Camera Basics written 5 years ago

The basic premise of a camera is that light comes from a source (the sun, lamp or flash), bounces of an object (whatever we want to photo), then passes through a lens and hits our photosensitive surface (be that film, or digital CCD), which then provides a mechanism to store a photo.

The above is obviously a simplification, with the main complication being introduced by the lens. To simplify things further we can remove the lens altogether to produce a pin-hole camera.

Pin-hole camera

A Pin hole camera is essentially a box with a small hole in one side, and a photosensitive surface on the other:

Here, light strikes the object and gets reflected into the small hole on the front of the camera. This then strikes the photosensitive surface (or Image Plane). The idea behind a pinhole camera is that the hole is so small that only one ray of light for each point in the scene can pass through to strike the image plane. This, in theory can produce pin-sharp images with an infinite depth of field.

There are a few issues with this though:

• Making a hole suitably small is difficult. The ideal hole might be just one photon wide, in a material of an infinitesimally small thickness. This is obviously impossible to construct, and even if it could be constructed would be of little use (see below).

• The smaller the hole, the less light passes through. Because light is a random process, you cannot guarantee that a ray of light will strike an object, then pass through the hole to strike the image plane. Consequently the smaller the hole, the longer the shutter needs to be open (sometimes days!), and the less use the camera is.

So this suggests we should be making the hole bigger, to let enough light in. But in order to take a reasonable picture with a larger hole, we need a lens.

Lenses

When light strikes an object, it is scattered in all directions, meaning that for one point in a scene, there are thousands of rays of light that have reflected from it. A lens makes use of this fact, and unlike a pinhole camera, where for each point in the scene only 1 ray of light is used, a lens takes many rays of light, and focusses them into one point.

So the above images shows 7 rays of light being reflected from the object, then being focussed onto one point on the image plane. Compared to our pinhole camera, that’s 7x more light! In reality it works out much better than that, as if the diameter of the hole is doubled, 4x the amount of light is let in.

There are lots of other good reasons to use a lens on a camera. Adjustable lenses allow the focal length of the lens to be changed, making different sections of the scene in focus. Other lenses allow you to zoom in on specific areas of the image.

References

• Wikipedia. Has a good introduction to pinhole cameras.

• Oatmeal box pinhole camers. An interesting method of creating a pinhole camera.

• Canon EOS 1Ds pinhole camera. An excellent site showing how someone made a rather sophisticated pinhole camera (with zoom!) for his digital camera.

• World’s biggest camera. The world’s biggest (pinhole) camera, built inside an aircraft hangar.

• Converging lenses. A highschool physics tutorial, for those that forgot what they were taught.

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