Canon EOS-3 Instructions Manual page 94

Flash Photography with Speedlite 550EX
High-Speed Sync (FP Flash)
When the Speedlite's high-speed sync lamp ( ) is on, high-speed sync
(FP or focal-plane flash) is enabled automatically. The Speedlite can then
synchronize at all shutter speeds, even those faster than 1/200 sec.
When high-speed sync is enabled, is displayed in the viewfinder to
indicate high-speed sync.
High-speed sync is effective in the following cases:
(1) When you want to use fill-in flash for a portrait and maintain
background blur with a large aperture.
(2) When you want to produce a catchlight in the subject's eyes.
(3) When you want to use fill-in flash to eliminate shadows
With conventional flash. With FP flash.
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