In this explanation, I will use a non-avi edit to demonstrate:
There are three pictures or "frames" to this animation:
1)
2)
3)
Frame (1) is held for a few seconds since people obviously keep their eyes often for longer than they blink.
Now, here's the key point: when people blink, their eyelids close at a faster rate than when they open back up. So what does this mean for our animation? Well, to put it simply, because the eyelids close at such a fast speed, the next frame should have the picture of the fully closed eyes (frame 2) instead of the half-closed eyes.
And then of course, the last frame is the half-closed/half-open eyes pic to reflect the slower speed of openning eyelids.
This concept of skipping frames to reflect speed is applicable to most, if not all, animations that need to display varying speeds to make the animation more convincing. To get a true grasp of the concept here's a project for you:
Animate a bouncing ball. Note: a ball hitting and bouncing off the ground happens a lot faster than when it reaches the high peak of its bounce to arc back toward the ground. So this means you'll have less pics of the ball hitting the ground, and more pics of the ball in the air (especially at the top of its bounce).
Here's a nice sample of the bouncing ball animation: http://www.idleworm.com/how/anm/01b/bball.shtml
