axial gradient index of refraction lens elements are used in a lens system , for instance for a photographic objective lens , to correct monochromatic and chromatic lens aberrations , thereby reducing the number of lens elements required to achieve good optical performance in the overall lens design compared to for instance a lens system using aspheric lenses . the gradient index lens elements are bi - convex and / or meniscus type lens elements . the large refractive index gradients as well as large dispersion value gradients provide aberration correction in the overall lens system . a high aperture ratio lens system uses six lens elements where the first and last lens elements are positive power lens elements having refractive index and / or dispersion gradients along the optical axis .