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Thread: why do you lose autofocus points as aperture tightens?

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    why do you lose autofocus points as aperture tightens?

    i can understand this concept if the diaphragm was constantly restricted down to a small f-stop, as the blades would physically block light-of-sight from the scene to the autofocus sensor. however, the diaphragm blades remain fully open inside the lens while focus is attained, then constrict to the specified aperture a split moment before the shutter opens. why do most cameras warn that periphery focus points are inoperable at small f-stops apertures?

    (example, lots of cameras show only the center AF point operable at f/8)

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    Moderator Dave Humphries's Avatar
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    Re: why do you lose autofocus points as aperture tightens?

    Hi,

    I agree that doesn't sound right unless 'they' are talking about;
    Other causes of reaching f/8 like using a teleconvertor? (So the figure is more hypothetical that literal)
    Using a manual iris controlled lens? (probably rare if it auto focuses, I'll grant)
    Using Depth of Field Preview?
    Clutching at Straws? (no, that's me, now )

    Can you be more specific?
    Perhaps provide a link or two to where you've seen this?


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    Re: why do you lose autofocus points as aperture tightens?

    Quote Originally Posted by jopezu View Post
    i can understand this concept if the diaphragm was constantly restricted down to a small f-stop, as the blades would physically block light-of-sight from the scene to the autofocus sensor. however, the diaphragm blades remain fully open inside the lens while focus is attained, then constrict to the specified aperture a split moment before the shutter opens. why do most cameras warn that periphery focus points are inoperable at small f-stops apertures?

    (example, lots of cameras show only the center AF point operable at f/8)
    I just read that recently, but am not sure where. I too thought it sounded odd.

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    Re: why do you lose autofocus points as aperture tightens?

    http://www.cambridgeincolour.com/tut...-autofocus.htm

    in the middle of the page linked above, there's a little interactive part showing the AF points diminish in number as a higher f-stop is selected.

    to quote, "For SLR cameras, the number and accuracy of autofocus points can also change depending on the maximum aperture of the lens being used, as illustrated above."

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    Re: why do you lose autofocus points as aperture tightens?

    Read the sentence that you quoted again. It says that autofocus may have issues with lenses with MAXIMUM apertures smaller than f/8 or even f/5.6. It did not say that a lens with a maximum aperture of say f/2.8 will have autofocus issues when SET to an aperture of f/8.

    John

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    Re: why do you lose autofocus points as aperture tightens?

    My understanding of phase detect auto focus systems is that the auto focus sensors are often at the bottom of the camera and receive their view of the image through a secondary mirror behind a semi-translucent part of the main mirror. They are therefore not getting as much light as the main sensor or viewfinder and hence the lens is kept wide open until auto focus is achieved to give the system the best chance of focussing properly. As John says above, it's the maximum aperture that matters.

    Dave

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    Re: why do you lose autofocus points as aperture tightens?

    It cannot just be a matter of the amount of light that the lens receives: in most cases,
    the camera manages to focus in low light conditions if there's enough contrast (and fails
    if there's no contrast, even in bright light).

    Could the increased depth of field play a role, so that the AF system can't get a precise
    enough position to achieve focus lock?

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    Re: why do you lose autofocus points as aperture tightens?

    As already stated, the loss of AF points is when the MAXIMUM aperture is smaller, and the crucial point there is the exit pupil of the lens. The secondary mirror or light level are not relevant, but the phase detection system and its way of working.

    To analyse the sharpness of the image, the phase detection receptors are receiving light from different parts of the exit pupil of the lens. half of the sensor segments aiming at one point of the exit pupil, the other half at another. When stopping the lens down, these points may be obscured by the diaphragm, so that phase detector AF sensors will not receive any light. Phase detectors that are away from the centre of the image will lose one side of the lens before the other, while the one in the middle will lose both at the same time. Therefore, those further out will lose their ability to analyse juxtaposition of left/right parts earlier than the one in the middle. Typically this may be at wider than f/8 for the ones off centre, but at about f/8 for the centre point.

    Hence, the OP conception is erroneous. No phase detection points are lost when using a smaller aperture with an automatic lens, as measurement is always done with the lens wide open.
    Last edited by Inkanyezi; 30th January 2013 at 06:54 AM.

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    Re: why do you lose autofocus points as aperture tightens?

    Quote Originally Posted by Inkanyezi View Post
    As already stated, the loss of AF points is when the MAXIMUM aperture is smaller, and the crucial point there is the exit pupil of the lens. The secondary mirror or light level are not relevant, but the phase detection system and its way of working.

    To analyse the sharpness of the image, the phase detection receptors are receiving light from different parts of the exit pupil of the lens. half of the sensor segments aiming at one point of the exit pupil, the other half at another. When stopping the lens down, these points may be obscured by the diaphragm, so that phase detector AF sensors will not receive any light. Phase detectors that are away from the centre of the image will lose one side of the lens before the other, while the one in the middle will lose both at the same time. Therefore, those further out will lose their ability to analyse juxtaposition of left/right parts earlier than the one in the middle. Typically this may be at wider than f/8 for the ones off centre, but at about f/8 for the centre point.

    Hence, the OP conception is erroneous. No phase detection points are lost when using a smaller aperture with an automatic lens, as measurement is always done with the lens wide open.
    Thanks Urban for that explanation. It makes sense to me now. My comment about the amount of light was just an assumption on my part and I can see now that it can't explain the loss of some AF points. As you point out, it must be caused by the optics of the phase detect system itself.

    Dave

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