When Autofocus Fails; Manual Focusing - Canon EOS 1Ds Mark III Instruction Manual

When Autofocus Fails

Autofocus can fail to achieve focus (the focus confirmation light <o>
blinks) with certain subjects such as the following:
Subjects difficult to focus
Low-contrast subjects
Example: Blue sky, solid-color walls, etc.
Subjects in low light
Extremely backlit and reflective subjects
Example: Car with a reflective body, etc.
Overlapping near and far objects
Example: Animal in a cage, etc.
Repetitive patterns
Example: Skyscraper windows, computer keyboards, etc.
In such cases, do one of the following:
(1) With One-Shot AF, focus an object at the same distance as the
subject and lock the focus before recomposing. (p.83)
(2) Set the lens focus mode switch to <g> and focus manually.

Manual Focusing

If you press the shutter button halfway during manual focusing, the active
AF point and the focus confirmation light <o> in the viewfinder will light
when focus is achieved.
86
Set the lens focus mode switch to
1
<MF>.
Focus the subject.
2
Focus by turning the lens focusing
ring until the subject looks sharp in
the viewfinder.