In my last post, I discussed how the old curve I used with Gray Scale negatives was too contrasty for RGB negatives when making my gum prints. The old curve was based on creating eight distinct points (representing the 8 printable steps I can achieve with gum) and altering the input and output of percentages of black ink.
In Gray scale mode, the Input levels and Output levels are represented in terms of ink percentages rather than light values in “Pigment/Ink %” (on left), where white represents 0% ink and black is 100% ink. In RGB mode, by default, Photoshop displays the Input levels and Output levels gradients in the Curves dialog box from black to white in “Light (0-255)” (on right): black represents areas of no light, a brightness value of 0, and white, the maximum amount of light possible, at level 255.
To come up with a new curve, it was easier for me to work with the “Light (0-255)” graph. I got the new input numbers for the new curve by using the old curve in RGB mode. I just needed to find the new output numbers.
Fortunately, much of the preliminary work was done when I came up with the old curve so it wasn’t necessary for me to “start from scratch.” Originally I used a densitometer to measure the percentages of black ink on a grid of 100 shades of gray printed on Pictorico Ultra Premium OHP Film and compare those readings with measurements from a Stouffer 21-Step Sensitivity guide. See post from July 4, 2011 for more information:
I enlisted a former student of mine from Queens College and aspiring gum printer, Vanessa Fischer, to help come up with a new working curve. I made a printout of the 100 shades of gray for the purpose of writing notes.
I identified those squares of percentages of black ink that corresponded to the old curve along with the correlating densitometry readings. Using the eyedropper tool in Photoshop, I recorded the number that appeared in the “INFO” window in RGB and repeated for all 8 squares corresponding to the 8 steps in my curve.
In the example square below, the bottom number (0.87) represents the densitometry reading, the middle number (85) is the percentage of black ink from the old curve and the top number (54) is taken from the “INFO” window—which has now become the new output numbers.
Below is a comparison of the old and new curve. The new curve is shown on the right in red below.
Vanessa was able to try out the new curve before I had a chance to print. Once I saw her result (below), I was convinced it was going to work.
The new curve.