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.
The tool supports raw photos of a Bayer or X-Trans sensor. Other raw types and non-raw formats are not supported.
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.
- In the Raw tab, activate the Film Negative tool.
- Optionally, you can try to automatically set more accurate red and blue ratio values. To do so, click the "Pick neutral spots" button, then click on a neutral light and dark spot in the photographed scene.
- These spots should have had no color tint in the original scene, they should have a neutral hue. Keep in mind that these spots might not appear neutral in your photographed film negative until you use the White Balance tool.
- The spots should differ in brightness, and should not be clipped.
- Picking the spots needs to be done only once per film roll, then the processing profile can be copied and pasted onto the other photographs from the same roll. This allows you to use any image in the whole roll to pick the neutral spots.
- White-balance the photo. Picking the white balance off a spot which should be neutral in hue, if the image has one, is easiest.
- Picking the neutral spots changes the raw data's values in the pipeline before the white balance tool takes effect; therefore it is recommended to white-balance the photo after having picked the spots. If you white-balanced it before picking the spots that is fine, but you might want to re-white-balance it again afterwards.
That's it as far as correcting the negative goes. Resume adjusting the photo just as if it was a normal "positive" raw photo.
3.1 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.
3.2 Red ratio
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.
3.3 Blue ratio
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.