In imaging optics, a field lens is a positive-powered lens or group of lenses that comes after the objective lens and before the image plane or the eyepiece, serving to change the size of the image^[1][2] or to provide image-space telecentricity. It is used for the reduction of detector size and, in instances needing high optical gain factor, it can correct aberrations through its several elements.^[3] Optical systems that feature multiple image planes are at risk of a potential problem, which involves the inability on the part of succeeding relay lenses to capture a cone of light from the primary objective lens.^[4] The field lens - by behaving as a variably angled lens - solves this problem by bending or refracting the cone of light back into the succeeding relay lens.

In X-Ray microscopy, the field lens is used to produce parallel and homogeneous illumination of the stencil.^[5]