Many of the controls use a scale that ranges from 1.00 to +1.00, with the image's original value sitting in the middle at zero. The Exposure control, for example, darkens the image significantly at 1.00, but doesn't turn it black. When you hold Option, that range changes to between 2.00 and +2.00, letting you darken the photo even more (or go the other direction and brighten a dark photo).
This feature is also useful when you're looking for more pop or an extreme treatment for a drab photo. In the images below, I've taken a photo of dried leaves and pushed the contrast to its initial maximum value of 1.00. With the Option key held, however, I can push that higher and get a more dramatic effect.
Multiple levels of Levels
The Histogram is good for identifying the color and tonal values in a photo, but it's there just for reference. The Levels adjustment, however, lets you manipulate those values in some sophisticated ways. Choose Levels from the Add menu to view it.
Looking at a histogram, the left side represents dark values (with black at the far left) and the right side represents light values (with white at the far right). The colored areas within indicate the distribution of red, green, and blue (RGB) pixels within the scene. If you want to isolate and edit any of those channels, click the options menu that appears when you move your mouse cursor over the controls. You can also choose Luminance to view only the brightness values.
The teardrop-shaped handles at the bottom of the Levels histogram control (from left to right) the black point, midtones, and the white point. To brighten an image, for example, drag the white point to the left--the values to the right of the white point get pushed to their full luminance, increasing the overall brightness of the photo (see below).
Drag the white point to make the photo brighter.
Similarly, dragging the black point makes the image darker, and dragging the midtones lightens or darkens the values that fall between the light and dark extremes; the smaller handles that flank the midtones control affect shadows (left) and highlights (right). Sometimes, for instance, it may be better to adjust the midtones to brighten an image to avoid clipping, which is when pixels are pushed all the way to pure white or black.
There's more to the Levels adjustment, however. The handles at the top of the Levels adjustment allow you to fine-tune the edits made with the bottom handles. In the image below, I've reduced the white point setting (by moving it to the right) so the lightest areas aren't blown out, and then also dragged the top-center control to brighten the midtones. The result is a brighter, more saturated sky, but also detail in light areas such as the pyramid-topped building in the center of the skyline. You can also hold Option and drag a top handle to also move its connected bottom handle in unison, maintaining the relationship between the two.
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