Originally Posted by Michael R 1974
I agree with almost everything you've said. [The exception is the shorter exposure range (< 20 seconds) you've suggested.]
Test strips are (or should be) simple. If you have some darkroom experience and have been shooting, developing, and printing for awhile, chances are you can walk into your darkroom and already have the timer set to an exposure that will give you a decent 1st work print at a certain degree of enlargement without even bothering to run a test. That lightbulb probably clicked on long ago.
But, to someone starting out, running tests strips is often confusing and seen as a waste of time and resources. If they don't understand why something has gone wrong, chances are they will abandon any sort of systematic method and just try random guessing that ends up having no consistency. Then, confusion really sets in and they may abandon the whole darkroom experience.
I have worked with beginning students in a community college darkroom for over 23 years and I know where they stumble. You would be surprised at how many never realize that "the exposures go from obviously too dark to obviously too light" until they have that specifically pointed out to them (perhaps more than once). The more common experience is that they get a range of exposures that are all too light and then they choose the darkest of those to use. Many are satisfied just to get any image. Many never end up getting proper dark values or contrast in their prints because they don't get the obviously too light/dark thing, and yet they may be satisfied with that print without understanding the further potential. I always tell them to shoot for that middle area on the strip because if one end is obviously too light and the other obviously too dark, the proper exposure has to be somewhere in between on their test. I never tell them to print the first or last strip on a test sheet unless I qualify it with "how do we know the next time out might not be better? Adjust the f/stop or extend the time to shift that presumed correct strip to the middle and run another test to be sure."
I usually use a full sheet of paper (and never less than 1/3rd) to run tests because I want to gauge what the light is doing over the entire image. That also comes in very handy when determining dodge/burn times as you have suggested. Running 1/4"-wide strips is false economy and I think leads to an ultimate waste of paper. If an incorrect area is chosen to make a small test strip, a print is subsequently run, discovered to be incorrect, and perhaps at that point confusion sets in. At least a full sheet + 1/4" will have been wasted along with the time taken to print and process both. The whole sheet approach helps prevent this kind of waste and provides a lot more exposure information all in one cycle.
If I am printing for myself, I don't want to be printing at less than 24 seconds unless I absolutely have too (e.g, when making very small enlargements from medium format negatives). I want some time to accurately be able to burn & dodge. I don't want to feel rushed. I'd rather double a 15-second exposure to 30 seconds than to attempt to manipulate things at shorter times that would double any error and not be as controlled and accurate. Have I really wasted 15 seconds initially when making the longer 30-second print? Or, have I saved myself a reprint and all the time associated with exposing and developing two (or more) prints when the longer first time would have required only one cycle?
Whether someone uses a geometric vs arithmetic strip to me is not a big issue. I personally think the geometric test provides better information, but admit it is harder to grasp at first. What I don't like is when a method is suggested to a novice printer in such a way that they never realize what is going on or make any connections about reciprocity between time and apertures. Why would anyone even try to make a good print at 5 or 10 seconds? Set the timer way out to some multiple of the interval at least twice as long in duration as the starting point (e.g., 40, 45, 50 seconds or higher), then after or 20 seconds have elapsed, start progressively covering the paper and move the card every 5 seconds until the time runs out. That will almost ensure a good, readable test if anywhere close to the correct f/stop. The method usually works because it covers over a full stop in exposure time. And that's the benefit of a geometric test as well. But with the geometric test a constant visual change is occurring as exposures get longer. With an arithmetic test, the visual effect is becoming less and less noticeable as exposure time increases. So, there is a danger for beginning printers to have an exposure at the very short end being not enough to see and then having big changes shortly afterward (- is that first visible strip 5 or 10 seconds, or was the last part of the paper even given an exposure...), while at the opposite end the strips are getting less and less and less distinct. As long as they realize what is happening and why, that's fine.
So the big message is don't waste paper on very short exposures or small strips of paper and if doing an arithmetic strip, understand that the exposure rate of change is decreasing the further out one proceeds with a constant time interval.
Another benefit of learning the geometric f-stop sequence is that information transfers to other photographic concepts once learned. Walk into a lighting studio and move a light from 11' to 8' and know it is suddenly twice as bright on the subject and then figure out lighting ratios with little difficulty. Or, figure out a bellows extension factor in your head without having to be that Manhattan Project dude.