Canon Close-up Lenses are for quick and easy close-up photography. The close-up lens is screwed on to the front of a lens like a filter. Close-up lenses 500D has two achromatic elements (the 250D lens also)  to correct chromatic aberrations. It does not degrade the high optical performance of EF lenses. Close-up lens 500 has only one element. The 500D is optimized for lenses of 70-300mm.