You bring your eye close to the small opening, and the scene appears at the back of the box upside down, the sky resting on the ground and left swapped with right. This inversion always comes as a surprise, and yet it follows from a very simple rule about the way light travels.
The answer comes down to one idea that you can trace with your finger, with no formula or complicated optics, the same idea that makes every pinhole camera work.
Light travels in straight lines
It all starts from something you can check every day without thinking about it: a ray of light goes straight ahead. When the sun shines through a gap between two shutters, you can see that sharp, straight line crossing the room, and it is exactly this behaviour that explains the reversed image.
From every point of the lit scene, rays set off in all directions. The wall of the dark chamber stops almost all of them, except those that find their way through the small hole.
Why the rays cross at the hole
The hole is so narrow that, for each point of the scene, it lets through only a thin thread of light. And since these rays go straight, they are forced to cross as they pass through the opening.
Imagine a tree in front of the box. The ray that leaves its top travels down in a straight line towards the hole, passes through it while carrying on in the same direction, and so strikes the bottom of the back wall. At the same moment, the ray that leaves the foot of the tree rises towards the hole and ends its course at the top of the back wall.
The two paths have crossed as they passed through the opening, like two roads crossing at a single point. The top ends up at the bottom, the foot ends up at the top, and the whole image flips over.
Top to bottom and left to right at once
This crossing does not work in just one direction. What applies to top and bottom applies in exactly the same way to left and right: the ray coming from the left of the scene comes out on the right of the back wall, and vice versa.
The image is therefore reversed top to bottom and left to right at the same time. It is a complete rotation of the projection, not a simple mirror image, which explains why text placed in front of the box appears both upside down and back to front.
To turn the image the right way up, you therefore only need to rotate it by half a turn: with a pinhole photo, you turn the print or the scanned file through 180 degrees, without needing a mirror.
| Point in the scene | Image at the back |
|---|---|
| Top | Bottom |
| Bottom | Top |
| Left | Right |
| Right | Left |
The same phenomenon as in the eye
Our own eye works this way: the lens of the eye projects a reversed image onto the retina, and it is the brain that turns it the right way up for us.
The dark chamber, however, has no brain to correct anything, and that is why you see the scene just as the light lays it down, upside down.
This phenomenon of an image projected through a hole has been observed and described since Antiquity, long before photography, from Mozi to Ibn al-Haytham. The camera obscura was long used to observe or trace this reversed image, without ever recording anything by itself.
And for the rest of the mechanism
Inversion is only one facet of what goes on behind the small hole. The sharpness of the image, the depth of field and the exact role of the diameter belong to the complete optical mechanism, detailed in the article that explains how a pinhole camera works.
Just remember that the smaller the hole, the thinner the thread of light coming from each point and the sharper the image, until diffraction, which spreads light out through an opening that is too narrow, blurs it again.
To see it with your own eyes, nothing beats trying it: all you need is a closed box with a hole in it to find, at the back, this reversed image that light draws all by itself. The Sténoflex pinhole mini-lab lets you check all this for yourself, with the box, the paper and the baths supplied.
Éric Marais — Photographer, author of two books on pinhole photography (published by Pearson) and creator of the Sténoflex. He has run introductory workshops in pinhole photography and analogue developing from the very beginning.