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Thread: Question about autofocus

  1. #1

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    Question about autofocus

    Hi,

    I'm a new member of this forum. I got interested in photography about a year ago, so I'm pretty much a beginner. I've been reading some of the great tutorials on this site and I have a question about the one on autofocus ("Understanding camera autofocus").

    In the middle of the text, it says:
    Further, since the central AF sensor is almost always the most accurate, for off-center subjects it is often best to first use this sensor to achieve a focus lock (before recomposing the frame).
    But a bit further down it says:
    Although the central autofocus sensor is usually most sensitive, the most accurate focusing is achieved using the off-center focus points for off-center subjects. If one were to instead use the central AF point to achieve a focus lock (prior to recomposing for an off-center subject), the focus distance will always be behind the actual subject distance—and this error increases for closer subjects.
    To me this seems to be contradictory. Which one is correct? If it's the second, then I would like to understand why this is the case.

    Thanks,
    Georg

  2. #2

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    Re: Question about autofocus

    I think what is meant here, Georg, is that when there is a very shallow depth of field the method of focusing, holding the focus, then recomposing the scene can produce an out of focus shot at the new spot where the photo will be taken.

    For example, when focusing on a bird in a tree there is a great risk that using multiple focusing points will automatically focus on a branch in front of the bird so using just the centre focusing point will give the most accurate focus.

    Then if I want to get a better balanced background. I can focus on the bird and hold the focus (by a number of ways) then recompose the shot to give a better background before finally taking the shot.

    With a good depth of field, say several feet at least, the bird will still be in focus and I will obtain the required background.

    The problem comes when the depth of field is very shallow. For instance when photographing insects with a macro lens my distance of sharp focus may be as little as half an inch, or less.

    So in this case, if I held the focus and recomposed the shot everything would now be out of focus because I have changed the distance between my camera and the subject by a greater amount than the depth which was in sharp focus.

    In cases like this, using multiple focus points may work better because my camera would focus on the closest point.

    But with the case in question I think the subject was a portrait shot with shallow depth of field; but the same principle applies. For instance, focusing on the eyes then moving the camera slightly to give a better angle but where the eyes aren't now in the centre of the shot.

    In this case, the eyes would now be out of focus.

    But, it also mentions in that tutorial something about the human eye being the best focusing aid with difficult subjects. Using manual focusing of course.

    Hope this makes some sense. It is quite a simple theory but I am struggling to put it into words.

  3. #3
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    Re: Question about autofocus

    They're both correct since each passage is describing a different aspect of focusing error . The central autofocus point is almost always the most accurate point, but when you move your camera to recompose, you're effectively changing that focusing distance -- causing a "focus-recompose" error. In those cases, using a non-central autofocus point can be more accurate (usually with close more moving subjects).

  4. #4
    Moderator Dave Humphries's Avatar
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    Re: Question about autofocus

    Quote Originally Posted by Geoff F View Post
    In cases like this, using multiple focus points may work better because my camera would focus on the closest point.
    I'd clarify this by saying Geoff means using a single focus point, just not the centre one (select the one nearest/on the subject after composing)

    If you use any "multiple" focus point mode, the camera starts making guesses for you and that's when you end up with sharp tree branches and soft birds

    I have the T-shirt, if anyone is interested

    PS Usually for macro, you'll end up cropping significantly anyway, so I tend to use the centre focus point, put that on the subject's eye and shoot, then 'compose' in PP by cropping.

  5. #5

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    Re: Question about autofocus

    Lately in many (but certainly not all) situations I've been using spot focus and spot meter, setting them on whatever is the subject, holding them, recomposing, and taking the shot. Seems to work for many purposes - maybe not macros of bugs!

  6. #6

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    Re: Question about autofocus

    Thanks a lot for the clarifications!

    Georg

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