Levels can change the color range or "exposure" of an image, including gamma or light adjustments. It can do this with individual color channels (red, green, and blue).

Levels duplicates the effect of increasing or decreasing the "exposure" of an image or how long the camera shutter is open. In the bygone days of film, the amount of light in the image was determined by shutter speed. The longer the shutter was open, the more light information was included in the final image. Too much light and you would get nothing but a bright white image.

Adjustments with Levels are done by adjusting the input and output of the white, black, and gray points. The white point is the brightest color that appears in the image. The black point is the darkest color. The gray point represents the average color of the entire image. With Levels, you can adjust all these points:

Levels has two parts: Input and Output. Input (on the left-hand side) represents your original image while Output (on the right-hand side) represents the image after your adjustment.

Notice the black and white boxes on the left-hand side? These are color swatches, each with a numeric representation. These can be adjusted using numeric inputs, the slider, or by double-clicking on the color swatches.

The small triangles are sliders that will allow you to change the light and the dark points of the image:

If you use the sliders on the left, they will fall or rise depending on where you are on that gradient. This represents the amount of lightness or darkness that will be going through the image.

Now, if you double-click on any one of the color swatches, you will see a color palette appear as shown in the following screenshot. This will allow you to change the color of the light let in either at the in point or the outpoint of an image. By doing this, you are telling Levels that any color in these boxes needs to be brighter or darker than the selected color:

All adjustments are done on all channels (red, green, and blue). If you uncheck any of the RGB checkboxes, you can adjust just one color at a time.

The best advice given about Levels is just to play with it. You will find that the smallest of adjustments can make all the difference.