Originally posted by JensE Log to base 2.

I see; thanks.

IOW log

2 (aperture reading squared x number on the shutter knob). Because you're dividing by 1/x of a second, simply multiply by x.

It gets even easier because all of the whole-stop aperture numbers are easy square roots anyway. The hardest part is the log, and even then, exponents of 2 should not be too hard to memorise and extrapolate as required. A quick mental calculation will probably get you in the ballpark most times, and IIRC Sunny Sixteen is EV

ISO 10015, and EV

ISO 1000 is f/1 for 1 second at ISO 100.

ETA Wikipedia. EV (any ISO) = EV at ISO 100 + Log

2(chosen ISO/100)