I found a useful command while making time lapse videos with a digital camera.
I set the camera to take a picture once every 30 sec. It converts those pictures into an avi file.
The problem was the hour I spent testing the time lapse only produced 120 'frames' - the avi was playing back at 30 frames per second.. end result being a 4 second video that flashed by the screen.
My first attempts to slow this video down resulted in a 4 second video with a slower frame rate but it truncated the movie and most of the 'frames' were missing.
mencoder -speed .25 -ovc copy movie_in.avi -o movie_out.avi
The end result of this command is a longer video file, because the speed was slowed down, this way each picture 'frame' is on the screen longer. The -speed option is expressed in a percentage of the original speed.
Adjust the .25 to 1 and you have the output .avi the same speed as the original..if you set -speed to 3 the framerate is faster.