About Decentered Lenses: a shield-type lens, particularly a spherical, decentered lens, will give you a wide open optical field with optimum clarity for superior vision. A decentered lens has a similar spherical curve on the outside of the lens, as it does on the inside. This makes the lens thickest in the middle, and thinner at the edges. This, in part, helps with making a lens distortion-free.