The name of the opening that lets light into any camera is called

the aperture

Explanation:

The diaphragm or aperture is a part of the lens that limits the ray of light that enters the chamber. It works like the iris of the human eye, opening or closing to allow more or less light to enter as needed.

The openness of the diaphragm is what is called the diaphragm opening.

Diaphragm aperture is measured in f numbers. The jump from one value to the next is called a step.

The typical f-number scale is as follows, although it does not show all the possibilities, since the different steps depend on the design of the objective