Αperture

When a photographer says that he took a shot with f/5.6, he states about the amount of light that will pass through the aperture diaphragm, not about the size of the hole (entrance pupil). The same entrance pupil’s size does not implies that will pass the same amount of light if we refer to different focal length lenses. So, in order to could compare the amount of light in all lenses, we use the f/number which is a value that expresses the relative aperture.
See at below photo in order to understand better:

In this picture, I photographed, in one shot, 3 of my old, manual focus, lenses at the same aperture  f/5.6. As you can easily see, the diameter of the entrance pupil is different in each lens, but the aperture is the same, it is f/5.6. So, although the size of the hole is different in all 3 lenses, the amount of the light that will pass through is exactly the same.
In the above photo you can measure the diameter of the entrance pupil with the help of the meter: 4.3 mm for 24mm, 8.9mm for 50mm and 24.1mm for 135mm lens. Right?
Now, you can calculate the f/number by this equation: N = f/D, where N is f-number, f is focal length, and D is entrance pupil diameter.
In our example:
24/4.3= 5.6
50/8.9=5.6
135/24.1=5.6
Amazing? No, it’s only mathematics and physics!