Canon EOS 20D Instruction Manual page 70

When Autofocus Fails (Manual Focusing)
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.
Manual Focusing
Focusing ring
_I_ On the lens, set the focus mode
switch to <MF>.
'_j_,_Focus the subject.
_ Focus by turning the lens focusing
ring until the subject is in focus in the
viewfinder.
] !f you press the shutter button halfway during manual focusing, the active
AF point and the focus confirmation light <0 > in the viewfinder will light
when focus is achieved.
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