I'm using ffmpeg in flash builder (Action script 3) to convert a video to a time lapse of the video. I've worked out how to use setpts to do this, but the resulting output file is the same length as the input (with just the last frame displayed after the time lapse), I want the duration to be changed based on the settings passed to ffmpeg. 

I'm trying the below through the command line first:

```
ffmpeg -i input.mp4 -filter:v "setpts=0.5*PTS" output.mp4

```

Any help would be greatly appreciated!