When we talk about the tonal range of an image, what we’re referring to is the amount of contrast or detail in the image. Every single image you work with, whether it’s from your digital camera or an exiting print that you scan in, will have a range of pixels from the lightest pixels, which are white, to the darkest pixels which are black.
You can improve an image very quickly by adjusting it’s tonal range and one of the tools we use in Photoshop to do this is the Levels. The Levels control allows you to adjust highlights, midtones and shadows of an image.
1. Open the image that you want to improve or adjust the tonal range of.
For this example I’m using a picture of a partial face of a smoker which looks a little faded. The blacks in the image are not truly black and the midtones are also a bit “wishy-washy”.
2. Choose Image > Adjustments > Levels to open the Levels dialog box.
When the Levels dialog box opens make sure that the Preview check box is selected so that you can see how your image is affected when you make changes.
The ideal histogram is one which extends across the full width of the graph with reasonably uniform peaks and valleys in the middle portion. If you’re want to get some special effects the Levels tool is great for playing with, but for this exercise, you’re just trying to improve the contrast overall.
The leftmost or black triangle (A) below the histogram corresponds to the shadows, the middle or grey triangle (B) corresponds to the midtones, or gamma, and the rightmost or white triangle (C) corresponds to the highlights in the image.
3. In the Levels dialog box, drag the left triangle to the right until just as far as the histogram indicates that the darkest pixels appear.
You will notice that when you drag the slider, the image starts to change. The Input Levels above the graph change too. Take a look at the Histogram palette, the left portion of the graph now stretches to the edge of the frame. This indicates that the darkest shadow values have shifted closer to black.
4. Drag the middle triangle a short distance to the left side to lighten the midtones.
Watch the changes in the image window and in the Histogram palette graph to determine how far to drag the middle triangle.
5. Drag the right triangle to the left to the point where the histogram indicates that the lightest colors begin. In this particular image there is a peak on the right hand side of the graph. This is caused by the light area behind the man’s shoulder and ear in this image.
6. When you’ve played around with the sliders and the image looks good to you click OK to apply the changes. The picture should look much improved.
To give you another example of how effective the levels can be, I took this old photograph which looked very faded and used the Levels to add contrast. Obviously the image could do with some more retouching to lose the yellowness and some other problems but you can see the vast improvement in the image on the right just from taking the same steps as above.