Optimizing Tonal Range

Most images require some optimization of tonal range and contrast. The appropriate Photoshop tool can be found under Image r Adjustments r Levels. (You will be using this dialog a lot, so learn the keystroke ^^-(T] (Mac: @-(T]) now!) This is one of the most important Photoshop functions; it allows us to alter the contrast in our images by setting the black point (figure 4-20, slider ®), the white point (slider ®), and the overall brightness (slider ©).

The dialog displays the distribution of brightness (luminance) values within the image from black (o, on the left) to white (255, on the right). The height of the curve represents the frequency with which each luminance value occurs.

If an image contains all luminance values between black and white, the histogram curve will stretch from the far left almost all the way to the far right of the diagram.

The histogram curve in our example is empty below 10 and above 220, which means that not all possible luminance values occur. We can improve image contrast by shifting the black point (slider A) to the right until it sits at the edge of the curve (here with a value of approximately 10). We then shift the white point towards the 220 value in a similar way using slider ®. This significantly increases image contrast, as shown by the changes in the preview

Figure 4-20: The "Levels" dialog initially displays the image histogram.

image. The colors also appear richer. All pixels that have luminance values below the black point are now displayed as pure black, and all that are above the white point are displayed as pure white.

This step maximizes the tonal range of your image. You should check to see whether the changes actually make a visual improvement.

Figure 4-21: We start by setting the black point and the white point.

Figure 4-22: Pressing the key while moving the white and black point sliders shows the detail that will be clipped in the preview image.

Figure 4-22: Pressing the key while moving the white and black point sliders shows the detail that will be clipped in the preview image.

Phil Lindsay gave us this tip: Be especially careful when setting the white point, as it is all too easy to cut off fine detail in highlight areas (recognizable by a fine black line towards the right of the histogram curve).

Don't increase contrast too much, as it is much more difficult to reduce contrast later than it is to increase it in small steps. Psychologically, a version of an image with higher contrast will attract your attention more easily than a low-contrast version of the same image, whether it is actually better or not. Printing on matte paper produces lower contrast images than printing on glossy paper. Personal and regional taste also plays a role, and Europeans generally prefer slightly less rich colors than Americans. Smaller prints (up to A3 size) are generally printed on glossy paper worldwide, while most people prefer to use matte paper for large-format prints.

Moving the white or black point sliders while holding down the |M| key produces a real-time preview of just the clipping in the image - all other image data is masked. Generally, it is better not to clip any image data at all (except for the odd pure white or pure black pixel).

The central slider in figure 4-20 (©) adjusts overall brightness in your image and is also known as the Gamma slider. Use this slider with moderation and great care.

You can adjust tonal range simultaneously in all color channels or individually for red, green, and blue via the drop-down Channel menu ©. Individual channel adjustments can lead to color shifts and should therefore be applied carefully.

Figure 4-23 shows our image before and after setting the black and white points.

Figure 4-23 shows our image before and after setting the black and white points.

Figure 4-23: The source image is shown on the left. The image on the right shows the result of shifting the black and white points.

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