50mm lens vignetting

I think that I have a problems with my Canon 50mm f/2.5 Compact-Macro lens. I compared it against a 40mm pancake lens and against a 24-85mm zoom lens. The 40mm lens shows no vignetting at any aperture. The 24-85mm zoom shows vignetting at f/4.0 (wide open), but the vignetting goes away when I stop the aperture down. But the 50mm starts off with vignetting at f/2.5, but the vignetting gets worse when it is stopped down.