How it works · 01
Light and the pinhole
Why a hole in a dark box makes a picture, why it is upside down, and why the hole can be too small as well as too big.
Try it: a pinhole camera
- f-number (depth ÷ hole)
- Blur from the hole’s size
- Blur from diffraction
- Sharpest hole for this depth
Straight lines make an image
Light travels in straight lines. Every point of a lit scene scatters light in all directions, but a small hole lets through only a thin pencil of rays from each point. That pencil lands at one spot on the back wall, so the wall shows a pattern of light that matches the scene: an image. Rays from the top of a tree pass through the hole and continue downward, so the image is upside down and reversed left to right. This is the camera obscura.
The trade-off: bright or sharp
A bigger hole lets in more light, but each scene point now lights a disc as wide as the hole, and the discs overlap into blur. A smaller hole is sharper and dimmer. Below a certain size, light spreads out as it squeezes through the hole (diffraction), and the image blurs again. Lord Rayleigh worked out the best compromise: a hole of diameter about 1.9 √(fλ), where f is the distance to the screen and λ the wavelength of light. For a box 100 mm deep that is about 0.45 mm, which works out to roughly f/220. That is why pinhole photographs need long exposures.
The lens: the way out
A lens bends every ray from one scene point back to a single image point, so a large opening can still give a sharp image. That is why the Petzval lens of 1840 could make portraits in seconds while a pinhole needs minutes. The opening’s size relative to the focal length is the f-number: each step from f/2 to f/2.8 to f/4 halves the light.