Apple Aperture 2 - Manual (Page 441)

Chapter 15 Making Image Adjustments 441
III
Adjusting the High Tonal Width of the Highlights & Shadows Controls
You use the High Tonal Width parameter controls to set the range of highlights that
Aperture adjusts with the Highlights controls. Increasing the High Tonal Width
parameter value widens the range of highlight tonal values that are adjusted toward
the midtones. If you want to limit the highlights adjustment to a range of highlight
values that are closer to white, you decrease the High Tonal Width parameter value.
Before High Tonal Width adjustment After High Tonal Width adjustment
(increased high tonal width)