Negatives are images with reversed lightness and hues, such as those produced by film cameras. RawTherapee 5.7 introduced the Film Negative tool to make developing raw photos of negatives simple.
In the negative image, each channel value is proportional to a power of the reciprocal of the corresponding channel in the original exposure (see Photographic Film - Basics for more info). Each channel value is raised to a different exponent, depending on the film type, age and possibly other factors, such as shooting conditions. These exponents can be specified in order to better adapt the correction process to the characteristics of each film. To simplify manual tweaking, these three R,G,B exponents are specified as one "reference" exponent (which gets applied to the Green channel), and two ratios of the Red and Blue exponents to the reference. Default values should produce reasonably good results out-of-the-box with some common Kodak film types like the ColorPlus 200 or Gold 200.
- Open a raw photo of a negative (non-raw photos are not supported).
- In the Raw tab, activate the Film Negative tool.
- White-balance the photo. Picking the white balance off a spot which should be neutral in hue, if the image has one, is easiest.
- Optionally, you can try to automatically set more accurate red and blue ratio values. To do so, click the "Pick white and black spots" button, then click the whitest and blackest spots in the actual image - these can be spots in the actual image captured on film, or exposed and unexposed areas of the film frame itself. Experiment to find which works best for you. This needs to be done only once per film role.
That's it as far as correcting the negative goes. Resume adjusting the photo just as if it was a normal "positive" raw photo.
Reference exponent (contrast)
Exponent applied to the Green channel, and proportional to the other exponents applied to the Red and Blue channels. Changing this value alters the general image contrast without altering its colors. The default value is good for an average-contrast negative image. In case of a very faint, or incorrectly exposed negative, this value may have to be increased. In case of a very high contrast negative, the converted, positive image could reach clipping, so this value will need to be decreased.
Ratio of the Red channel exponent to the reference exponent. This coefficient indicates how "bent" the Red channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.
Ratio of the Blue channel exponent to the reference exponent. This coefficient indicates how "bent" the Blue channel transfer curve is, with respect to the Green transfer curve. Changing this value alters the color characteristics of the correction, while keeping the general image contrast.