Canon EOS M Instruction Manual page 146

f: Changing the Depth of Field
If the picture will be too dark, the "30"" (30 sec.) shutter speed
display will blink. If this happens, tap to display the setting screen
and then tap [Y] (or turn the <5> dial to the left) to set a lower
aperture f/number or increase the ISO speed.
If the picture will be too bright, the "4000" (1/4000 sec.) shutter
speed display will blink. If this happens, tap to display the setting
screen and then tap [Z] (or turn the <5> dial to the right) to set a
higher aperture f/number or decrease the ISO speed.
Using Speedlite 90EX
To obtain a correct flash exposure, the flash output will be set To obtain a correct flash exposure, the flash output will be set To obtain a correct flash exposure, the flash output will be set
automatically to match the set aperture (autoflash exposure). The automatically to match the set aperture (autoflash exposure). The automatically to match the set aperture (autoflash exposure). The
shutter speed will be set automatically between 1/200 sec. - 30 sec. to shutter speed will be set automatically between 1/200 sec. - 30 sec. to shutter speed will be set automatically between 1/200 sec. - 30 sec. to
suit the scene's brightness.
In low light, the main subject is exposed with the automatic flash, and In low light, the main subject is exposed with the automatic flash, and In low light, the main subject is exposed with the automatic flash, and
the background is exposed with a slow shutter speed set automatically. the background is exposed with a slow shutter speed set automatically. the background is exposed with a slow shutter speed set automatically.
Both the subject and background look properly exposed (automatic Both the subject and background look properly exposed (automatic Both the subject and background look properly exposed (automatic
slow-speed flash sync). If you are handholding the camera, keep it slow-speed flash sync). If you are handholding the camera, keep it slow-speed flash sync). If you are handholding the camera, keep it
steady to prevent camera shake. Using a tripod is recommended. steady to prevent camera shake. Using a tripod is recommended. steady to prevent camera shake. Using a tripod is recommended.
To prevent a slow shutter speed, under [A1: External Speedlite To prevent a slow shutter speed, under [ To prevent a slow shutter speed, under [
control], set [Flash sync. speed in Av mode] to [1/200-1/60 sec. Flash sync. speed in Av mode Flash sync. speed in Av mode
auto] or [1/200 sec. (fixed)] (p.221). 1/200 sec. (fixed) 1/200 sec. (fixed)
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