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302 “Custom” (Gear icon) Menu Settings

6.28.1 FREQUENTLY ASKED QUESTIONS
Q: “Hey, I thought Autofocusing was a closed-loop process, so even if the
camera and lens were out of tolerance, the feedback loop should still
compensate for that and focus properly! Why is this feature even needed at
all?”
A: Well, really, it’s not closed-loop, since the light from the lens to the
phase-detect array (located above the semi-transparent mirror) takes a
different path than the light from the lens to the sensor. In theory these 2
paths are exactly the same distance; however if either the image sensor or
the phase-detect array were to become mis-aligned (for example, if the
mirror weren’t perfectly snapped into place), the path to the phase detect
array would be shorter and all of your images would be unsharp. There can
be other causes for this error as well (usually related to tolerances), and so
this feature is welcome by sharp-shooters.

6.28.2 HOW TO TEST
There is a right way and a
wrong way to test for
Backfocus issues. The
wrong way can be seen in
Figure 6-21, where you
just have a ruler with a
continuous scale at a 45

degree angle from the
camera. There are several Figure 6-21: The wrong way to test for
reasons why this test is frontfocus / backfocus issues is looking at a
unideal: the center continuous scale on a ruler or tape measure at
focusing sensor is actually a 45-degree angle to the camera.
larger in size than the
small square you see in the viewfinder, so you never know which part of
the rectangle it will focus on if there’s more than one choice. In some cases
the pattern can easily confuse the autofocusing algorithms (especially when
you’re testing in poor light conditions, such as under compact fluorescent
light).

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