I've never done a zoom effect in a film, so take my thoughts with a rather large grain of salt, but I have done basic camera movements like dolly shots and the like. I'd assume that the best thing to do is move the camera, but very very slightly each frame, to achieve a smooth zoom. If I were you, I'd try physically moving the camera to see how smooth that looks, and if you're unhappy with the results, zoom in post-production instead. (Or if you're more comfortable in post-production softwares, do it in post-production first and see how that looks; I just personally find doing as much as possible in the actual filming easier than trying to figure out programs.)

I do it manually, also because I have no idea how to do it with a comnputer editing program. But well, I use a videocam, so it allows me to zoom slowly.

Very impressive results. Comparing that video with tests that I've done, I have this to say: It is a difference of style. Doing it all with the physical camera gives it the stop-motion feel, whereas doing it in post-production gives it a more professional feel (but also more out of place compared with the animation). I suppose it all depends on what sort of mood you're going for, and whether you're more comfortable with stop-motion or whatever editing program you may have.

Ok, so I would just hold the camera still and animate, then After Effects (or whatever) would implement the zoom effect afterwards. The only problem I see with this is that your footage loses quality, as you systematically delete pixels around a certain section of your images. But then, it won't be a problem for me as my native uncompressed resolution is about 2 GB for 4 seconds of video. Crazy, I know!

Thanks for your replies, guys.

A pan is just the camera turning on a fixed spot. You can't say it looks better than zoom, they are two different camera movements.

Is there a free program that can pan and keyframe?

I have it but how does it work?

Yep if you shoot in high resolution then there's no problem! Just do the zooming in the post-production.

I zoom and pan while animating, and in post. I feel that just doing it in post exclusively makes it look a little fake, unless you do it REALLY well. Digital zooming, will cause you to lose some quality, and you can't get all of the camera movements that you can get while animating.