Canon Speedlite 90EX - Manual (Page 12)

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In this mode, the camera will automatically adjust the flash output level to achieve standard
exposure for the set aperture.
If the background is dark, like a night scene, a slow sync speed will be used to obtain a
standard exposure of both the main subject and background. Standard exposure of the main
subject is obtained with the flash, while a standard exposure of the background is obtained
with a slow shutter speed.
• Since a slow shutter speed will be used for low-light scenes, using a tripod is
recommended.
• If the shutter speed display blinks, it means that the background exposure will be
underexposed or overexposed. Adjust the aperture until the shutter speed display stops
blinking.
Select this mode if you want to set both the shutter speed and aperture manually.
As the camera will automatically adjust the flash output level according to the aperture you
set, the exposure of the main subject will be standard. The exposure of the background is
obtained with the shutter speed and aperture combination you set.
W : Aperture Priority AE
q : Manual Exposure COP
Y