Does anyone know how to calculate how far a flash fill will cover in on a lets say a sunny 16 day - what is the formula?
by Administrator
Sunny 16 is an approximation for ambient light exposure and is
unrelated to flash coverage. Flash coverage/distance is not affected by
levels of ambient lighting, only distance(inverse square law) and flash
Guide Number(power).
Distance = GN / f stop at ISO 100
at ISO 200 multiply by 1.4
at ISO 400 multiply by 2
So on a sunny day, the max distance of a 550EX, using f/16, is about 4 Meters at ISO 100.
Flash fill indicates that you are using ambient light as your
primary light source and flash to just fill in some shadows under the
eyes, etc. That means you should also be dialing in -FEC to the tune of
-1 to -1 and 2/3 as long as you stay within that max distance. Outside
that distance the flash will be maxing out and may give erratic
results.
Ron
