This Project was super simple and my 9 year old son did most of the work and even hosted a video about it. This project can be done for under $5.
A camera obscura is simply a pinhole or a lens in a dark room that projects an upside down image onto a wall of the world outside.
Step 1: Measurements
The first step is to measure the distance from the window to the wall. Ours was 3.3 meters. The lens we will need would be a focal length of 3.3 meters but eye doctors don't use focal length as a measurement, they use diopter. The is simple to figure out by dividing 1 over the focal length. This gave us .299 but these lenses are only made in .25 steps. We rounded ours down to .25. If you had the exact diopter for the room, the image would be sharper. We went to the optometrist in town and when we explained what we were trying to do, they just gave us the lens. Make sure you ask for a + or a converging lens. A - or diverging will not work for this.
The only other thing you have to measure is the size of the window frame.
Step 2: The Window Covering
We measured the window frame and figured we could cover the window with 4 sheets of Bristol board. We got 4 sheets at 2 for a dollar at the local dollar store. We taped the sheets to the correct size but when we held it up to the window we discovered Bristol board is not as opaque as we thought. We decided to cover the paper with more paper by making a collage of pictures but it still was letting light through. We went back to the dollar store and bought 4 more sheets of black. This did the trick.
Step 3: The Pinhole
Even though we had a lens, we wanted to try the pinhole first, Just to see the difference. So we poked a hole through the bristol board and hung it up by taping it to the window frame. While it did work, It was very faint and required us to sit in the dark and wait for our eyes to adjust before we could see anything. Time to try the lens...
Step 4: The Lens
We created a small tube of Bristol board to hold the lens by friction and taped that to a small board. We cut the pinhole bigger to accommodate the lens and taped it all together. The reaction you hear in the video is 100% real. We were amazed at how bright and clear the image was. I guess it shouldn't be too surprising. Our pinhole was about 1/8th of an inch across and our lens was about 2.5 inches. That is a difference of .012 square inches to 4.908 square inches or over 400 times more light coming in. The whole project can be done in about 15 minutes and for less than $5. What are you waiting for?