> If the 550EX has a guide number of 180 at ISO 100 in feet, how do I
> properly increase the guide number for ISO 400?
The guide number for ISO 400 film is 360 feet (180 * 2) at 105mm
coverage. However, be aware that Canon overrates their guide numbers by
up to 1 stop to include indoor wall reflectivity.
The increase of film speed from ISO 100 to 400 is a four-fold increase,
or 2 "stops". Since light falls off at a ratio of the square of the
distance, it take a four-fold increase to illuminate double the distance;
hence double the guide number.
See by web page: http://www.kjsl.com/~dave/gn.html
Dave Herzstein
[EMAIL PROTECTED]
http://www.kjsl.com/~dave
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