Thorlabs' N-BK7, UV-fused silica, calcium fluoride, and zinc selenide bi-convex lenses are popular for many finite imaging applications. Bi-convex lenses are most suitable where the object and image are on opposite sides of the lens and the ratio of the image and object distances (conjugate ratio) is between 0.2 and 5. For more information on these substrates, please visit our Optical Substrates page.