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).
Z=100 (ISO value)
Y=125 (shutter time)
X=0 (EV adjustment, clear sky on a sunny day)
Z=200 (base ISO for several sensors)
Y=500 (shutter time = 1/500)
X=2 (overcast sky)
F= 5.7 (the closest F-stop will be 5.6).
Edit:I just realise that the STOP instruction is not really needed; AVIEW already pauses the display until a key is pressed.
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