A rough way to estimate exposure is the "Sunny F16" rule, which determines that the following works for a sunny day with a clear sky:
ISO = 100
1/t = 100
F = 16
The following program computes the aperture value (F-stop) given the following inputs:
Z-register = ISO value
Y-register = 1/t (taking 125 as base, because digital sensors like slight underexposure)
X-register = EV adjustment (0 = clear sky, 1 = slightly cloudy, 2 = overcast, 3 = heavy overcast, 4 = rain or sunset)
The output produced is the desired F-stop value. The user is expected to press R/S to end the program.
N.B. currently the program assumes that FIX 6 must be set as the last step (that is what I use, because of date and time display).
Examples:
Z=100 (ISO value)
Y=125 (shutter time)
X=0 (EV adjustment, clear sky on a sunny day)
XEQ FSUN
F= 16.0
Z=200 (base ISO for several sensors)
Y=500 (shutter time = 1/500)
X=2 (overcast sky)
XEQ FSUN
F= 5.7 (the closest F-stop will be 5.6).
Code: Select all
LBL "FSUN"
FIX 01
2
X<>Y
Y^X
*
X<>Y
/
SQRT
17.8885
X<>Y
/
"F= "
RND
ARCL X
AVIEW
STOP
FIX 06
END