Negative exposure at least can be pretty sloppy, so earlier cameras could happily get away with pretty coarse speed spacings. Especially when you consider that mechanical shutters can be off by quite a significant margin. However if you look at a modern(ish) electronic camera like my RZ, the shutter speeds are 8, 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/400. Those values are nominal I suspect because no one really wants to see 1/32, 1/64, 1/128 and 1/256 on their dial; other than that the values are pretty much exactly powers of two. 1/400 is the odd duck just because the shutter physically can't do 1/500.
Originally Posted by Lionel1972
In terms of printing, there's absolutely no benefit to using the 2.8, 5.6, 11 sequence over the 2, 4, 8 sequence - both are spaced by factors of two (whole stops). I was suggesting you use 2.8, 4, 5.6, 8, 11, etc, which gives you half-stop steps. In my experience, half-stop steps are a good way to do a test strip at moderate (grade 2 or 3) contrasts. 8 steps of half stop covers a LOT of exposure range (e.g. 4s to 45s) to get you in the ballpark even with an uncertain negative. Once you get more confident, you'll find yourself doing 4-step strips at 1/4 stop intervals.
There is nothing special about the sequences I've illustrated, they're just (IMHO) the easiest ones because they're powers of two. You could just as reasonably use 5, 7, 10, 14, 20, 28, 40: still separated by a factor of sqrt(2) and therefore at half-stop spacing. You'll look at your strip, maybe you like one exposure, maybe want to go somewhere between two steps for a bit better rendition of subtle highlights. If so, you can so a second test-strip spanning the gap between two previous exposures. Say you like the look of the 11s and 16s exposures. You might want to try 1/8 stop steps: 11.3, 12.3, 13.5, 14.7, 16s. Yes, that's getting into finicky times but once you start doing fine prints at higher grades, you'll find that you need to be that finicky to ride the balance on highlights. With such fine exposure spacings though, there is very little benefit to doing exact logarithmic timing unless you're using an f/stop timer; you might as well use 11.5, 12.6, 13.7, 14.8, 15.9 or whatever.