The cropped image is not a function of the lens, it's due to the camera you're using. It's relative to the full format images produced by cameras like Canon's 5D. Basically your effective focal length (on all of your lenses) is going to be longer than the lens actually says. Here's a review of the 100mm L lens, which illustrates this difference under the "Angle of View" heading: http://www.dpreview.com/lensreviews/canon_100_2p8_is_usm_c16/
I'm assuming you have a kit lens though that came with your camera. If that's the case, set it to 28mm and that will be the same focal length as the lens you're looking at (albeit with a much smaller maximum aperture).