Complete on-axis focusing formula for any conic mirror

The paraxial-ray spherical mirror focusing formula, 1/s + 1/t = 1/f, is generalized to a focusing formula valid for any conic mirror surface for any ray angle emanating from the source so long as image and object are on axis (s is the object distance; t is the image distance; f is the focal length). The complete focusing formula for any conic mirror is readily reduced to simpler but complete forms for parabolic and spherical mirrors. All forms reduce to the standard formula in the paraxial approximation.