Biconcave LensesBiconcave Lenses have negative focal lengths. They are often used to expand light or increase focal lengths in existing systems. Specification: Material: Crown glass, high index glass, or custom materialsFocal length: +/-1%Diameter: +0.0/-0.03Center thickness: +/-0.05Centering Accuracy: 1"Figure (sphericity): 1~3Surface Quality: 40/20Coating: Optional