Concave lenses do not by themselves form images, but they can be used to shift an image away (to the right) from an existing lens or lens system.
A (spherical) concave lens will cause light from an infinitely distant object to diverge. If these divergent rays are extended back toward the light source (eg, to the left of the lens for a source object to the left), they will intersect at a “virtual” focal point—say, at a distance f to the left of the lens (Figure 1-10). In keeping with the sign conventions above, the power of this lens is given by P = 1/f (with f in meters). Here, f is a negative number, and the power of the concave lens is likewise a negative number.
Concave lenses can be combined with other immediately adjacent lenses. The power of such a lens system follows the same simple addition formula that we introduced for convex lenses:
P = P1 + P2
where, now, we must track the algebraic signs carefully.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.