Microlens

Spherical plano convex lenses are useful when parallel beams are converged or lights from sources are converted to parallel beams, i.e., at infinite conjugate ratios.

Spherical biconvex lenses are useful when lights from sources are converged or images are relayed to other optical systems, i.e., at unit conjugate ratios.

Since these lenses are designed at the wavelength 587.6nm (yellow helium line [d]), the focal lengths of them vary at other wavelengths.