Apply levels adjustments
You can adjust the levels in a photo to control the tonal range and contrast of the photo. You can adjust settings for the black point, shadows, midtones, highlights, and white point. You can also change the look of specific colors (red, green, and blue).
Double-click a photo, then click Edit in the toolbar.
Click Adjust in the toolbar.
In the Adjust pane, click the disclosure triangle next to Levels.
Do any of the following:
Auto correct the levels of a photo: Click the pop-up menu under the Levels icon, then choose Luminance, RGB, or the color you want to change, then click Auto.
Manually adjust levels: Drag the handles of the histogram to make the adjustments you want. Option-drag a handle to move both the top and bottom handle together.
Black point: Adjusts the point at which black areas become completely black (no detail can be seen).
White point: Adjusts the point at which white areas are completely white.
Shadows: Adjusts the lightness or darkness of shadows to reveal the details you want.
Highlights: Adjusts the highlights to the brightness you want.
Midtones: Adjusts the lightness or darkness of midtone areas of the photo.
Change the color cast of a specific color in a photo: Click the pop-up menu under the Levels icon, then choose the color you want to change. Drag the handles to adjust the brightness or darkness of that color in the photo.
Change the histogram to display Luminance values (instead of RGB values): Click the pop-up menu under the Levels icon, then choose Luminance. For example, you might want to change the histogram to luminance values when working with black and white photos. The histogram handles perform the same functions as when the RGB histogram appears.
Note: You can also drag the top handles of the level controls to change the range of adjustment. For example, to adjust only the lightest of highlights, move the top handle further to the right. To fine-tune an adjustment, hold down the Option key and then drag the bottom handle. This moves both bottom and top handles in unison until you get the results you want.