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When we do an AutoFocus Micro Adjustment session, the general opinion is to set the focusing target at 25 x focal length (or somewhere in the range of 5-50 x focal length).

Why is so? What happens if we "disobey"?

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  • \$\begingroup\$ No idea and actually never heard of that. More important is that I have seen lenses have different focus errors depending on the distance, so I try to calibrate at a typical distance. If it is a macro lens for example, I put the chart close. \$\endgroup\$
    – Itai
    Commented Jan 17, 2013 at 14:01

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Two reasons. The first is that the distance is more-or-less in the middle of the lens's focus range. Too close to nominal infinity (which is very easy to get to with short lenses) and you won't be able to accurately determine the point of focus on the calibration scale, so you want to make sure the target is close enough. If you're too close to the target you may find that focus errors cover the whole visible scale. 5 to 50 times focal length is a pretty wide range to work with.

Second, it really helps if the scale is about the right size in the image. If you're too far away for the focal length, the scale will be too small, and the markers will be difficult to read. If you're too close, you'll either crop out the scale or have to recompose off of the focus target to be able to see the scale, and that movement kind of invalidates the calibration.

It's not about optical science or anything weird like that, it's just a practical way of using the equipment.

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