I don't know, I didn't really double-check the math.
The next numbers of seconds in third-stops according to an old Master light meter, 40 - 50 - 64 - 80 - 100
Reciprocity failure is a factor to keep in mind.
I also believe the idea that you get consistent steps using f/stop printing breaks down a little when you change paper grades.
Because 1/3 stop "change" at Grade 2 is a gentle difference... While it's a rather dramatic difference at Grade 4.
Still I'd prefer f/stop differences than 5 - 10 - 15 - 20 - 25
p.s. I made the 11x14 print last night using Looten's 5 - 10 - 20 - 40 - 80, and it sort of caught me off guard. The vintage neg was kind of thin compared to my usual. I had to use f/22 to make that test strip.
Exactly! The only thing I don't have with percentages that you have with f-stop timing is the headache! :p
Originally Posted by Bill Burk
As Bob points out, the percentages and the stops are not exactly equivalent (and reciprocity failure is always a factor at longer times), however percentages are consistent and flexible and have all the advantages and none of the drawbacks of f-stop timing. I have two sequences, 20% and 30%, that I use for test strips and have simply memorized them. No programming, no math, nothing but counting seconds.
Seriously, it seems to me that so many f-stop timing schemes (and timers!) are overly-complicated for the task at hand. Percentages are easily figured and rounded, and I can just use my metronome to time with and watch my work. Plus I have the advantage that I can apply the same system to dodging and burning, which is rather difficult with f-stop timing (how do you dodge 1/3 stop anyway?).
And, when scaling a print up or down, I can find a base exposure and then use the percentages I've recorded to figure a starting point for manipulations for the new size. I say starting point because different size prints often require very different approaches to get the same "feel."
When I worked in the 80s doing huge murals we worked with a mag formulation that considered reciprocity , and therefore once we established a set time at one magnification
then we could calculate fstop differences without adjusting the timer unless in extreme changes.
Quite frankly I cannot tell you all how this was done today as my mind is old and feeble.
I am a strong proponent of keeping my times within 10 -18 seconds so that the math will work out in % 's, but since we all have different types of enlargers. and different types of negatives our times may vary.
I do use three bulbs on my enlargers .. 75w, 150 w and a 250 ww depending on the negative therefore keeping my times to the sweet spot as well keeping in the middle areas of my apeture without going to the extremes.
I have heard of some here doing 2 minute exposures which if find impractical.
I also utilize ND to lower the power of my 11 x14 and 8 x10 enlargers so that I am almost always in the sweet spot.
I just finished 4 different negatives for mural silver prints and was able to print at two stops down and between 12 and 16 seconds.
This way I can use predictable % printing as some here suggest to do.
Also I use glass carriers and will adjust the apeture once focused to change density if my time is in the sweet spot.