Apple Aperture 2 - Manual (Page 430)

430 Part III Performing Image Adjustments
To adjust levels in an image based on luminance:
1 Select an image.
2 Show the Adjustments inspector or the Adjustments pane of the Inspector HUD.
3 By default, the Levels histogram is off. Turn it on by selecting the Levels checkbox.
The Levels histogram appears.
4 Choose Luminance from the Channel pop-up menu.
5 Do one of the following:
 Drag the Black Levels and White Levels sliders to where they touch the outside of the
histogram graph, constraining the image to its new black and white points.
 Select the numbers in the Black (B) and White (W) fields, then enter a value from 0 to
1.00. By default, the black point is set to 0.00, and the white point is set to 1.00.
The tonal values for shadows and highlights in the image are updated, resulting in
more defined blacks and highlight values as well as increased overall contrast.
White Levels slider Black Levels slider
Drag the Black Levels
and White Levels sliders
to where they touch
the outside of the
histogram graph.