I don't know whether to laugh or cry...
Let's say you know the correct f/stop for ISO 100 film. You therefore
automatically know the correct f/stop for EVERY OTHER FILM SPEED. You
don't need a calculator for that.
To put it directly... If the lens opening for 400-speed film is, say,
f/8, then what is the correct opening for 200-speed film? For 800-speed
film? For 640-speed film? For 100-speed film?
This is Photography 101. _Anyone_ involved in photography should know
this without having to ask.
Please don't flame me with "why are you dumping on someone who needs
help?" I ask, why don't people bother to learn about the things that
interest them? I recently visited Border's Books to find photo books I
could recommend to a friend who was getting started in photography.
Border's had _dozens_ of books, both technical and artistic. There is no
lack of information for anyone who makes the slightest effort to look
for it.
I urge the person who posted this question to get some books on basic
photographic theory, or find someone who can teach him.
>>>>>
Guide numbers _can_ be confusing.
If you need a guide number for an unlisted film speed, you multiply a
known guide number by the _square root_ of the ratio of the unlisted
film speed to the known film speed.
For a 1/3-stop increase in speed, you increase the guide number by 10%.
For a 2/3-stop increase in speed, you increase the guide number by 25%.
For a 1-stop increase in speed, you increase the guide number by 40%.
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