Correcting the tonal range usually requires two steps. In the first step, you reset the white and black points to make the range of values between the lightest part of the image and the darkest part of the image as large as possible. In the second step, you adjust the gray point to control how much of the total tonal range falls above and how much below the middle value.

Setting the white point and the black point is often relatively straightforward, since the shot will include an area that should obviously be very light and another area that should be very dark. You simply look for what should be the lightest area of the image and adjust controls until it becomes as light as possible, and then do the same for the area that needs to be black. You can dramatically improve the quality of shots taken using insufficient or excessive light just by making white and black point adjustments.

In some cases, however, the shot should have less range of brightness (for example, when the whole scene was originally in shadow or was shot at sunset). In such cases, you need to be careful to expand the range as much as possible without making parts of the image unrealistically light or dark.

Avoid clipping any significant part of the image. You want the range between your lightest value and your darkest value to be as large as possible, but in most circumstances you don't want to lose detail by reducing all your very light values to white or all your very dark values to black.

Do not use intense reflected spots of light (known as speculars) to judge where your white point should be. If you do so, you define white by an artificial standard that probably occurs in only a tiny fraction of the image. A true white object such as an item of clothing might appear dull and gray by this standard.

You have a number of choices for controls to use to make white point and black point adjustments, including the Gain and Offset sliders in the Hue Offsets subdividing tab of the HSL group and the Master curve in the Curves tab. All of these controls can be set automatically using the Auto Black, Auto Contrast, and Auto White buttons. If necessary, you can begin with an automatic correction and then fine-tune the adjustment manually.

Once you have established the range from the brightest part of the image to the darkest part, you can adjust the gray point if necessary. When you make a gray point adjustment, you define how much of the overall tonal range is between black and midgray, and how much is between midgray and white.

The most obvious effect of a gray point adjustment is that it either lightens or darkens the overall look of the image. Large adjustments of the gray point toward either the black point or the white point are almost always undesirable because they leave the whole image much too dark or too light.

Smaller, well-chosen gray point adjustments, however, can be useful for fine-tuning the overall brightness of the image. Also, since a gray point adjustment expands the tonal range on one side of the midpoint and contracts it on the other, it can be useful for improving contrast and detail overall. For example, some images look better if more contrast is available in the range between gray and white, even though the price paid for that extra contrast is a reduction in contrast between gray and black.

The main control for making gray point adjustments is the Gamma slider in the HSL group.