In After Effects, as you apply new layers to your composition, by default they will inherit the composition time setting. The heart of animation is timing, motion, and storytelling with your action. Use the timeline settings to tell your story.
When you create a new composition, you set duration, a length for your movie. As you add new elements to your composition they will by default be set for the entire duration but this is rarely what you want. This is an animation, a story and your elements will come in and out of view as they play their roles.
In the lower left corner of your timeline panel you see three icons for setting timeline options and displays. The one on the far right, with a 'brackets' icon '{}' hides and displays the time settings for each layer. These settings are 'in', 'out', 'duration', and 'stretch'. The time values default is '0:00:00:00' with the last setting being frames. A setting of '1:20:15' would be one minute, twenty seconds, and fifteen frames. The number of frames per second depends on your media and format but for DV and NTSC (National Television System Committee), very popular formats, there are 29.7 frames per second.
If you add some new text or a shape they will occupy the entire timeline, the entire duration but you can click on the duration setting and change this. You can also enter new values for the 'In' and 'Out' time settings. If you click on the 'In' or 'Out', you open an input box with these settings. However, if you click on 'duration', a box pops up with a stretch factor and duration.
In a similar perspective, if you adjust the 'In' or 'Out' time, when your element enters and leaves the animation, you see its timeline bar move to the right or left, the duration adjusted but the 'stretch' factor does not. This is an important distinction because you will nearly always adjust the timing of each element, when it appears and leaves your movie but 'stretching' it is actually adjusting its real time value.
Said another way, if you have a short clip of a hummingbird whiz by that lasts 3 seconds, adjusting either the 'In' or 'Out' value determines where it enters and leaves your animation. If you adjust the duration or 'stretch' value you actually shorten or lengthen the 3 second clip.
Above your timeline, the time is displayed in seconds. Under File->Project Settings you can change this display to feet or feet plus frames. It is easy to 'time' your players with these tools and information, easy to adjust the timing of your action. Another display adjustment that is very useful is directly below the timeline bars. It shows a large and small triangle that lets you zoom in on the time window. As you move it to the right, you are specifying a more narrow 'time window' which is almost always necessary for fine tuning, close interactions, and tightly sequenced events. ( Tom Womack )
