Learning how to adjust the tonal values of an image is an essential part of post processing. Deciding the levels of brighter and darker parts of the image can help leading the viewer’s eyes towards the main subject and you can set the overall contrast level. Some images may benefit from high contrast with deep blacks and large areas of dark shadows combined with white highlights and a bright main subject, while others require soft tones and low contrast.
Darktable automatically applies a base curve by default. This is an attempt to give a starting point close to what a jpg image out of the camera would be like. The base curve is a little bit different for each camera, but you are free to apply any curve as you see fit. You can also change it as you wish or decide not to use it at all. The base curve brightens the midtones and compresses the detail in the highlights a bit, so this can be valuable to know if you want to preserve as much detail in your highlights as possible.
Adjusting tonal values
Start off with a general exposure adjustment if needed, to center the histogram and make room for further adjustments.
Tone curve is a very useful tool that I often use several instances of for different parts of the image. Set the black- and white-points and general contrast with the first instance, then use masks to adjust the contrast and brightness level of parts of the image separately. Black and white point can also be set with the Levels module if you prefer.
Darktable has an abundance of tools for adjusting tones, many more than I have mentioned here. Some, like Exposure, Tone Curve and Levels are well known from most image processing software. Others, like the Zone System and Fill Light, are more obscure, but they definitely deserve their place in this magnificent piece of software.
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