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I have a Sigma 150mm F2.8 EX DG OS HSM APO Macro lens mounted on my Canon 6D. Even if AF mode is set to minimum-to-infinity range, if my subject is way out of focus (too far or too near), like, say, ring is set for 60cm and subject is at 6m, the lens will not even attempt to auto-focus. If I manually rotate the focus ring a bit so the image will start to become a little sharp, the AF will grab the focus almost instantaneously.

This does not happen in any of my other lenses, but none of them is macro. Is this the intended behavior so I'll have to live with it, or is it a malfunction so I can claim my warranty?

  • Do you have the problem with other lenses as well? If so, there could also be a setting in your body which is turned on by accident.. – mmumboss Mar 13 '14 at 8:15
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Take a look at Why can't my SLR autofocus on certain parts of a scene?

While the basic premise is different — there, parts of the scene which don't have much contrast no matter how well-focused they are — I think the basic explanation is the same. When you are so far out of focus that everything is a blur, the AF system has nothing to lock on to, and therefore can't decide which way to go. When you adjust manually so some detail is apparent, it can then work.

This isn't really a matter of intentional behavior — I'm sure the designers would love for focus to magically work in all situations — but it is the reality of the practical world.

Assuming AF is accurate when you do get focus, I don't think you have a defect. On the other hand, it is possible for lens elements to be misaligned so that correct focus can't be obtained, which could present itself in a similar way (except that the final result would be wrong).

  • That seems like an odd choice of behavior. Seems like the more logic option would be to choose a direction to move the focus in to see if anything gets contrasty rather than just do nothing. – rfusca Mar 13 '14 at 1:22
  • @rfusca - the lens/camera probably would hunt initially before moving using contrast-detection AF (in live view), but in phase-detect the camera expects some information regarding the displacement of details (much like split-prism focus aides in MF cameras or in a rangefinder). A tele-macro with a wide aperture can easily get things so far out that the direction indication is missing altogether (a detail may appear in only one or potentially none of the sensel lines for that AF spot). Remember that the camera is driving; the lens can get the stop position wrong, but it doesn't start itself. – user2719 Mar 13 '14 at 2:35
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    Stan, thanks for the technical explanation. On my part, I might have been inaccurate. The camera might actually be trying to focus, because I can hear some faint noise. However it probably gets locked on moving back and forth, as the steps it moves are too small to gain any de-blurring. Therefore, it might be doing cycles around the wrong focus point. If it is the camera that is driving, then it is its error minimization algorithm at fault, as it does not increase the step magnitude each time that it sees that there is no improvement. – galanom Mar 17 '14 at 10:32

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