Finding the First White and the Last Black

Knowing the exact location of the highest (lightest) and lowest (darkest) values in your image can be very valuable information. It is useful in almost every facet of color workflow management from original capture to final output. For example, it can help you understand how your image will look when:

you upload it to your web site... you make an ink jet print... your color lab prints it... your client prints it...

When you open an RGB image, Photoshop assigns a number value between 0 and 255 to the Red, Green and Blue color values in each pixel of the image. 0,0,0 = pure black and 255,255,255 = pure white.

The Threshold command converts grayscale or color images to high-contrast, black-and-white images. The Threshold command is useful for determining the lightest and darkest areas of an image.

To use the Threshold command to identify representative highlights and shadows: 1. Open the Threshold dialog box. On the menu bar select Image > Adjustments > Threshold

2. Select Preview.

3. To identify a representative highlight, drag the slider to the far right until the image becomes pure black. Drag the slider slowly toward the center until some solid white areas appear in the image. Tip: Use the Up an Down arrow keys to move the slider one threshold level at a time.

Place a color sampler, a crosshair "target", on the first white area that appears. Note: When you select Threshold from the Menu Bar Photoshop defaults to the Eyedropper Tool.

To set a color sampler "target" use your mouse to place the eyedropper over the selected area. Hold the Shift key down. This will change the Eyedropper Tool into the Color Sampler Tool. Click and a color sample target appears on your image and adds another readout to the bottom of the Info palette.

4. To identify the darkest shadow, drag the slider to the far left until the image becomes pure white. Drag the slider slowly toward the center until some solid black areas appear in the image, and place a color sampler "target" on one of the areas. Click Cancel to return the image to RGB and close the Threshold dialog box without applying changes to the image.

You may set a maximum of 4 "targets" to sample different areas of your image.

To remove a sample "target", simply drag it off the screen, or hold down the Option key (Mac) / Alt key (Win) and click on the sample, or click the Clear button on the tool command bar when the Color Sampler Tool is selected.

Note: The color sampler "targets" are vector images and will not print. You can use the Info palette readouts from the two target samples to determine your deepest shadow and brightest hightlight.

While this technique may look complicated, it really isn't. In fact after you've used this technique on a few images you'll be amazed at how quickly it can be accomplised, literally as fast as you can click and drag the threshold slider.

Once we know where the first white and last black are located we can quickly and easily color correct the image. See the lesson 3 Click Color Correction, no trial and error, no guess work, and as a result we will end up saving a great deal of time.

A bonus is that we'll be able to use the information to fine tune the shadow and highlight detail of our print outputs... but that's another lesson.