Autofade

A very useful expression to automatically fade in and out based on the layers length.

After Effects CS3
Transitions
Expressions

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If there are no markers, the transition variable is used (where is says "transition=20", this is in frames.) If there are *2* markers, the first marker is used for end point of the fade in, and the second marker is used to define the start of the fade out.

//Autofade: Add to opacity

transition = 20;       // transition time in frames
if (marker.numKeys<2){
tSecs = transition / ( 1 / thisComp.frameDuration); // convert to seconds
linear(time, inPoint, inPoint + tSecs, 0, 100) - linear(time, outPoint - tSecs, outPoint, 0, 100)
}else{
linear(time, inPoint, marker.key(1).time, 0, 100) - linear(time, marker.key(2).time, outPoint, 0, 100)
}

by Harry J. Frank, graymachine.com

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