What is A Camera Obscura?

On the most basic level the Camera Obscura is a darkened space (be it a room or small box) with an aperture pointed at a brightly lit subject. This aperture can consist of either a pinhole or a lens. It works on the principle of light traveling in straight lines and bouncing off an object and into the aperture. This is the same principle that allows people to see and how a regular camera works. As light hits an object it bounces off in all possible directions but in straight lines. By utilizing an aperture only specific light rays (the ones that bounce back into the aperture) are able to go through the pinhole or lens and form a projection. Thus within the Obscura (camera or eye) an image is created on the surface opposite the aperture.

Camera Obscura
Camera Obscura

The image that is created is shown upside down (as the light rays from the top of the image travel straight to the bottom of the projection). The brightness of the image is determined by the size of the aperture. If a pin hole is used the issue of focus is prominent. Here one must determine the aperture size based on a balance of brightness and focus. If the pinhole is large then the projection will be bright, but focus will be lost. Alternatively if the pin hold is small the image will be dim, but closer to in focus. Another option for the aperture is the use of lens. Lenses are great because they allow the brightness of the largest pinhole with the focus of a small one. The lens focuses the light to a certain pre-defined distance (the drawback of a lens) and allow a very bright and focused projection.
The Camera Obscura can be used either a live projection tool or to take images either by photographic means or simply by drawing or paining the projected image.

For more clarification see this video: