When a system is designed to work with objects at infinity or at large distance, a convenient measure of the light-gathering power is the so-called aperture ratio or focal ratio or f-ratio.
The f-ratio Q is a positive number defined in the image space from an axial point at infinity in the object space. Denoting D the diameter of the parallel beam in the object space and efl the effective focal length - assumed positive - in the image space, Q = efl/D. In the Gaussian approximation, the ratio Q is two times the absolute value of the maximum aperture angle u'max,
A system with a focal ratio Q = 3 is usually denoted f/3.
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