(Part 1: Background)
In alternative processes, the print is only ever as good as the negative (film or “desktop” negatives). Gum prints require the negative to be the same size as the final print; which was a challenge (to say the least) using traditional darkroom methods. With film, density (the dark and light portions of the film) and contrast (distinct separation of tonal values) are controlled by exposure and development. The “Characteristic Curve” of film illustrates the increase in negative density as exposure and development increases. The three portions of the curve represent the different areas of the negative that shows detail: the “TOE” portion are the light areas in the negative and represents the shadows, the “STRAIGHT-LINE PORTION” are the midtones and the “SHOULDER” portion are the dark areas in the negative and represent the highlights in final image. The overall “curve” of each negative could be manipulated by adjusting (increasing or decreasing) the exposure and/or development of the film.
Digital technology (Adobe Photoshop and Epson desktop printers) has made it possible to produce quality, consistent, predictable negatives effortlessly. Before I began printing out digital negatives, I had to create a gray scale with ink on Pictorico OHP Premium Transparency Film that was the equivalent to the gray scale with film. Using a Stouffer 21-Step Transparent Guide, I measured the 8 steps that the Gum Bichromate process can print (from #2-#9) with the X-Rite Densitometer Model 331C and came up with a numerical value for each step. I then created a gray scale with wedges of varying percentages of black ink (from 2% to 100%) and with the Densitometer I measured each wedge to come up with a numerical value for the different percentages of black ink. Finally I identified the percentages of black that come close to steps #2-#9 on Stouffer 21-Step Sensitivity guide (give or take .02).
I am now able to apply this information in order to alter or manipulate the density and contrast of digital negatives by simply adjusting the “input” and “output” using “CURVES” in Adobe Photoshop (similar to increasing or decreasing exposure and development with film).
NEXT: THE NEGATIVE (Part 2: Curves)