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AF-A Setup 159

So that’s what the 4 focusing modes do. What the AF-A setup menu
variable does is allow you to redefine the last setting (“A”) from the
description above to the following description:

A Direct Manual Focusing (DMF) mode. This is identical to “S”
mode above, except once the camera has focused on a subject,
the autofocus mechanism is disengaged and you are free to turn
the lens’ focusing ring manually to fine-tune the focusing.
“Are you saying that the camera’s autofocus mechanism is fallible?” Well,
when it nails it really nails it; however there are situations (like when
shooting flowers via macro mode on a windy day) when the subject shifts
just a little and it’s faster to fine-tune things manually.
But wait! As mentioned in Chapter 2, this DMF feature when coupled with
the Peaking Level function (Section 2.1), even if you don’t choose to tweak
the focusing manually the camera will highlight all of the areas in focus for
you, acting as a sort of meta-focus-confirmation. It’s quite reassuring.
(Plus, if you did decide the focusing needed tweaking, it’s VERY easy to
focus manually with the Peaking Level feature enabled!)

TIP #1: If you attach an SSM lens (such as “kit” 16-50 f/2.8 SSM), then DMF
mode is ALWAYS available to you, regardless of the setting of this menu
variable. Just turn the lens’ focusing ring at any time. There is a benefit to
enabling this feature with an SSM lens attached, though: it will enable the
Peaking Level function as described above.

TIP #2: If you attach an SAM lens then DMF is NEVER available to you (due to
the way the internal focusing motor is designed) and hence this option will be
greyed out in the menu. Another good reason to avoid SAM lenses. 
TIP #3: In DMF mode, if your subject is close it’s sometimes easier to tweak the
focusing by moving slightly closer or further away rather than turning the
focusing ring.