Operation Manual

In addition to adjusting the tonal values of an image for overall shadows, midtones, and
highlights, you can also color correct an image by adjusting the levels of the red, green,
and blue channels independently. Aperture provides a way to adjust levels for each color
channel, allowing you to control the color tonality in an image.
You can also brush the Levels adjustment on selected parts of an image. For more
information, see Making Brushed Adjustments.
About Working with Auto Levels Controls
You use the Auto Levels buttons in the Levels adjustment to analyze the image and adjust
the levels based on that analysis. This is a good way to make a quick correction to an
image before fine-tuning the Levels adjustment with the manual adjustment controls.
There are two ways to perform an Auto Levels adjustment. For more information, see the
following sections:
• Using the Auto Levels Combined Button to Adjust Combined Luminance
• Using the Auto Levels Separate Button to Adjust RGB Channels Individually
You can also adjust the tolerance of the white and black clipping points of the Auto Levels
adjustment. For more information, see Setting the Auto Levels Options.
Using the Auto Levels Combined Button to Adjust Combined Luminance
When you want to quickly adjust the levels of an image based on total luminance
values—red, green, and blue channels combined—you choose Luminance from the
Channel pop-up menu in the Levels adjustment and then click the Auto Levels Combined
button. Red, green, and blue channels are adjusted by the same amount based on the
total luminance. You use the Auto Levels Combined button for luminance when you want
to correct the contrast of the image without modifying the color cast of the image.
Before Auto Levels Combined adjustment After Auto Levels Combined adjustment
To automatically adjust the levels of an image based on total luminance
1 Select an image.
555Chapter 17 Making Image Adjustments