The Levels command enables you to adjust the brightness of pixels, hence relocating the brightness values of an image. This is done by manipulating the controls which represent the shadows, mid-tones and highlights in the image. In effect the Levels sliders are used to expand or compress the current tonal range. The Histogram represents the tonal range of the image and measures how many pixels are colored with each of the tones between 0 (black) and 255 (white) (see Histogram for more explanation).
Access the Levels command in the Adjust sub-menu under Image.
In the "Levels" dialog you can set the following:
Set the channel in Channels, choosing from Luminosity (red, green and blue combined), Red, Green, or Blue. Note that the histogram changes to reflect your setting.
The Input Level boxes display values for shadows and highlights, thereby determining the range of pixels that will be turned black or white. You set these values by dragging the sliders just below the boxes.
For example, if you use the left-hand slider, which adjusts the shadow value in the left-hand box, to set a value of 60, then all pixels with a brightness value of 60 or less will become black. Likewise, using the right-hand slider to set a highlight value of 200 in the right-hand box causes all pixels with a brightness value of 200 and higher to become white. The range of values is from 0 to 255.
The middle box and slider control the brightness of the mid-tones. The default value is 1 and the range through which you can move the slider is 0.09 at the right side to 10 at the left side. Moving towards the lower value darkens and towards the higher value lightens all the tones of the image that aren’t either black or white (these parts of the image stay the same).
The Output Level slider and boxes are used to compress the tonal range. This is done by restricting the darkest and lightest tones, hence causing a decrease in the number of possible tones in between the new darkest and lightest tones. The effect of this is to decrease the contrast in the image.
Set the shadow value by moving the left-hand slider to the right: a value of 50 reduces the darkest tone to 80.4% of black i.e. 0 is black and 255 white, so a setting of 50 represents a reduction of 19.6%. Set the highlight value by moving the right-hand slider to the left: a value of 200 increases the darkness of the lightest tone to 21.6% of black (alternatively, 78.4% of white). The tonal range is adjusted to fit in between the new limits of 50 and 200, there being nothing outside of this range in the 0 to 255 scale.
Any changes you make are immediately applied to the image on screen. Click on Cancel to cancel these changes and exit the dialog; otherwise click on OK.
Note that the Levels command isn’t as accurate or powerful as the Curves command (see Curves).
See: