Canon Speedlite 320EX - Manual (Page 21)

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The flash head can be extended or retracted manually to match the lens
focal length. As the focal length of EF lenses and EF-S lenses differs,
extend or retract the flash head while referring to the table below (the
flash head position in relationship to the lens focal length).
When using an EF-S lens with a focal
length of 32mm or longer or an EF
lens with a focal length of 50mm or
longer, extend the flash head for
greater flash output and longer flash
range.
When the flash head is extended, the
<TELE> lamp on the Speedlite lights
or blinks.
When the <TELE> lamp blinks, retract
the flash head to its normal position. If
you take a picture while the <TELE>
lamp is blinking, the periphery of the
picture may appear dark.
Switching the Flash Coverage
Flash Head Position and Lens Focal Length
Flash Head Position Normal Position Extended Position
Lens Focal
Length
EF-S Lens 15mm or longer 32mm or longer
EF Lens 24mm or longer 50mm or longer
Normal position
Guide number 32/105 (ISO 100 in m/ft.)
Extended position
Flash head position
Guide number 24/79 (ISO 100 in m/ft.)
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Y