Canon EOS 1DS MARK II 9443A009 Instruction Manual (en)

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Autofocus can fail to achieve focus (the focus confirmation light <
o
>
blinks) with certain subjects such as the following:
Subjects difficult to focus
(a) Low-contrast subjects
Example: Blue sky, solid-color walls, etc. 
(b) Subjects in low light. 
(c) Extremely backlit and reflective subjects
Example: Car with a reflective body, etc. 
(d) Overlapping near and far objects
Example: Animal in a cage, etc. 
(e) Repetitive patterns
Example: Skyscraper windows, computer keyboards, etc. 
In such cases, do one of the following:
(1) Focus an object at the same distance as the subject and lock the 
focus before recomposing. 
(2) Set the lens focus mode switch to <
g
> and focus manually. 
When Autofocus Fails (Manual Focusing)
If focus cannot be achieved with the AF-assist light of a Speedlite or ST-E2, 
select the center AF point. An off-center AF point may not be able to achieve 
focus.