Aperto

Meet Aperto, a camera lens who opens his iris wide to let in more light.

The f-number of a lens is its focal length f divided by the diameter D of its opening, N = f/D. A 50 mm lens with a 25 mm opening is at f/2. Close the iris to 12.5 mm and it becomes f/4.

The light reaching each point of the image goes with the area of the opening divided by the square of the focal length. That means it goes as 1/N². Each full stop on the usual scale, f/1.4, f/2, f/2.8, f/4 and so on, multiplies N by √2 and halves the light.

A small f-number lets in a lot of light, so you can use a fast shutter in dim places. It also gives a shallow depth of field that blurs the background behind a portrait.

A large f-number keeps more of the scene sharp from front to back, which landscape photographers like. Your eye works the same way. Its pupil changes from about f/8 in bright sun to about f/2 in the dark.