Canon SPEEDLITE EL-1 Advanced User's Manual page 44

Auto Zoom Support for Image Senzor Size
EOS DIGITAL cameras have three sizes of image sensors, and the effective shooting angle
of view of the attached lens varies depending on the size of the image. The Speedlite
automatically recognizes the image sensor size of the EOS DIGITAL camera and
automatically sets the flash coverage that is ideal for the effective shooting angle of view of
a lens for the focal length range of 24 - 200 mm.
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