When the Speedlite 550EX is set to the high-speed sync mode (), it can synchronize at all shutter speeds, even those faster than 1/250 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 daylight sync 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 daylight sync to eliminate shadows