Determine Image Quality with Photoshop CS5 Histograms
Before making image adjustments, look at the image’s histogram, which displays an image’s tonal values, to evaluate whether the image has sufficient detail to produce a high-quality image. In Photoshop CS5, choose Window→Histogram to display the Histogram panel.
The greater the range of values in the histogram, the greater the detail. Poor images without much information can be difficult, if not impossible, to correct. The Histogram panel also displays the overall distribution of shadows, midtones, and highlights to help you determine which tonal corrections are needed.
A full histogram indicates a smooth transition from one shade to another in the image. When a histogram is spread out and has gaps in it, the image is jumping too quickly from one shade to another, producing a posterized effect. Posterization is an effect that reduces tonal values to a limited amount, creating a more defined range of values from one shade to another.
If you have a bad histogram, we recommend that you rescan or reshoot the image. Make sure your scanner is set for the maximum number of colors.
If you have a good histogram to start with, keep it that way by not messing around with multiple tone correction tools. Most professionals use the Curves feature — and that’s it. Curves (choose Image→Adjustments→Curves) adjusts levels (brightness and contrast) and color balance, all in one step.
If a Warning icon appears while you’re making adjustments, double-click anywhere on the histogram to refresh the display.
Don’t panic if your histogram is smashed all the way to the left or right. The bars of the histogram represent tonal values. You can break down the types of images, based on their values, into three key types:
High key: An extremely light-colored image. Information is pushed toward the right in the histogram. Color correction has to be handled a little differently for these images to keep the light appearance to them.A high key image is a light image.
Low key: An extremely dark image. Information is pushed to the left in the histogram. This type of image is difficult to scan on low-end scanners because the dark areas tend to blend together with little definition.A low key image is a dark image.
Mid key: A typical image with a full range of shades. These images are the most common and easiest to work with.A typical image with a full range of values is a mid key image.