In order to find the optimum exposure and development times it is possible to make exposure tests.
There are many variations on the method but Graham Saxby's method works well if you have your film lying flat in a single beam reflection setup. (Any other setup will only take a slight variation using a moving slit or tape.) Calculate your best estimate at the proper exposure time. Place 4 coins on the film. Remove one coin after 1/4 of you estimate, remove one more coin after 1/2 of your estimate, remove one more coin at your estimate, remove the last coin at double your estimate and then shut off the exposure at 4 times your estimate.
If you use tape, you can remove tape at the proper times. This makes strips of differing exposure, if you cut the plate across the strips and develop each piece for a different time you can see the effects of exposure and developing times.
Some tips from ErichRose
After screwing around with one inch vertical strips I came up with my own little system: I was shooting 4x5s so I made a black cardboard mask that had one quadrant missing (~2x2.5). It was cut just a bit smaller than the plate so it would nestle in front of the plate between the edges of the plate holder, then it was tacked in place with hot glue. I started with the upper left corner, then upper right, lower right & bottom left. In other words clockwise* around the plate. I usually did 1/2 the calculated exposure, 1x, 1.5x and 2x for the first test.
The beauty of the quadrants was the ease of lining up the card. I just flipped it around or rotated it. I had minimal overlap on the exposures. It also gave me a better field of view for assessing the exposure. The "one inch" strips often ended up being 3/4, 1-1/4, overlapped, a thin strip unexposed, etc. I could easily live without the fifth exposure. After a while you get a feel for how much the exposure will need to be even if one of the four isn't quite right.
I also made good use of that fine point Sharpie marker and labeled all test plates immediately. And I kept good notes. Which I am re-reading twenty years later. Just wish I was going to be working again with 8E75 so I wouldn't have to do so much over agian.
- It's a good habit to make a habit of always doing things in the same order: clockwise, left to right, top to bottom, etc. When working in the dim safe light this will make it easier to keep everything straight.
Notes from Ed Wesly
- ErichRose is on the right track, by using a quadrant system. This takes into account the natural Gaussian fall-off of the spread laser beam, unlike the strip methods where the peripheries are seeing different flux energies at the edges compared to the centers. The quadrant captures the radially diminishing exposures, so that there is a realistic comparison of same intensities in the center of the plate but with different exposure times.
- Another point to make is that the exposure times should be logarithmic, like the response of the eye. In other words, a series like 1", 2", 4", 8", 16" ... as appropriate would be good, or for finer tweaking, 1", 1.4", 2", 2.8", 4", 5.6", 8", 11", 16" ...