What is Camera; f-number?

A camera is a converging lens of focal length $f'$ forming a real image on a sensor, at $\overline{OA'} \approx f'$ for distant subjects; focusing moves the lens. A diaphragm of diameter $D$ limits the beam; the f-number is $N = f'/D$ (written $f/N$). The angle of view is $2\arctan(\ell/2f')$ for a sensor of width $\ell$.