Plano-concave lenses diverge a collimated beam from a virtual focus and are commonly used in Galilean-type beam expanders. Given their negative focal length and negative spherical aberration, plano-concave lenses can be used to balance out aberrations of other lenses in the system. Thorlabs provides plano-concave lenses fabricated from N-BK7, N-SF11, UV-fused silica, calcium fluoride, and zinc selenide. N-SF11 offers a usage range that is slightly shifted to the red compared to N-BK7. However, N-SF11 glass is associated with more dispersion because it has a lower Abbe Number than N-BK7 glass. For more information on these substrates, please visit our Optical Substrates page.