Re: Re: Reciprocating

----- Original Message -----
From: "Stephen Harris" <smhcine@email.com>

> So again thanks and 2 questions: How is the formula
> derived

Go here http://curvefit.com/goodness_of_fit.htm and look at the 2 graphs
at the bottom of the page (never mind the text) we are going to assume the
black dots on the graph are the reciprocity corrected values for different
times, the curve that has been drawn is the curve that in "average" BEST FIT
those values, you could do that manually or you could use some math to do
it. The math equation that describe that curve is known as BEST FIT CURVE
and that equation can be used to extrapolate values that are beyond the 5
points of data we have, meaning that if you have 5 values of reciprocity up
to 100 seconds, if you get the best fit equation you can easily find the
value for 15 minutes just by plugging that time into the equation of the
best fit curve. The more points of data you have the better the best fit
curve will be. There are many ways to get to the best fit curve equation.
This stuff is taught in high school (at least in my currently earthquake
stricken country of El Salvador) so any good high school math book could be
a good source if you want to find the nitty gritty details of it. Of
course, there are also programs that do the work for you.

> what, exactly, is the sunny 16 rule?

It is a well know fact that if under clear SUNNY skies you photograph an
average scene (gray card for instance), the needed exposure will be "f/16"
and a speed in seconds of de reciprocal of the ISO speed of the film (or
paper for us pinholers) you are using. For instance, if you are using
TMAX-400, you would need f/16 and 1/400 second. Knowing the above, if you
are not shooting at f/16 you can just find a pair of exposure values
(aperture and time) that would give you an equivalent exposure, i.e. if you
are using f/128 (which is 6 stops away from f/16 -f/22, f/32, f/45, f/64,
f/90, f/128-), you would need to double the time 6 times (1/200, 1/100,
1/50, 1/25, 1/12, 1/6) you your new exposure would be f/128 and 1/6 seconds
using TMAX-ISO400.

Hope I am not confusing you.

Guillermo
Received on Wed Jan 17 12:16:39 2001