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For those of you here who may want to give this AFMA method a try, having done it myself, I would like to add a few helpful notes ...
1. Put the tape measure (which is held up by the pencil) right up against the box, so that it's actually touching the box. And affix the target right to the edge of the box, abutting the tape measure. This will help when it comes to analyzing the results later on.
And will also help when you position the tape measure's "1' " marking exactly in parallel with the target. (Tape the target top and bottom to the box so that the target stays flat to the box.)
2. You can easily make your own focus target, as I did. Just cut a 2.5" x 2.5" square of white copy paper and take a thickish black marker and draw an "X" on on the paper, like this:
The actual target I made was an "X" with a "+" sign on it, like this:
3. The recommended distance from camera to target is 25X the focal length, though I've heard you can use anywhere from 5X to 50X. What I've found is that the farther back you are, the easier it is to tell if focus is off. For example, at 3' from camera to target, focus will be off 1/2" but move back to a 6' camera-to-target distance, and the focus will be off 1".
(When calculating the recommended 25X distance, it was said to ignore the crop factor of the smaller sensored cameras in your calculations ... full frame or crop makes no difference.)
My actual camera to target distance was 17X the focal length, which seemed to work out fine.
Here's an online calculator to convert lens focal length in millimeters to inches:
http://www.convertunits.com/from/mm/to/inches
Any questions, don't be shy.
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