Control Negatives

Establishing Exposure and Contrast

TruNeg uses a Control Negative to find the exposure and contrast parameters for the printer and material being used.

Copy of Control Negative

The purpose of the Control Negative is to find the exposure time and printer density settings that makes a print on the photographic material that has a satisfactory black and a clean white and a range of highlight tones.

Printers and alternative materials fall into two broad categories. Printers with dye based inks which make negatives of low contrast under UV light and printers with pigment inks that have high contrast. Similarly, cyanotypes and dichromate sensitized materials are “contrasty”, that is they need negatives of lower contrast than negatives for salt, Van Dyke Brown etc.

Generally dye based printers cannot, without using sandwiched negatives, make satisfactory prints on salt, Van dyke Brown etc. and may need the ink density or intensity increased to make cyanotype, carbon or gum bichromate prints. Conversely, pigment based inks usually need the inking density reduced 10 to 20% to make a good test print of the Control Negative on gum bichromate, carbon and cyanotypes.

To print the Control Negative if the approximate exposure time is not known make a test print with three exposures a stop apart with the first you think will be too short and the third too long. E.g.  4, 8, 16 minutes.

If lucky, one test strip will have a clean white, some nicely graded highlights and a good black. A good black will mean that some of the dark tones on the left hand side of the print will not be distinguishable from the adjacent clear film strip. If working with gum bichromate the Dmax of the first exposure will not be high, however as long as some dark tones cannot be separated from the adjacent clear film and some can and there is a clean white a some highlight tones then a good control negative print has been achieved.

From the first test print re-adjust the exposure and printer contrast until a good test print is achieved. See example below.

Example of control neg. test print for Van Dyke Brown

There is no perfect test strip, any print that has a clean white, good maximum density with at least one tone distinguishable from black will work as well as any other.

Finding T1, T2 andT3

T1

When the test strip is dry find the first tone that is just white by comparing the tone against the adjacent white paper.  The RGB value of that tone is the “just white” tone and labelled T1. 

T2

With the  monitor set to the same settings as when editing the image,  open the TruNeg step wedge with the background set to mid grey and compare the test print with the step wedge on the monitor to find the print tone with the same relative contrast to white as the T2 tone on the monitor. The RGB value of that tone is called the “Threshold”[1] and labelled T2. 

Do not overthink this, if in doubt choose a slightly lighter tone.

T14

T14 is the first visible tone from black and should have the equivalent contrast to black as T14 has to black on the monitor. Again, do not overthink this, if in doubt choose a slightly darker tone.

The monitor should be set to a gamma of 1.8 and RGB 16 should be clearly separated from RGB 0.  Keep the monitor contrast and brightness relatively low to be closer to the characteristics of reflective prints.

Adjustment for Different Photo Editing Programs

For some reason different programs use different scales in the curve dialogue. Photoshop goes from zero to 255 , GIMP goes from zero to 100 and Affinity zero to 1. TruNeg deals with this by providing a cell in the second row of the Control Panel where the maximum curve dialogue value can be entered to adjust the TruNeg scale to match the editing software.

[1] This is acknowledged as a misnomer as the term threshold is more properly used to describe the first detectable tone in a silver gelatin negative.