K9 plano-concave lens plano-concave lens focal length is a negative focal length, parallel beams through a plano-concave lens will diverge like, I produced the lens, good uniformity, no texture, impurities, bubbles. Product parameters: Material: K9 Dimension Tolerance: +0.0 /-0.2mm Surface Accuracy: λ / 4@632.8nm finish :60-40 center offset :3-5 arc min Clear Aperture:> 90% Chamfer: 0.3mm × 45