Black out a room, let the daylight in through one small hole, and the world outside appears on the opposite wall: the street, the trees, the clouds moving, the cars driving past, all in colour and upside down. That is a camera obscura, Latin for "dark room", and it is the oldest camera there is. It takes an afternoon, a roll of black plastic and some tape.
It works because light travels in straight lines. Light from the top of a tree passes through the hole and lands low on the wall; light from the bottom lands high. Every point outside sends its own ray through the hole, and together they draw the picture, flipped top to bottom and left to right.
What you need
- A room with one window that looks out on something in daylight: a street, a garden, a skyline
- Black plastic sheeting, blackout fabric or thick cardboard, enough to cover every window
- Gaffer tape or black duct tape, a lot of it
- A craft knife and scissors
- A square of cardboard and a sheet of aluminium foil for the hole
- A white wall, or a white bedsheet to hang as a screen
- A towel or draught excluder for the gap under the door
- Optional: a cheap magnifying glass or the lens of reading glasses, for a sharper, brighter picture
- Optional: a camera on a tripod, to photograph the result
Step by step
Choose the room and the day
The view matters more than the room. Pick a window that looks out on something lit by the sun, while the room itself stays in shade: the brighter the scene outside and the darker the room, the better the picture. A small room works better than a big one, since the picture is brighter on a wall that is close to the hole.
Wait for a sunny day. On an overcast day it still works, but the picture is faint and takes longer for your eyes to find.
Black out every window
Cover every window completely with the plastic, fabric or cardboard, and tape all the edges down. Then turn off the lights and look for leaks: every crack of light washes out the picture. Tape over the gaps, put a towel along the bottom of the door, cover the standby lights of chargers and screens.
It is fine if it takes a while. The darker the room, the more the picture glows.
Make the hole
In the covering of the window with the view, cut a square opening of about 5 cm. Tape a piece of foil over it and pierce a round hole in the foil with a pencil, about 1 cm across. Foil lets you change the hole later without cutting the window covering again.
The size of the hole is the trade-off of the whole thing: a smaller hole makes a sharper but dimmer picture, a bigger hole a brighter but blurrier one. Begin with 1 cm, and try 5 mm and 2 cm to see the difference. For a room about 4 metres deep the sharpest hole is only about 3 mm, but most people like the brighter picture of a bigger one.
Set up the screen
The picture falls on the wall opposite the hole. A white wall works well; a patterned one or a bookshelf does not, so hang a white sheet there if you need to. To see a smaller but brighter picture, hold a sheet of white card or a white board closer to the hole and move it until you like what you see.
Let your eyes adjust
Close the door, sit down and wait. At first you will see almost nothing. After five minutes the shapes appear; after fifteen to twenty your eyes have adapted to the dark and the picture is full of colour and detail. Do not look at your phone while you wait, its light resets your eyes.
The picture is upside down and mirrored. The sky is on the floor, the street on the ceiling, and anything moving outside crosses the wall in the opposite direction.
Sharpen it with a lens
For a picture that is brighter and sharper at once, use a lens instead of a small hole. Cut a hole in the covering the size of the lens, 3 to 5 cm, and tape the lens over it. A lens only focuses at one distance, its focal length, so the screen has to stand that far from it.
Reading glasses tell you their focal length: a +1 lens focuses at 1 metre, a +2 lens at 50 cm, a +0.5 lens at 2 metres. Hold the white card at that distance from the lens and move it back and forth until the view outside snaps into focus. A magnifying glass works the same way; find its focal length by focusing a distant window onto a sheet of paper.
Photograph it
To keep what you see, put a camera on a tripod facing the wall, switch off the flash and set a long exposure: a few seconds at ISO 800, or up to 30 seconds at a lower ISO. A phone in its night mode works too. Long exposures make people and cars into ghosts and pull out colours that the eye barely sees.
Turn the photo the right way up afterwards, or leave it upside down: the photographer Abelardo Morell made a famous series of rooms with the outside world spread over their beds and walls, and the strangeness is the point.
Safe with the sun
Never look at the sun through a lens. If the sun itself is in the view, a lens focuses it into a hot point that can scorch paper or fabric, so do not leave a lens in place unwatched, and keep the screen away from the bright spot. A plain hole is safe.
Taking it down
Gaffer tape comes off cleanly; duct tape can pull paint and leave glue, so put painter's tape on the frames first if the room is not yours. Keep the cut covering of the window with the hole: the next time it takes ten minutes.
A very old idea
The Chinese philosopher Mozi described a picture turned upside down by a small hole more than 2,400 years ago. Aristotle noticed the round images of the sun that fall through the leaves of a tree. The Arab scholar Ibn al-Haytham studied the dark room around the year 1000, and Johannes Kepler gave it the name camera obscura in the early 1600s. Painters later used portable boxes with a lens to trace a scene, and when someone found a way to fix that picture with chemicals, the camera obscura became the photographic camera.
Try it smaller
The same thing works in a cardboard box with a hole at one end and tracing paper at the other, or on any sunny day under a tree: the bright spots on the ground are pictures of the sun, and during a partial eclipse every one of them turns into a crescent.