Create a trail of images

This example expression instructs a layer to be at the same position as the next higher layer in the Timeline panel, but delayed by a specified amount of time (in this case, 0.5 seconds). You can set similar expressions for the other geometric properties.

After Effects CS3
Useful things
Expressions

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1. Start with two solid-color layers that are scaled to approximately 30% of the composition size. (See Create or modify a solid-color layer.)
2. Animate the position of the first layer. (See Creating and modifying motion paths.)
3. Select the second layer, press P to reveal the Position property, and Alt-click (Windows) or Option-click (Mac OS) the stopwatch button to the left of the property name.
4. Enter the following in the expression field:

thisComp.layer(thisLayer, -1).position.valueAtTime(time - .5)

5. Duplicate the last layer five times by selecting it and pressing Ctrl+D (Windows) or Command+D (Mac OS) five times.

All layers follow the same path, and each is delayed 0.5 seconds from the previous.

(Min. Version: After Effects CS3, Category: Useful things, Type: Expressions)