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Thread: Auto Focus advice needed

  1. #1
    arith's Avatar
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    Auto Focus advice needed

    How do you use AI focus? What is the difference between AI and AI servo? Normally I take static subjects but yesterday I just walked into a fashion show, set AI then composed on thirds like an idiot.

    Auto Focus advice needed

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    Re: Auto Focus advice needed

    Steve,

    The photo taking experience is usually the opposite in that my subject is moving. I have to get used to using a stationary tripod and hope that my subject moves into my line of view.

    I use a Nikon camera and single servo is used for stationary subjects, while AF-C is used for moving subjects. There is also a selection for AF-Area Mode (closest subject, dynamic area, or single point). Closest subject would be used for a moving subject and one of your focus points would be chosen by the camera when locking on a subject.

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    Re: Auto Focus advice needed

    For slowly moving subjects, with a decent amount of focus depth, I just shoot normally. Assuming there is only a split second between focusing and shutter click.

    AI Focus locks onto a static subject then 'tracks' that subject if it moves, providing of course that you hold the shutter button halfway.

    AI Servo locks onto a moving subject and tries to get/hold a good focus despite the movement. The focus can 'hunt' a little while attempting to achieve a sharp focus so you sometimes have to pan and follow the subject while the lens searches for a focus.

    Personally, I prefer to just use the centre focusing point for these occasions; to prevent accidental focusing on the wrong area. But some bird photographers, for example, prefer multiple points when focusing on flying birds.

    In your example, there appears to be two likely answers. The area behind the two women is in good focus so probably your camera has focused on that point, and your depth of field was shallow. Or your shutter speed was very slow and the foreground subjects have moved too fast.

    With either AI Focus or AI Servo you should have achieved good focus here, providing you actually focused on the intended target, the women, and not the background; and didn't recompose the shot. This is where 'hold focus and recompose' can fall down; although that chiefly applies to fast moving subjects.

    I suspect that normal settings would have actually worked better here.

  4. #4
    arith's Avatar
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    Re: Auto Focus advice needed

    Quote Originally Posted by Shadowman View Post
    Steve,

    The photo taking experience is usually the opposite in that my subject is moving. I have to get used to using a stationary tripod and hope that my subject moves into my line of view.

    I use a Nikon camera and single servo is used for stationary subjects, while AF-C is used for moving subjects. There is also a selection for AF-Area Mode (closest subject, dynamic area, or single point). Closest subject would be used for a moving subject and one of your focus points would be chosen by the camera when locking on a subject.
    Well in the past it would be manual; and I would be expecting the models to walk into focus. This new stuff is harder to use John. I know what I did wrong just don't know how to get it right.

  5. #5
    arith's Avatar
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    Re: Auto Focus advice needed

    Quote Originally Posted by Geoff F View Post
    For slowly moving subjects, with a decent amount of focus depth, I just shoot normally. Assuming there is only a split second between focusing and shutter click.

    AI Focus locks onto a static subject then 'tracks' that subject if it moves, providing of course that you hold the shutter button halfway.

    AI Servo locks onto a moving subject and tries to get/hold a good focus despite the movement. The focus can 'hunt' a little while attempting to achieve a sharp focus so you sometimes have to pan and follow the subject while the lens searches for a focus.

    Personally, I prefer to just use the centre focusing point for these occasions; to prevent accidental focusing on the wrong area. But some bird photographers, for example, prefer multiple points when focusing on flying birds.

    In your example, there appears to be two likely answers. The area behind the two women is in good focus so probably your camera has focused on that point, and your depth of field was shallow. Or your shutter speed was very slow and the foreground subjects have moved too fast.

    With either AI Focus or AI Servo you should have achieved good focus here, providing you actually focused on the intended target, the women, and not the background; and didn't recompose the shot. This is where 'hold focus and recompose' can fall down; although that chiefly applies to fast moving subjects.

    I suspect that normal settings would have actually worked better here.
    Cheers Geoff; I was thinking does it track when only the centre point is used for focus. I set my camera to act like an old 35mm as much as possible; then I don't have to learn new things. But I like AF even though I can do without everything else.

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    Re: Auto Focus advice needed

    It looks to me like the camera chose the guy wearing the WW-II U.S. Army hat as point of focus. I often found my earlier DSLR cameras picking the wrong point of focus; especially noiceable when I was using a long lens. I have not modified my 7D focus from the factory default but, it seems to do a better job of focusing...

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    Re: Auto Focus advice needed

    I use Nikon so the terminology is different but I think AF-S and AF-C are the counterparts to AI and AI-servo. If that is the case then AI will only allow a photo to be taken if it is in focus and AI-servo will allow a photo to be taken regardless if focus is achieved or not. However, neither mode determines where the focus point is so the wrong part of the photo may be the part that is focused on. You need to select that separately. Some choices, depending on camera model, are single point, 3-point, 9 point, 11-point, 39-point, 51-point. Your best bet is to select the least number of points that will provide focus (in other words you have to experiment).

    In the case of the photo you posted, focus was achieved but not where you had anticipated. Either you need more focus points so that some of them cover the subject you want in focus (and hope the camera "knows" which is the important part of the photo) or you can use focus and recompose (aim a single focus point on the subject, hold shutter button half way down and then compose the photo).

    Bottom line - auto focus is not as automatic as the manufacturers would have you believe.

    Edit - here is a link to a guide you may find useful for autofocus. It's written for Nikon but Canon probably has the identical features but named differently and located differently. The concepts should work the same for both.

    http://www.nikonsupport.eu/europe/Ma...uide_En_02.pdf
    Last edited by benm; 3rd July 2011 at 11:49 PM.

  8. #8
    Shadowman's Avatar
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    Re: Auto Focus advice needed

    Quote Originally Posted by benm View Post
    I use Nikon so the terminology is different but I think AF-S and AF-C are the counterparts to AI and AI-servo. If that is the case then AI will only allow a photo to be taken if it is in focus and AI-servo will allow a photo to be taken regardless if focus is achieved or not. However, neither mode determines where the focus point is so the wrong part of the photo may be the part that is focused on. You need to select that separately. Some choices, depending on camera model, are single point, 3-point, 9 point, 11-point, 39-point, 51-point. Your best bet is to select the least number of points that will provide focus (in other words you have to experiment).

    In the case of the photo you posted, focus was achieved but not where you had anticipated. Either you need more focus points so that some of them cover the subject you want in focus (and hope the camera "knows" which is the important part of the photo) or you can use focus and recompose (aim a single focus point on the subject, hold shutter button half way down and then compose the photo).

    Bottom line - auto focus is not as automatic as the manufacturers would have you believe.

    Edit - here is a link to a guide you may find useful for autofocus. It's written for Nikon but Canon probably has the identical features but named differently and located differently. The concepts should work the same for both.

    http://www.nikonsupport.eu/europe/Ma...uide_En_02.pdf
    Check your Auto focus mode, you can choose closest subject, single point, etc.

  9. #9
    arith's Avatar
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    Re: Auto Focus advice needed

    Quote Originally Posted by rpcrowe View Post
    It looks to me like the camera chose the guy wearing the WW-II U.S. Army hat as point of focus. I often found my earlier DSLR cameras picking the wrong point of focus; especially noiceable when I was using a long lens. I have not modified my 7D focus from the factory default but, it seems to do a better job of focusing...
    Cheers Richard; I set mine on centre single point only and I think it is possible to choose only a single point or auto.

  10. #10
    arith's Avatar
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    Re: Auto Focus advice needed

    Quote Originally Posted by benm View Post
    I use Nikon so the terminology is different but I think AF-S and AF-C are the counterparts to AI and AI-servo. If that is the case then AI will only allow a photo to be taken if it is in focus and AI-servo will allow a photo to be taken regardless if focus is achieved or not. However, neither mode determines where the focus point is so the wrong part of the photo may be the part that is focused on. You need to select that separately. Some choices, depending on camera model, are single point, 3-point, 9 point, 11-point, 39-point, 51-point. Your best bet is to select the least number of points that will provide focus (in other words you have to experiment).

    In the case of the photo you posted, focus was achieved but not where you had anticipated. Either you need more focus points so that some of them cover the subject you want in focus (and hope the camera "knows" which is the important part of the photo) or you can use focus and recompose (aim a single focus point on the subject, hold shutter button half way down and then compose the photo).

    Bottom line - auto focus is not as automatic as the manufacturers would have you believe.

    Edit - here is a link to a guide you may find useful for autofocus. It's written for Nikon but Canon probably has the identical features but named differently and located differently. The concepts should work the same for both.

    http://www.nikonsupport.eu/europe/Ma...uide_En_02.pdf
    Cheers Ben, I'll have a look at that.

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