Time = 2^stops. So 0 stops is 1s, 2 stops is 4s, 3.5 stops is 2^3.5 = 11.314s.
Originally Posted by SMBooth
To make a more complicated example, and illustrate how dodges are computed, consider:
base = 3.5 stops = 11.314s
dodge 1 = 0.5 stop = 2^(3.5-0.5) = 8s (i.e. dodge for 3.314)
dodge 2 = 0.25 stop = 2^(3.5-0.25) = 9.514s (i.e. dodge for 1.8s)
Therefore, the base exposure once dodges are subtracted is 11.314-3.314-1.8=6.2s. When you perform the dodges, the non-dodged areas will get the originally intended base exposure of 11.314s.
The reason the base exposure is expressed in stops is it makes it easier to scale your paper size, or to shift the whole exposure darker/lighter by a certain tone. Say you figure out a good print at 8x10 and want to print 16x20, you just add two stops. And when you do a test strip in stops, the values will be equi-spaced, e.g. 2.0, 2.25, 2.5, 2.75, 3.0, 3.25, 3.5, 3.75. If you converted all of those to seconds, you'd get some really annoying numbers. It's also very easy to apply drydown correction; the software just needs to subtract a configured value, e.g. 0.08.
It also makes it easier to do stuff like grade changes when printing split-grade. My timer doesn't directly support that (i.e. it won't recompute the times for a grade change, or do a grade change while preserving a particular zone), but the use of stops to represent each exposure time makes it relatively simple for the user.