[kortex]

In optics, there's a thing called the circle of confusion (1). It's the locus of points that maps to a single point source of photons. A "perfect" focus maps a point source to a point on the image: perfect contrast. Ability to focus is basically how tight you can get this circle, for a focal distance. (this is all super simplified, perfect lenses, no aberration, etc).

The smaller the aperture, the more constrained the light field, thus the smaller the circle of confusion (at the expense of light collection). That's how pinhole cameras work in fact.

Shrinking the aperture also increases the depth of focus. Pinhole cameras have ~∞ depth of field because there is ~1 path every photon can take (ignoring hole width, diffraction, etc).

1 - https://en.m.wikipedia.org/wiki/Circle_of_confusion