Apple Aperture 2 - Manual (Page 431)

Chapter 15 Making Image Adjustments 431
III
You can also use the Gray Levels slider if you need to adjust the brightness values of
the midtones while limiting the brightening effect in the black and white points. The
brighter areas of shadows and darker areas of highlights are affected, but the effect
tapers off as it nears the dark blacks and bright whites.
To adjust the midtone levels in an image, do one of the following:
m Drag the Gray Levels slider until the brightness values in the image’s midtones are correct.
m By default, the gray point is set to 0.50. Select the number in the Gray (G) field, then enter
a value from 0.02 to 0.98 until the brightness values in the image’s midtones are correct.
Before Gray Levels adjustment After Gray Levels adjustment
Grey Levels slider