pinhole

a small hole that lets the outside world in, upside down
barely any light, and the hole is so small it diffracts the sharpest hole in the ladder sharp, a little past the sweet spot softening softer soft softer still wide open, bright and out of focus
what lands on the wall
aperture0.15 mm
blur on the wall0.56 mm
geometry 0.15 · diffraction 0.54
compared with a full moon51% of it
blur on the wall (mm) against hole diameter. The low point is 0.28 mm; the dots are the eight settings in the slider — the bright one follows your hand.

This is a camera with no lens. Light passes through a hole 0.15 mm across and falls on the back wall of a dark box, 120 mm deep. Everything is inverted — the sky at the bottom, the ground at the top — because light travels in straight lines and crosses itself at the hole.

With no lens, the hole itself is the picture of every point. A point of light outside lands on the wall as a disc the size of the hole — the projection is essentially one to one. So the rule is: the wall shows you the hole.

Except it doesn't, quite, and that's the whole thing. A hole is also a gap that light bends around. Shrink it and the bent light spreads more. At 0.15 mm the hole should give the tightest disc in the ladder, and instead it gives a blurrier picture than 0.30 mm — more than half of its blur isn't the hole at all, it's diffraction. Rise through the slider and geometry wins for a while and then loses its own argument.

The sharpest setting is not the smallest one. It's the second. Nobody put it there: it falls out of the box's depth and the wavelength of light, and you can compute it — d = √(1.22·λ·v) = 0.28 mm for this box. Drag the slider past it in both directions and watch the edges lose and then regain their sharpness. The dip in the middle of the dial is the dial.