Canon Speedlite 580EX II Instruction Manual page 22

Auto Zoom for Image Size
EOS DIGITAL cameras have one of three image sizes. The lens'
effective focal length will differ depending on the camera's image size.
The Speedlite automatically recognizes the EOS DIGITAL camera's
image size and automatically sets the flash coverage for lens focal
lengths from 24mm to 105mm.
When the Speedlite is attached to a compatible camera, <s> will
appear on the Speedlite's LCD panel.
Auto zoom can be disabled. (C.Fn-09 → p.27)
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