This quantity must be the same for each component of an apochromatic triplet. In one approximation, the reciprocal of this quantity equals the index of refraction, minus one, times the difference in the reciprocals of two radii. The inverse of this quantity is measured in diopters and is referred to as power. This quantity is divided by the diameter of the pupil in order to calculate a namesake stop. This quantity over this quantity minus the distance to the (*) object, gives the magnification. This quantity equals half the radius of curvature for a concave mirror. An object placed outside this distance for a convex lens produces an inverted, real image. FTP, name this quantity, the distance from the lens to the point where the light rays converge. 
This quantity must be the same for each component of an apochromatic triplet. In one approximation, the reciprocal of this quantity equals the index of refraction, minus one, times the difference in the reciprocals of two radii. The inverse of this quantity is measured in diopters and is referred to as power. This quantity is divided by the diameter of the pupil in order to calculate a namesake stop. This quantity over this quantity minus the distance to the (*) object, gives the magnification. This quantity equals half the radius of curvature for a concave mirror. An object placed outside this distance for a convex lens produces an inverted, real image. FTP, name this quantity, the distance from the lens to the point where the light rays converge. 
This quantity must be the same for each component of an apochromatic triplet.
focal length [or f; prompt on partial answer] 