Canon EOS 50D - Manual (Page 130)

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Notes about Custom Functions
Live View shooting will disable some Custom Function settings (p.173).
If [8C.Fn II -4: Auto Lighting Optimizer] (p.177) is not set to [3:
Disable], the image will look bright even if a decreased exposure
compensation has been set.
Notes about lenses and flash
The focus preset feature on super telephoto lenses cannot be used.
FE lock is not possible when the built-in flash or external Speedlite is
used. Modeling flash is not possible when external Speedlite is used.
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