When the 540EZ is used with and EOS camera, you can select whether to have the flash fire as soon as the shutter opens (first curtain sync) or immediately before the shutter closes (second curtain sync)
When using a also shutter speed with a moving subject, second-curtain synchronization will provide a more natural effect because the flash exposure occurs at the end of the subject movement rather than at the beginning providing a "trailing blur" effect that emphasized motion.

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To set the 540EZ to second curtain sync mode, press the Plus and Minus button simultaneously
- The second-curtain sync indicator
appears in the LCD panel and the flash enters second-curtain sync mode
- Each time the Plus and Minus button are simultaneously pressed, the synchronization mode alternates between first-curtain sync and second-curtain sync. When first-curtain sync is selected, the second-curtain sync indicator disappears from the display
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