At 22:32 2/12/02, Dave Dougherty, who sold his "Shipman" book, now asks the
question:
Fellow Zuiks,
Guide numbers are usually expressed as a function of 100asa. As light is, I
think, subject to the inverse square rule, does anyone remember the formula
to convert a GN from 100asa to say 400asa, etc. I had it in a Shipman book
which I sold, having failed to write down the formula before I did so.
Dave Dougherty
To convert a GN for one film speed to the equivalent GN for another film speed:
GN2 = GN1 * SQRT(ISO2/ISO1)
GN1 = known guide number rating at "ISO1" film speed
ISO1 = film speed for which guide number is known (GN1)
GN2 = desired guide number for "ISO2" film speed
ISO2 = film speed for which guide number is desired
SQRT = Square Root
Example:
If the flash is rated at GN 105 for ISO 100 (in feet), its GN for ISO 160
film is:
GN2 = 105 * SQRT(160/100)
GN2 = 105 * SQRT(1.6)
GN2 = 105 * 1.265 [approximated]
GN2 = 133 [approximated]
Now use ISO 200 film:
GN2 = 105 * SQRT(200/100)
GN2 = 105 * SQRT(2)
GN2 = 105 * 1.414 [approximated]
GN2 = 148 [approximated]
or, the previously found GN and its film speed could also be used
GN2 = 133 * SQRT(200/160)
GN2 = 133 * SQRT(1.25)
GN2 = 133 * 1.19 [approximated]
GN2 = 148 [approximated]
-- John
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|