DVD encoding uses a hell of a lot of processing power up and coupled with having your screen on full brightness then that battery's gonna be eaten up pretty quickly I'm afraid.
If you really wanna do processor intensive tasks on a MBP then plugging it in would be best. If you have the battery menu bar option enabled and show the remaining time you'll see that it changes depending on what you're task you're doing. So browsing the web might give you 3.5 hours remaining but as soon as you pop in a DVD and start watching it might fall down to 1.5.