Solving the Digital Negative Problem

Advanced negatives: According to John Riches, developer of TruNeg, creating an accurate digital negative is more complex than simply inverting an image in editing software. Standard photo programs invert pixel values arithmetically, but this does not replicate how a true photographic negative responds to light. As a result, inverted negatives often produce weak shadows, uneven midtones, and excessive highlight contrast. John shares his thoughts.

Writer and photography / John Riches

The Authentic Digital Negative and Why the Inverted Image Fails

To explain why the editing program’s negative doesn’t work, or how to turn the positive into an authentic photographic negative, we need to understand what makes a true digital photographic negative.

By convention, the standard photographic negative, whether analogue or digital, captures a brightness range of eight stops, meaning the brightness doubles with each stop, from the darkest to the brightest recorded tones. 

The silver gelatin negative, for the most part, increases in density by regular amounts for each stop increase in the subject brightness.

In the 16-bit positive digital image, RGB 17 is considered the first distinguishable tone from black, and pixel values increase from 16 to 255 with a factor of 1.41 for each stop change in exposure. (Table 1):

If the positive image increases by a factor of 1.41 at each stop, then for a negative to exactly mirror the positive, it must decrease from 255 to 16 by that same factor at each stop. (Table 2):

However, in practice, the negative range that prints the eight-stop subject from black to white varies across printers and processes, and the factor will change accordingly.

Definition of the True Digital Negative

A true digital negative is a negative in which the pixel values decrease by a constant factor for each stop increase in exposure, the factor being a function of the negative range.

Why the Inverted Negative Fails

The traditional inverted negative fails because the photo editing programs invert the image by subtracting the positive pixel value from 255. Table 3:

This results in a negative with very small, uneven factors in the shadows and midtone stops, along with large jumps in the highlights. It explains the faint, low-contrast shadows and midtones, as well as the excessive contrast in the highlights of inverted negatives.

Plotting the inverted negative in blue (Table 3) against the true negative in yellow (Table 2) shows the extent of the problem.

The Authentic Photographic Digital Negative

The figure above illustrates the constantly changing proportion between the positive exposure stops and negative pixel values. For example, in the correct negative, the range between stops one and two is 52 RGB, but the range between six and seven is 9 RGB, making it difficult to calculate the intermediate negative values.

But if the logarithms of the stop inputs are plotted against the logarithms of the negative, the graph becomes a straight line, indicating an inverse proportion between the positive and negative logarithms.
Therefore, if the parameters of the negative range are known, the negative value B of any positive pixel A can be calculated using the laws of inverse proportion.

However, performing these calculations requires a dedicated program to handle the inputs and logarithms, and to work alongside the photo editing program for initial image editing, file conversion, and printing, which has all led to the development of the TruNeg application.

How it works

Here is a video of the process, or see the images below.

Here is an example for making a salt print:

Save the preset for future prints, convert and print the negative, and print the print.

 John Riches is an Australian landscape photographer whose work was collected in 1979–80 by the National Gallery of Victoria and the Bibliothèque Nationale de France. He served as a judge for the Australian Professional Photography Awards from 1984 to 1989 and was a senior lecturer at the Australian College of Photography from 1987 to 1993. From 2000 to 2020, he worked as an architectural drafter designing houses around Apollo Bay. More recently, while making digital negatives for carbon printing, he began researching how photographic images are recorded and how digital negatives function, leading to the TruNeg concept.

John riches app: TruNeg: The app has three main features:

• It produces authentic photographic negatives that capture the full range of shadow and highlight tones from just two inputs obtained from one test strip, eliminating the need for densitometers, scanners, linearisation procedures, etc
• Provides profiles for Argyrotype, Carbon (A. Dichromate)/Gum Bichromate, Classic Cyanotype, Salt, and Van Dyke Brown, and I hope to add Kallitype in the near future.
• Creates an interactive plot for users to adjust highlights, shadows, etc., to fine-tune the print to their personal preferences.

The app is modestly priced at US$35.00, and previous purchasers of the spreadsheet program are encouraged to contact TruNeg to receive a code to download the new app at no further cost.