rack focus is a nice effect but when I do it the image kind of shifts. Increases and decreases a bit like I'm using the zoom, which i'm not. I don't recall seeing this when I've seen similar shots on tv. am i doing something wrong? I'm using a Sony DSR 500 if that helps.

When you focus, you move a lens in a lens assembly. Depending on the build of the lens assembly, zooming may or may not also happen at the same time.

It's called lens breathing. Most lenses suffer from this to some extent. Really expensive lenses have measures to prevent this from happening.

Most 2/3" zooms breath when adjusting focus. The Canon wide angle zooms don't breath to a noticeable extent. What can work with some shots is to use the macro for the rack focus, setting the focus ring for the distant object and pull using the macro - it's very much an eye focus job.

Double check the back-focus adjustment on the lens.