Increase or decrease the overall brightness of an image.
This module has two modes of operation:
- Set the exposure, black level and clipping threshold manually
- automatic (RAW images only)
- Use an analysis of the image’s histogram to automatically set the exposure. Darktable automatically selects the exposure compensation that is required to shift the selected percentile to the selected target level (see definitions below). This mode is particularly useful for automatically altering a large number of images to have the same exposure. A typical use case of automatic mode is deflickering of time-lapse photographs.
- Choose the mode of operation (automatic/manual).
- compensate camera exposure (manual mode)
- Automatically remove the camera exposure bias (taken from the image’s Exif data).
- exposure (manual mode)
- Increase (move to the right) or decrease (move to the left) the exposure value (EV).
- The color picker tool on the right sets the exposure such that the average of the selected region matches the target lightness defined in spot exposure mapping options.
- clipping threshold (manual mode)
- Define what percentage of bright values are to be clipped in the calculation of the exposure and black level correction. Use the color picker to sample a portion of the image to be used for this calcuation.
- percentile (automatic mode)
- Define a location in the histogram to use for automatic exposure correction. A percentile of 50% denotes a position in the histogram where 50% of pixel values are above and 50% of pixel values are below that exposure.
- target level (automatic mode)
- Define the target level for automatic exposure correction (EV) relative to the white point of the camera.
- black level correction (manual and automatic modes)
- Adjust the black level point to unclip negative RGB values.
Note: Do not use the black level correction to add more density in blacks as this can clip near-black colors out of gamut by generating negative RGB values. This can cause problems with some modules later in the pixelpipe. Instead, use a tone mapping curve to add density to the blacks. For example, you can use the relative black exposure slider on the scene tab of the filmic rgb module, or establish a deeper toe in the base curve module.
🔗spot exposure mapping
The spot mapping feature is designed to help with batch-editing a series of images in an efficient way. In this scenario, you typically develop a single reference image for the whole batch and then copy&paste the development stack to all of the other images in the batch.
Unfortunately, the light often changes slightly between shots, even within the same series captured in the same conditions. This can be the result of a cloud passing by the sun in natural light, surface reflections having less “shine” from a different angle, or simply due to unavoidable variability in the mechanical diaphragm aperture. Each image will still need some individual fine-tuning if you want a perfectly even look over the whole series, and this can be both time-consuming and frustrating.
Spot exposure mapping allows you to define a target brightness, in terms of exposure, for a particular region of the image (the control sample), which you then match against the same target brightness in other images. The control sample can either be a critical part of your subject that needs to have constant brightness, or a non-moving and consistently-lit surface over your series of images.
The mapping process consists of two steps.
🔗step 1: set the target
There are two ways of setting the target brightness for your control sample:
- if you know or expect an arbitrary lightness for the control sample (for example, a gray card, a color chart, a product or a logo of a specified brightness), you can set its L value directly, in CIE Lab 1976 space,
- if you simply want to match the development of your reference image, set the spot mode to measure, then enable the color picker (to the right of the exposure slider) and draw a rectangle over your control sample. The input column will then be updated with the lightness value of the control sample before the exposure correction, and the target column will show the resulting lightness of the control sample after the current exposure setting is applied.
If you reset the lightness value, the default value is 50% (middle-gray) – this can be useful to quickly set the average exposure of any image.
Note that the target value is not reset when you reset the module itself, but is stored indefinitely in darktable’s configuration and will be available on next launch as well as for the next image you develop.
🔗step 2 : match the target
When you open a new image, the spot mode is automatically reset to correction. Using the color picker attached to the exposure slider, you can then directly reselect your control sample in the new image. The proper exposure setting required for the control sample to match the memorized target lightness will be automatically computed, and the setting will be updated in the same operation.
This operation can be repeated as many times as you have images in your series with no further work.
Note 1: Trying to match lightness of moving parts of a subject across frames can prove tricky because they can have legitimate changes in their illumination as their orientation changes with respect to the main light source. For example, a part of the face can be fully lit in some frames and partially shaded in others. An inconsistently-lit control sample will generally not provide a robust reference for lightness matching across a series and may result in more work than manually matching it with visual feedback.
Note 2: The exposure module works (by default) in the scene-referred, linear, camera RGB part of the pixel pipeline, before the input color profile is applied. However, the conversion from camera RGB to CIE Lab 1976 space relies on the input color profile. All of the lightness L metrics given in the spot mapping settings will use the input profile defined later in the input color profile module to perform the conversion accurately, but the conversion itself assumes a linear (RAW) signal and will not work for JPEG and PNG images (which are non-linearly encoded before the input color profile module). If you want to use this feature on non-RAW images, you will need to move the exposure module to after input color profile or to use the module order preset v3.0 for JPEG/non-RAW input.
Note 3: Perfectly matching your control sample against the target lightness may still not yield a similar perceptual result, even if the numbers are exactly the same. For example, if your subject sits in front of a background made of some bright parts and some dark parts, the ratio of bright areas / dark areas will affect the perception of contrast and brightness. If this ratio changes across your series, the subject brightness will not appear constant even though the lightness value is exactly constant. For more details, see the checker shadow illusion and the Chubb illusion.