A: Manual Exposure - Canon SPEEDLITE 320EX Manual De Instrucciones

f: Aperture Priority AE
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.

a: Manual Exposure

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Select this mode when you want to set
the aperture manually.
The camera will then automatically set
the shutter speed matching the aperture
to obtain a standard exposure.
If the background is dark, such as in 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.
Select this mode if you want to set both
the shutter speed and aperture manually.
Standard exposure of the main subject is
obtained with the flash. The exposure of
the background is obtained with the
shutter speed and aperture combination
you set.