In a message dated 01/24/2000 8:42:40 AM Eastern Standard Time,
wright@xxxxxxxxxxxx writes:
> hey, wait a minute.....ISO 400 is two stops faster than ISO 100, which
> makes for a GN of 128. If he used ISO 200 film, the GN doubles to 64,
> for ISO 400, it doubles again to 128. Isn't this correct?
No, the inverse-square law applies. Guide number increases by a factor equal
to the square root of the ratio of the film speeds. 400/100 = 4, the square
root of 4 is 2. So guide number doubles. For 200 speed film, the guide number
increases by a factor of 1.4 (the square root of 2). So for the T32, with 200
speed film, the guide number is about 45.
Hope this clears things up.
Paul Schings
< 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 >
|