A 13-element zoom system fitted with an optically structured Fresnel output lens specially developed for using framing shutter blades, geometric gobos, or an ovalization lens without the constraints associated with this type of lens. A subtle progressive diffusion system allows the level of diffusion to be adapted according to the application and accessories or optical elements used. The F.O.S. system also significantly improves colour mix and can obtain 10% more light than when using a conventional Fresnel lens.