Photoshop – Curves, Hue/Saturation, Color Management

Curves, Hue/Saturation, Color Management (credit to Clint Jukkala, Yale University, 2008 for text)

After resizing and saving a file, you need to evaluate the image and make changes in Density, Color and Contrast. Photoshop offers multiple ways to achieve this and a few of them are discussed below.

It is important to remember that what you see on the screen is not always what you will see on a print. Computer monitors often show a wider range of colors and luminance values than a printer can create. Using the Info Palette can help you predict how your image will translate into print.

The Info Palette

When you open Photoshop there is a palette in the upper left corner with two headings, Navigator and Info. If it is not visible choose, Window > Info. When you drag the cursor over any area of your image, the info palette displays the color values for that area in three colors: Red, Green and Blue. As you learned from looking at Histograms, 0 across all three colors means pure black, or in the case of a single color none of that specific color. 255 across all three colors means pure white, or in the case of a single color, an excess of that color.  

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Curves

You can change color balance, density and contrast using Curves. The horizontal axis of the graph represents the original brightness values of the pixels (input levels); the vertical axis represents the new brightness levels (output levels). In the default diagonal line, no pixels have been changed to new values so all pixels have the same input and output levels. (A) To change the midtones, click in the center and drag up to lighten or down to darken. To change the highlights, click in the upper left and drag left to lighten or down to darken. To change the shadows click in the bottom left and drag up to lighten or to the right to darken. Click on the circular icon at the bottom of the Layers palette and scroll up to choose Curves. The Curves window will appear. Move the curve to control density, color and contrast.  You can check and uncheck the Preview box to see what the image looks like before and after the changes.

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To make the image lighter or darker, click in the center of the curve and drag up (lighter) or down (darker)

Lighter                                                 Darker

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To control contrast, place two points on the curve, each 1/4 of the way in from the two corners.

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To increase contrast, drag the upper point up to brighten the highlights and the lower point down to darken the shadows. To decrease contrast, drag the upper point down to darken the highlights and the lower point up to brighten the shadows. You can check and uncheck the Preview box to see what the image looks like before and after the changes.

+ Contrast                                           – Contrast

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Controlling Color with Curves

After determining what color is in excess use the guide below to choose one of the three color channels in Curves. Remember that changing the curve affects brightness. If you make a large color adjustment, you may need to change the density of your overall image.

If the image is too… Do this.
Cyan Add Red
Red Subtract Red
Yellow Add Blue
Blue Subtract Blue
Magenta Add Green
Green Subtract Green

Evaluating Density, Color and Contrast

There are few hard and fast rules about making a good color print. A good memory of what the original scene looked like is a great guide but you can also consider slight changes to enhance the feeling of the picture. Density is probably the easiest thing to evaluate since we all have a good sense of how light or dark a picture should be. Color and Contrast are a but harder. To evaluate color it is helpful to locate an area of the picture that should be neutral. Look at the middle tones of the picture and try to see if there is an excess of any one color, this is called a color cast. Next try to determine whether this cast is warm (magenta, red, yellow) or cool (cyan, blue, green). You can use the Info Palette to help with this. Drag your cursor over an area that should be neutral. If all the numbers are close to the same, it will print neutral. If one color is in excess you get a sense of what your cast is. If you are still having trouble you can go to Image > Adjustments > Variations and Photoshop will display a range of possible color corrections. When evaluating contrast it might be helpful to remember the example of a “normal” histogram. Most images should have shadows and highlights that retain some detail with a range of middle tones in between. Use the Info Palette to see how much detail there is in the shadows and the highlights.

Changing Color Balance

After determining the color cast in your image you will need to use one of Photoshop’s tools to correct it.

Color Balance

Click on the circular icon at the bottom of the Layers palette and scroll up to choose Color Balance. The Color Balance window will appear. Determine whether the color cast of your image is located in the Shadows, Highlights or Midtones. Click the appropriate area and drag the slider to make the change. You can check and uncheck the Preview box to see what the image looks like before and after the changes. After clicking OK, you will see that a new layer has appeared in the Layers palette. This new layer holds the changes you have made without affecting the original background layer. If you click on it again, the Color Balance window will appear showing the changes you have made.

Selective Color

Selective Color allows you to choose a predefined color and adjust slider bars to change that color. This tool is especially useful for controlling whites, medium grays (neutrals) and blacks. Click on the circular icon at the bottom of the Layers palette and scroll up to choose Selective Color. The Selective Color window will appear. In the Colors pop-up window choose the color you want to change. Drag the desired sliders to the left or right to change that color. You can check and uncheck the Preview box to see what the image looks like before and after the changes.

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Hue/Saturation

Hue/Saturation, for our purposes, is used to control the saturation of specific colors in an image. Occasionally digital cameras record certain colors with too much intensity, or saturation. Greens and reds are most susceptible.  Click on the circular icon at the bottom of the Layers palette and scroll up to choose Hue/Saturation. The Hue/Saturation window will appear. In the Edit pop-up window choose the color you want to de-saturate. Drag the Saturation slider to the left to decrease that colors saturation. You can check and uncheck the Preview box to see what the image looks like before and after the changes.

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Brightness/Contrast

Brightness/Contrast allows you to control the density and contrast of an image. Click on the circular icon at the bottom of the Layers palette and scroll up to choose Brightness/Contrast. The Brightness/Contrast window will appear. Drag the sliders to make change the brightness and/or contrast. You can check and uncheck the Preview box to see what the image looks like before and after the changes.

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