Canon Speedlite 90EX - Manual (Page 13)

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 1/X sec. is the camera’s maximum flash sync speed.
Flash Sync Speeds and Apertures Used
Shutter Speed Setting Aperture Setting
V Set automatically (1/X sec. - 1/60 sec.) Automatic
X Set manually (1/X sec. - 30 sec.) Automatic
W Set automatically (1/X sec. - 30 sec.) Manual
q Set manually (1/X sec. - 30 sec., Bulb) Manual
COP
Y