How to convert aspect ratios

[techreactor]

What is the process/formula to convert from one aspect ratio to other while re-encoding. Lets say If I want to convert a 2.66:1 AR movie viewable in a 4:3 screen i.e 1.33:1 or 2.21:1 movie to a 1.77:1, so that I dont loose picture clarity niether should the movie become disfigured (thin or fat). I know letter box will show in a 4:3 screen, but sometimes it becomes too thin a letterbox to enjoy a movie.

How should the AR conversion be handled ?????

[tigerman8u]

if you're talking about a dvd the AR info to play the dvd is in the IFO file. most encoders (I use mainconcept) have the option to encode to 4:3 or 16:9