Making Negatives with a Densitometer
Before commencing, ensure that the monitor and printer are set up as detailed in the Settings page
Background
The Gradient finds the RGB and density for an exposure that produces “Just White” and “Just Black”(Dmax). These are entered into the program, which uses the inverse proportion between the positive and negative logarithms to calculate the target density of 10 in-between tones. The RGB values that produce these densities are found with the densitometer and recorded in the profiler. The program then plots the DLogRGB curve that reproduces the print target densities.
While it is probably wise to run a test strip, if you are confident the process is reliable and the image has been edited at a gamma of 1.8, you can go straight to print, as illustrated by the Van Dyke Brown and Classic Cyanotype prints below, which have been printed from uncorrected profiled negatives.
Instructions
The TruNeg Gradient
The Gradient is actually a 128-step logarithmic stepwedge between RGB 6 and 248, making the density difference between each step equal if correctly printed. It is labelled in “8-bit” notation to three significant figures, and each second step is numbered.
The stepwedge is 14.6 (37.1 cm) inches long to fit the long side of a quarter sheet of watercolour paper. It is 2.14 (5.43 cm) inches wide. Adjust the size if necessary, but keep it as large as possible.
The Gradient can be downloaded from the Downloads page.
If you do not know the exposure, it is easier to find the RGB “Just White” and “Just Black” points using the 3 ×9 inch Calibration Stepwedge. The stepwedge has three identical stepwedges, making it easy to do, say, a 3-, 6-, or 12-minute test to find a good exposure. If necessary, find an intermediate tone that satisfies the criteria, noting that there is no perfect exposure, just as long as there is a good Dmax and a clean white.
The Calibration Stepwedge can be downloaded from the Downloads page.
Once the white and Dmax are known, there is no need to print the full gradient.
The “Just White” point is the last step before any tone is visible. Rather than tracking down the very last step, run your finger down the stepwedge and find the first obvious white point. If in doubt, choose the slightly darker step, but be aware that this will be the lightest tone in the print.
The “Just Black” is the last tone that can be separated from the adjacent Dmax strip. Similarly, run your finger down the Dmax strip and stop at the first obvious “black” or Dmax point where the gradient merges into the strip. If in doubt, choose the slightly darker step. This establishes the darkest shadow tone without affecting the maximum density and should be just discernible from Dmax.
The Profiler
Open the Profiler in the app window and input the parameters for the “Just White” and “Just Black” RGB and density values. Press Next to continue.
The program calculates the ten in betwen target densities and displays them in bold text in the Profiler window. The densitometer is then used to find the densities, and the matching RGB values are entered in the adjacent fields.
The markings on the Gradient upper and lower scales are 5mm apart, which fits a densitometer with a 10mm outer diameter. The overlap with the tones on either side cancels out, and the readings have been found to be repeatable and reliable.
A small piece of card with a notch cut out helps to line up the densitometer.
When completed, save the file identifying the process and exposure time. The program adds the just white and black values, the overall gamma and appends “Profiled” to clearly identify the Preset. The Preset is saved as a Python Pickle file. At the same time, the recorded densities are saved in a separate file.
E.G. Kalli(3).pkl(19,4_121,9_0,665)Profiled
Kall(3)_density.pkl
The Preset curve should be viewed before conversion to check for input errors and smooth out any slight wobbles.
If there is an obvious error, such as a large spike in the curve, open the Edit_RGB menu, select the Preset (not the Density file), find the error and overwrite the entry and save the corrected Preset.
Note that if the curve has been tweaked, the tweak will need to be redone
Convert
Open the Convert menu, select the image and preset, then press OK.
Once the negative is completed, which can take 30 seconds or more, it is automatically exported to the Preset folder, ready for printing.
Print the Print
If you have confidence in the consistency of your process and the image has been edited at a gamma of 1.8, you can go straight to print. Otherwise, make a test strip and, if thought necessary, edit the image.
.1×12.5 inch (18×32 cm) Kallitype print made from a profiled negative without further correction. Photograph & print by John Riches