Choosing Composition Settings
Because compositions describe how layers are arranged in space and time, you must define a composition's spatial attributes, such as its frame size and pixel aspect ratio, as well as its temporal aspects such as its duration and frame rate. Composition settings allow you to specify these characteristics, in addition to the resolution or quality of the display of the Composition window. You may change any of the composition settings at any time.
The frame size determines the viewing area of the Composition window. Though you may position images in the workspace outside of this viewing area, only the elements within the visible frame will be rendered for previews and output (Figure 5, Figure 6 and Figure 7).
The frame size defines the dimensions of the viewable area of the composition. Over time, an element may move from the "off-screen" work area...
...and into the "on-screen" visible frame...
...and vice versa. Only elements within the visible frame appear in the final output.
Often, the frame dimensions of the final output determine the frame size of a composition. But if the composition is to be nested in another composition, the frame size may be larger or smaller than the pixel dimensions of the final output.
The Composition Settings dialog box provides a list of preset frame sizes, or you may enter a custom frame size. The frame size you choose is centered in a work space that is limited to the same maximum dimensions as imported image files. As with imported footage files, chances are you'll run out of available RAM before you exceed the maximum image size (up to 30,000 x 30,000 pixels, depending on the output option).
To set the frame size:
In the Composition Settings dialog box, do one of the following things: Enter the width and height of the frame size in pixels.
Choose a preset frame size from the pull-down menu (Figure 8).
Enter the frame dimensions, or choose a preset size from the pull-down menu.
If you are changing the frame size of an existing composition, choose an anchor point from the Anchor section of the Composition Settings dialog box.
You can enter a custom frame size that uses the same image aspect ratio of a preset frame size. First, choose a preset frame size that uses the image aspect ratio that you want to maintain. Check the Lock Aspect Ratio To checkbox and enter a custom frame size. When you enter a value for one dimension, After Effects automatically fills in the other, maintaining the same aspect ratio.