The Canon MP-E 65mm Macro Lens is an extreme macro lens. And it is unlike any other Canon lens I have used to date. And I recommend that you read the manual for this lens - it will help you understand its uniqueness. Canon lists the focal length for this lens as 65mm.

Maybe you've been playing around with macro photography, either with a dedicated 1:2 or 1:1 prime macro, or with a zoom lens that close focuses to about 1:3 magnification. Maybe you've even experimented with extension tubes or magnification filters to get even more macro action through your viewfinder.