Originally posted by jolepp A formula: EV = log2((f^2) / t) - log2(iso/100), this is handy enough with a calculator, without one can work the EV out by keeping in mind that EV 0 is f=1 t=1.0s @ISO 100 by definition, 1 EV unit = 1 stop and that higher values mean more light. For example f=2.8 t=0.5s is 3 + 1 = 4 stops brighter than the baseline or EV 4 (assuming ISO 100).
Jolepp (as is not uncommon) said a good thing...easy to remember & to work from..to wit:
EV 0 is f=1 t=1.0s @ISO 100 by definition - count f-stops up (brighter scene) and others down (dimmer scene).
Dave
PS it is easier for me to think in terms of speed rather than time (increase f-stops and/or speed-stops to admit less light.)
PPS I now wish the ISO people had started with ISO = 1 rather than ISO = 100, but at the time it was good to be consistent with the ASA definition.