Converting numbers and percentages to stops (Danger: Math!)

[480sparky]

To put in whole fractions I am just thinking it would be simple to use IF and specify 1/3, 1/2 or 2/3 as the result based on whichever of those falls closest to your decimal number. So halfway to 1/3 would go down to the full stop, then halfway between 1/3 and 1/2 display 1/3, then halfway between 1/2 and 2/3 display 1/2, then halfway between 2/3 and 1 display 2/3 and finally above that would display 1. If you need more precision than 1/3 stops then the VLOOKUP is better.

As you could have more than a full stop difference then I would first pull out however many full stops difference and then add that back with the fraction using Concatenate to read out something like: 2 and 2/3 Stops less light.

If I wanted to break it down like I imagine it to be, that would be a LOT of IF/THTEN statements. I'll just roll right into VLOOKUP and call it a day.

 

[Alexr25]

To get the time difference in f stops simply take the log to the base 2 of the times and subtract them.
To find the log base 2 of a number find the log base 10 of the number and divide it by the log base 10 of 2.

eg 1/30 = .033 sec, log(.033)= -1.477, log(2)= 0.301, log(.033)/log(2)=-4.9
1/60= .0167sec, log(.0167)= -1.178, log(2)= 0.301, log(.0167)/log(2)=-5.9
Difference = -4.9-(-5.9) = 5.9-4.9 = 1 stop.