Question about 580 zoom/diffuser with crop body

I have an XSi with a 580exii and the 18-55 kit lens. I'm unsure whether to use the built-in diffuser panel with the 18mm end of my lens or not.

I considered that the 580 head zooms down to 24mm coverage, but wasn't sure whether this accounted for my 1.6x crop body, making my 1.6x18mm into 29mm full-frame length.

When I'm using the 18mm end of my lens, do I need to be using the diffuser panel? Does the flash know I'm using a crop body?

Any help would be appreciated.

The 580EXII has the ability to know when a 24mm FL is mounted on APS-C vs. 24mm mounted on FF body. The 24mm zoom head position (without supplemental lens) ordinarily is for a 84 degree FOV with FF, so it will sufficiently cover your 18mm WA setting when used on APS-C, as that is only a 74 degree FOV (same as 28mm on FF/135 format). You only need to use the supplemental flash lens if using the Canon 10-22mm lens in the 10mm-14mm part of its range.

No diffuser panel needed with that lens on that body. It will cover 24mm focal length on a full frame camera, which is equivalent to a 15mm lens on yours.

Thanks very much guys! I hate pulling that thing out when borrowing other people's lenses.