Levels adjustment is used to adjust tonality and color balance of an image in shadows, mid tones, and highlights intensity levels. Level is mostly used to adjust contrast and color correction in an image. This is one of the most widely used tool in photography.
The histogram slider in the application can be used to map image’s input levels to output levels. Initially sliders are set to level 0 in case of black pixels and level 255 in case of white pixels.
The black histogram slider can be moved to map image’s pixel value to level 0 or white histogram slider can be moved to map image’s pixel to level 255. The remaining levels of the image gets redistributed between level 0 and level 255. This redistribution of levels lead to increased tonal range of an image and thus lead to increased overall contrast of the image.
These histogram sliders can be used to get desired tonal effect in shadows, mid tones and hight light intensity level of an image and 4 color channels, namely RGB (Gray color), Red, Green and Blue channel of an image. In the above image, mid slider is used to adjust tonality in gray color channel and mid tone intensity region of an image.
Similarly, left histogram slider is adjusted in above image to get desired tonality effect in red color channel and shadows intensity region of an image.
In the same line, right histogram slider can be used to adjust tonality in green color channel and highlights intensity region of an image.
In a nutshell, level is a perfect tool for making large global adjustments in an image. Level is different from Curve in that level adjustment is bit subtle that Curve adjustment.