Canon EOS 30D - Manual (Page 80)

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Autofocus can fail to achieve focus (the focus confirmation light
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 .
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 in the viewfinder will light
when focus is achieved.