use your maths skills and a calculator.
sizes:
4:3 = 720:480
16:9 = 854:480 = 1.78:1
widescreen mode 1.85:1 = 854:462 (leaving black bars of 18 ; 9 on top / 9 at bottom)
widescreen mode 2.35:1 = 854:363 ( leaving black bars of 117; 58 on top / 59 at bottom)
virtualdub does automatically center the image so top and bottom calculation is not necessary.
so you want to turn 4:3 to 1.85.1
720x480
--> 720 / 1.85 = 389 (that's the new height --> you have to crop 91 px from the video height; for centered crop: 46 from bottom, 45 from top)
get it to 854 width (use lanzcos3 for resize or avisynth which has the better lanzcos4)
--> 854 / 720 = 1.186 --> 389 x 1.186 = 461
as you can see we have 2 different values for height 1.85: 461 and 462. Resizing is not an exact science, but try to stay as accuarte as possible or you audience will notice.
Never use centered cropping, you will cut heads off and delete important information. Cropping is done scene by scene and sometimes even moving in scenes from top to bottom or vice versa or both to get the perfect frame (and even that is not guaranteed, because for some scenes there simply is no perfect cropping as they contain information on both top and bottom that should not be deleted). This is tough and takes super long.
Ergo: to get from 4:3 to 1.85 you have to crop 91 px from each frame for correct AR).
4:3 deleted scenes inserted into a true 1.85 video will show a very visible quality reduction, because you have to resize. While lanzcos is a pretty good resize filter (4 is better than 3), it is not even near perfect. You can achieve better results with the program video enhancer. Still the problem remains that your deleted scenes will always look worse than the original video, except if you degrade the original video to match the quality of the deleted scenes. Of course no one wants that, but it is the only way to get a consistent quality.
