Use the highest resolution file for best results (in the past, I was using 4×5 negatives and making high res scans, now I’m using the Canon 5D Mark II).
In order to get the curves I needed to print CMY negatives in Gum I came up with an ink jet gray scale using the Epson 1400 printer that was equivalent to the Stouffer 21-Step Sensitivity Guide. Knowing that Gum can only print a total of 8 steps (from step #2 to step #9), I used a densitometer (the X-Rite 331 Densitometer) to measure each of the steps (from 2-9) on the Stouffer 21-Step Sensitivity Guide and came up with a numerical value that I could use to match against an ink jet gray scale using Pictorico ULTRA PREMIUM OHP Transparency film. Making wedges in Photoshop and filling in each wedge with different percentages of black ink I came up with a ink jet gray scale that is equivalent to that of the Stouffer 21-Step Sensitivity guide (give or take .02). To apply this ink jet gray scale, I used Photoshop CS5 and in curves, I made 8 points (to represent the 8 steps that Gum can print) and changed the input and output accordingly:
Input 0% – Output 22%
Input 16% – Output 52%
Input 30% – Output 65%
Input 44% – Output 72%
Input 58% – Output 76%
Input 72% – Output 80%
Input 86% – Output 83%
Input 100% – Output 85%
“Pulling up” on the “top” of the curve (even 1%) to an output greater than 85% will increase the contrast of the negative while “pulling down” on the output below 85% will reduce the contrast. Using a different printer is one of those variables that will alter the results of the negative.
Making the Master Template
In Photoshop, I start with a “canvas” the size of the media I’m using (13×19 inches). I fill the background with 100% black (which will print white). In four corners I place the registration marks.
Then I place the four step tablets (YMC, Y, M, C) on the top of the negative making sure each one (except the YMC step tablet) is a separate layer that can be removed later.
Then I place an image as a “place holder” for positioning the negatives (also a separate layer to be removed later).
1. Filter↵Sharpen↵Unsharp Mask↵Amount = 100%, Radius = 1, Threshold = 0
2. Image↵Image Size W = 11 inches, H = 16.5 inches, resolution = 300 pixels
3. Image↵Mode↵Multichannel↵Split Channel
6. Image↵Adjustment↵Curves (see above)
7. Starting with the Yellow color separated negative, Drag the negative onto the Master Template creating a new layer
8. Lower the opacity of the new layer in order to see the layer underneath
9. Enlarge the image enough to see the individual pixels
10. Align the corners of the top layer with the corner of the bottom image
11. Change the opacity of the top image back to 100% and delete the bottom image
12. Delete the two layers of “C”, “M” step tablets
13. Keep the “YMC” step tablet
14. Flatten Image and SAVE AS “Y”
15. Repeat with the “C” and “M” negatives