Apple Aperture 3.5 - Manual (Page 289)

Chapter 7 Make image adjustments 289
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To set the gray point in an image: Select the Gray Point eyedropper tool to activate the Loupe,
position the eyedropper over a midrange color that is as close as possible to medium gray,
and click.
Click the Gray Point
eyedropper button
to activate the Loupe.
67% resize factor