Pinhole Camera

Drag the object and the screen, and change the size of the hole.

In between: a wider cone of light from each point gets through. The picture is brighter, but each point now makes a patch, so it is a little blurry.

Object distance: 24 cm
Screen behind the hole: 22 cm
Picture size: 0.9 × upside down
Each point blurred over: 15 mm

Hole Tiny (2 mm) Medium (8 mm) Big (24 mm)

What’s happening

Light travels in straight lines. Light from the top of the object goes down through the hole and lands at the bottom of the screen, and light from the bottom lands at the top, so the picture is upside down. No lens is needed, just a dark box with a small hole: a pinhole camera, or camera obscura (Latin for “dark room”). A smaller hole makes a sharper picture, but a dimmer one, because less light gets in.

Try this

Bring the object closer to the hole. What happens to the picture?
Slide the screen back. Does the picture get bigger or smaller? Sharper or blurrier?
Make the hole big. Why does the picture smear?
Where does the light from the object’s tip land? And from its foot?
At home: make a pinhole in one end of a shoebox and tape tracing paper over a hole cut in the other end. Point it at a bright window and look at the paper with a cloth over your head.

Learn more: picture size, sharp against bright, and suns under a tree

How big the picture is. The light from the tip and the foot of the object crosses in the hole, making two triangles of the same shape: one outside the box, one inside. With $u$ the distance from the object to the hole and $v$ from the hole to the screen:

\frac{\text{picture's height}}{\text{object's height}} = \frac{v}{u}

So the picture grows when the object comes closer, or when the screen is farther back. A pinhole camera has no focus to set: the picture is equally sharp (or equally blurry) wherever the screen is, and for things near and far at once.

Sharp or bright. A hole is never just a point. Light from each point of the object comes through the whole hole, of width $d$ , as a narrow cone, and makes a little patch on the screen:

\text{blur} = d \times \frac{u + v}{u}

The patches from neighboring points overlap, and that is the blur. A smaller hole makes smaller patches and a sharper picture, but lets in less light. Real pinhole photos take seconds, minutes, even hours.

Not too small. Shrinking the hole only helps up to a point. A really tiny hole makes light spread out as it passes through (diffraction), and the picture blurs again. The best width, with $\lambda$ the wavelength of light (about 0.00055 mm for green), is about:

d \approx \sqrt{1.9 \, \lambda \, v}

For a box 20 cm deep, that is half a millimeter: a hole made with a needle.

Eclipses: a pinhole in a card throws a small, safe picture of the Sun on the ground, so you can watch it turn into a crescent.
Under a tree: the gaps between the leaves are pinholes, and the round spots of sunlight under the tree are all pictures of the Sun. During an eclipse they all become crescents.
Dark rooms: a small hole in a window shutter throws the street outside, upside down, on the far wall of a dark room.
History: Ibn al-Haytham studied the camera obscura in Cairo around the year 1000, and centuries later painters used one to trace scenes.
A lens instead: put a lens in a big hole and it brings each cone back to a point. That gives a picture both bright and sharp, but only at the right distance: the single-lens camera.

See it at Science Playground

The Single Lens Camera