I recently discovered F-stop printing. A simple concept and incredibly powerful for darkroom printing. Especially useful for judging test-strips and making dodge & burning more repeatable.
Anyhow, about F-stop printing. Instead of thinking of exposures in seconds, you think of it in F-stop just like you would with your camera. For instance want part of the image to be 1/2 a stop darker, just burn with half a stop.
An easy concept, and for whole stops it’s easy just double or half the exposure time. However partial stops it gets a bit tricky, because it’s not linear. If you want half a stop over 4 seconds, it’s calculated as 4 x 2^(1/2) = 5,7 seconds. Not 6 seconds as one might first think. A small difference, but a difference, and the longer the times the greater that difference becomes.
Therefore when I’m out in the darkroom I now bring a couple of charts with me, on how to adjust exposure times in half or third stops, as well as my standard test strips.
I have two different types of series. One is of the type describing what actual exposure times partial stops are. Like this one for 8 seconds, and the half stops around 8 seconds
-2/2
-1/2
B
+1/2
+2/2
4,0
5,7
8,0
11,3
16,0
8 seconds adjustments in halfstops
But I also have another type of chart, that’s for test strips, and when I want to build up an exposure. Let’s say I want 4 seconds for the whole paper, but then I want to burn 1/2 stop on part of the paper. I do the 4 seconds exposure of the whole paper. Then I add on top of that 1/2 stop (1,7 s), therefore I need to know how much time I need to add. Therefore I have a chart for my base exposure and how to do so.
½ B
-1/2
2/2
B
+1/2
+2/2
2x B
2
0,8
1,2
4,0
1,7
2,3
8
Adjustments for 4 seconds, and how much to add for 1/2 stop. I I want 1/2 stop under 4 seconds, I do 2 seconds, add 0,8 seconds and that’s half a stop under 4. If I then want to go the whole way t 4 seconds I add another 1,2 seconds.
The same goes for my test strips. I do a base of 2 seconds for the whole paper, cover up part of the paper do 0,8 s, cover up a part and do 1,2 s etc. Until I’ve done my whole test strip.
2
2,8
4
5,7
8
11,3
16
0,8
1,2
1,7
2,3
3,3
4,7
Haf stop test strip that starts at 2 seconds and ends at 16 seconds. Start with 2 seconds, add 0,8 s, then 1,2 s, etc.
I hope that made sense. Here are the full charts that I’ve used. They aren’t perfect but I hope they make sense! Use them at your discretion, I think they are correct, but I don’t guarantee it.