When you capture an image with a digital camera or scanner, chances are that the tonal range in your image is lacking in highlights or shadows. An optimized tonal range is one in which the darks are completely black, the highlights are white, and the other tones are well distributed. In Photoshop, you adjust the tonal range of an image with the Levels option. Not only does adjusting tone ensure good contrast and detail, it can correct any unwanted color casts. Even if the image looks pretty good to start with, optimizing the tone can improve things even further.
Open the Info Palette
Open the image for which you want to adjust the tonality. Choose Window, Info to open the Info palette.
Choose Image, Adjustments, Levels to open the Levels dialog box.
Select the Black Point
Click the black Eyedropper icon in the lower-right corner of the Levels dialog box. Move the cursor over the image; the pointer changes to an eyedropper as it enters the image. Using the readings from the Info palette, find the darkest area of the image and click to set the black point.
Select the White Point
Click the white Eyedropper icon in the lower-right corner of the Levels dialog box. Move the cursor over the image; the pointer changes to an eyedropper as it enters the image. This time, find the lightest area of the image and click to set the white point.
Click OK to apply the effect. By setting the black and white points for the image, you are telling Photoshop what the darkest shadows and brightest highlights in the image are. Doing so establishes the tonal range of the scanned image. You can experiment with setting a black point that is not the darkest area of the image or a white point that is not the brightest area of the image to see what effect this "skewed" tonal range has.
Reselecting the Darkest or Lightest Area
Take your time when looking for the lightest and darkest points in the image. Selecting a pixel that is not close enough to the light or dark point can result in blowing out the highlights or shadows. If you make a mistake, press the Option key (Mac users) or the Alt key (Windows users) and click the Reset button (the Cancel button changes to the Reset button) to make another attempt.