Adobe After Effects - Manual (Page 210)

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You can adjust the exposure (in f-stop units) for previews with the Adjust Exposure control, which is located to the right of the Reset Exposure
button
at the bottom of a Composition, Layer, or Footage panel. Each viewer can have its own Adjust Exposure setting.
When the Adjust Exposure control is set to a value other than zero, the Reset Exposure button is orange .
The Adjust Exposure control doesn’t affect final output, only how video appears during previews. To make tonal adjustments to a layer that appear
in final output, use the Exposure effect.
The Adjust Exposure control is useful for finding the black point or white point in an image. For example, drag the value control to the right
(positive values) until the entire image is white except for one area; that area is the darkest area in the image.
To check the quality of a composite, drag the Adjust Exposure control far to the left and far to the right and look for places where the
composited elements differ too much in color or luminance. This technique—sometimes called gamma slamming—is useful for ensuring that a
composite will look good and be convincing in contexts other than the one in which you’re working. For example, a composite that is adequate
in a dark scene may be less convincing when the scene is color-corrected to brighten the scene.
To adjust exposure for a viewer, drag the Adjust Exposure control to the left or right, or click the control and enter a value in the box.
To reset exposure, click the Reset Exposure button. To return to the most recent nonzero setting, click the button again.
Safe zones, grids, guides, and rulers
In the Footage, Layer, and Composition panels, you can display safe zone margins, grids, rulers, and guide lines to align and arrange visual
elements. After Effects preserves guides when importing Photoshop files saved with guides.
Safe-zone margins, grids, and guides are not rendered, either for RAM previews or for final output.
The size of proportional grids increases or decreases when the composition size changes; the size of standard grid squares remains the same
regardless of composition size.
To change settings for safe-zone margins, grids, and guides, choose Edit > Preferences > Grids & Guides (Windows) or After Effects >
Preferences > Grids & Guides (Mac OS).
To show or hide safe zones, grids, guides, or rulers, click the Grid And Guides Options button and choose the appropriate item, or use a
menu command or keyboard shortcut in the View menu.
To toggle between showing and hiding the safe zones, Alt-click (Windows) or Option-click (Mac OS) the Grid And Guide Options button.
To make layer edges and mask edges snap to grids or guides, choose View > Snap To Grid or View > Snap To Guides.
To create a guide line, drag from either ruler.
To delete a guide line, drag it to a ruler using the Selection tool.
To delete all guide lines, choose View > Clear Guides.
To move a guide line, drag it using the Selection tool.
To lock or unlock guides, choose View > Lock Guides. Locking a guide prevents it from being accidentally moved.
To set the zero point (origin) for the rulers, drag the crosshair from the intersection of the two rulers (in the upper-left corner) into the image
area. Reset the zero point by double-clicking the intersection of the rulers. The position of the pointer measured from the new zero point is
shown in the Info panel as X' and Y' coordinates.
Dragging the zero-point crosshair
About title-safe and action-safe zones
Television sets enlarge a video image and allow some portion of its outer edges to be cut off by the edge of the screen. This kind of cropping is
known as overscan. The amount of overscan is not consistent between television sets, so you should keep important parts of a video image within
certain margins, in areas known as safe zones. Safe-zone margins represent the percentage of image dimensions not included in the safe zone.
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