The Levels tool in Capture One, when in the combined RGB mode, is used to map the tonal values in an image. Typically this is used to expand the tonal range shown in the Levels’ histogram. It is especially useful with low-contrast images such as those taken in flat lighting or heavily-diffused light (for instance, hazy or foggy daylight that contains compressed tonal values with few if any clipped areas). However, it will depend on your creative intent.
Selecting an Auto Levels in the combined RGB Channel mode, Capture One will attempt to set the shadow and highlight sliders to either side of the histogram, thereby increasing contrast and altering the brightness. Some small number of pixels are allowed to go to pure black or white, such as specular highlights, without substantially affecting the overall contrast.
Adjusting the shadow and highlight target sliders maps the pixel values at those points to the selected output values. If the output sliders or output values are left to the defaults, the points will be mapped to 0 and 255. In the example above, the Highlight input is mapped to 220 and the Highlight output is mapped to 244; the value range from 0-220 is remapped to 0-244 with no bias (it is a linear shift). Tones above 220 are set to remapped as well, in a linear fashion.
The Levels tool in Capture One differentiates from other applications in that the Input and Output tones are not necessarily pure black (0) and pure white (255) points. They can be; leaving the Output values to 0 and 255, while remapping the Input values as to 10 and 245 will clip the histogram on both ends. This is in contrast to the Curves tool, where the black and white points are absolute endpoints for the histogram values:
The middle-slider is provided to adjust brightness or gamma of the mid-tones. Moving it to the right compresses the shadows and lightens the highlights, while moving it to the left compresses the highlights and lightens the shadows. While the remaining RGB values are re-distributed to avoid color shifts, the combined RGB mode will not correct any inherent color in-balance.
The Levels tool can also be used to adjust the color balance of the image. If a color shift is present, you can adjust the Levels using the individual Red, Green, and Blue Channel mode instead. Adjusting each histogram end-point manually is possible, however, it is quicker to switch a channel mode and apply Auto Levels on the individual channels. This often provides a very realistic-looking result. A shortcut to the application preferences is provided from the Levels tool’s action menu.