The resolution of pit eyes can be improved a lot by adding a material to make a lens. This will reduce the radius of the blurring, and increase the resolution that can be achieved. The most basic form can still be seen in some gastropods and annelids. These eyes have a lens of one refractive index. It is possible to get a better image with materials that have a high refractive index which decreases towards the edges. This decreases the focal length and allows a sharp image to form on the retina.