Canon EOS 30D Instruction Manual

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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 <MF> and focus manually.
1
On the lens, set the focus mode 
switch to <MF>.
2
Focus the subject.
 
Focus by turning the lens focusing 
ring until the subject is in focus in the 
viewfinder.
When Autofocus Fails (Manual Focusing)
Manual Focusing
Focusing ring
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.