points "acceptably sharp" -- note that "acceptably sharp" is a malleable term, it doesn't always mean the same thing but it never means "perfectly focused".
    α900+VG|F20|2xF58|16-35,24-70,135Z|STF|70-400G|50,85 1.4|16,20,28,100M,80-200APO f/2.8|28-135|500f/8|1x-3xMacro|2xMFC-1000|Tiltall+RRS, Bellows, etc.

  3. #3
    Jamesmd
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    Re: Af question

    I thought it might take lens and f data , and then focus where DOF would be the best for what it measured in different points .

    The truth is I think I must of used it twice .

    cheers

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    Re: Af question

    Nope, nothing so fancy AFAIK. Phase-detect AF notices that the image is not in focus, then it moves the lens to where it thinks the image will be in focus. This seems to be an iterative process. I read somewhere that the specs usually call for the lens to focus plus or minus 1/3 total DOF, implying that it's not generally expected to nail perfect focus, just get it "good enough".
    α900+VG|F20|2xF58|16-35,24-70,135Z|STF|70-400G|50,85 1.4|16,20,28,100M,80-200APO f/2.8|28-135|500f/8|1x-3xMacro|2xMFC-1000|Tiltall+RRS, Bellows, etc.

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    Re: Af question

    Dave,

    I think you're right. Closest item usually gets the focus point in my experience.

    I wish Sony would update the focusing algorithims to do more of that Canon stuff. I usually use center point-and-recompose because I can never trust the wide angle mode.

    Greg

  6. #6
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    Re: Af question

    Maybe if they bent the little AF mirror behind the main one the Sony would focus more like a Canon I respectfully prefer the focusing prowess of the Nikon.
    Carsten - Website

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    Re: Af question

    While I agree that Nikon has the best overall AF and is best for tracking, nothing is better than the A900 if you only use the center AF point. In fact, a year or two ago, I saw a test where the A900 (or it may have been the A700) beat every pro Nikon and Canon DSLR for accuracy and speed with the center point. That double cross with a third f2.8 line is killer. Sony does need to get on the ball with multi-point, though.

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