Canon Speedlite 580EX II Manual page 23

Metered Manual Flash Exposures
When the Speedlite is attached to an EOS-1D series camera, you can
set the flash level manually for closeup subjects.
1
Set the camera and Speedlite.
Set the camera's shooting mode to <q> or <W>.
Set the Speedlite to manual flash.
2
Focus the subject.
Focus manually.
3
Set up an 18% gray card.
Place the gray card at the subject's position.
In the viewfinder, the entire spot metering circle at the center
should cover the gray card.
4
Press the <7> button.
The Speedlite will fire a preflash and the required flash output for
the subject is retained in memory.
On the right side of the viewfinder, the exposure level indicator
will show the flash exposure level for the correct flash exposure.
5
Set the flash exposure level.
Adjust the Speedlite's manual flash level and the
camera aperture so that the flash exposure level
aligns with the standard exposure index.
6
Take the picture.
Remove the gray card and take the picture.
This feature works only with EX-series Speedlites with manual flash coupled
with an EOS-1D series camera.
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