How Can a Hole Make a Picture? Optics Without a Lens
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A small hole in one side of a dark box projects an inverted image of whatever is outside onto the opposite wall. No lens is involved, the principle was described a thousand years ago, and the trade-offs are unavoidable.
Why it works
Light travels in straight lines, so each point on an object outside sends rays in all directions, and only those passing through a small hole reach the inside of a box. Because the hole is small, the rays reaching any particular spot on the far wall come from one small region of the object, which is what makes an image rather than a uniform wash of light. Rays from the top of the object arrive at the bottom of the wall and rays from the left arrive at the right, so the picture is inverted and reversed. Nothing bends the light and nothing focuses it, which means the image is formed at every distance behind the hole and does not need adjusting, unlike a lens image.
The trade-off at the centre
Hole size controls two things that pull in opposite directions:
- •A smaller hole means each spot receives rays from a smaller region, which sharpens the image
- •A smaller hole also admits less light, which lengthens the exposure required
- •Below a certain size, diffraction spreads the light and the image blurs again
- •The optimal diameter depends on the distance from hole to screen and on the wavelength of light
- •For a typical box that optimum is around half a millimetre, which is larger than people expect
- •Depth of field is effectively unlimited, with near and distant objects equally sharp
The long history
The effect has been known and described for a very long time. Observations of the phenomenon appear in Chinese texts from the fifth century before the common era and in Greek sources. The eleventh century scholar Ibn al-Haytham gave a systematic account with experiments, using it to argue that light travels in straight lines and enters the eye rather than issuing from it, which was a substantial correction to earlier theories. Renaissance artists used a room-sized version, a darkened chamber with a hole in the shutter, to trace projected scenes, and the addition of a lens and mirror produced the portable device that is a direct ancestor of the camera. Astronomers used the same arrangement to observe the sun safely, projecting its image rather than looking at it, and large architectural instruments were built into cathedrals for exactly that.
Making one
Building a working camera from the principle needs almost nothing and the practical details decide whether it works. Any light-tight container serves, from a biscuit tin to a room. The aperture is best made by piercing thin metal foil rather than the container itself, since a thick material makes a tube rather than a hole, and the edges must be clean because a ragged hole scatters light. Blackening the interior prevents reflections washing out the image. Exposure times run from seconds to hours depending on light and on what is recording the image, which is why such pictures show moving objects as blur or not at all and why photographs of busy streets can come out empty. Exposures lasting months, taken with a can and a sheet of photographic paper, record the sun's path across the sky as a series of arcs.
Where the effect turns up
The principle appears in places that have nothing to do with photography. Gaps between leaves act as small apertures and project images of the sun onto the ground, which is normally invisible because overlapping circles of light look like dappled shade, and becomes obvious during a partial solar eclipse when every patch becomes a crescent. A hole punched in card is the standard safe way to watch such an eclipse. The same optics explain why squinting improves vision for someone who needs glasses, since narrowing the aperture reduces the blur from a poorly focused lens, and why pinhole spectacles appear to work while admitting very little light. Imaging at wavelengths that cannot be focused by lenses, including some X-ray astronomy, uses aperture-based techniques descended from the same idea.
The takeaway
Only rays passing through a small hole reach the far wall, and each spot receives light from one small region of the object, which forms an inverted image without any lens. Making the hole smaller sharpens the picture and admits less light until diffraction blurs it again, with the optimum around half a millimetre. Gaps between leaves do the same thing, visible during a partial eclipse.