Scleral or semi-scleral lenses may be used for increased comfort and visual acuity. Since they are more rigid, gas permeable lenses and scleral lenses are able to create a smooth, round shape around the cornea, creating a smoother surface for better vision. Scleral or semi-scleral lenses have a larger diameter which covers the entire cornea and reaches over into the white part of the eye, which is known as the sclera. Many patients find these more comfortable than regular RGPs and find that they move around less when the eyes move. The main disadvantage of these rigid lenses is that for some, they are somewhat less comfortable than soft lenses and they must be continually refitted as the shape of the eye changes.