The Pinhole Camera

You may know that I work for a software company that is highly respected for its digital imaging products. As a result, I’ve been trying to learn more about photography. For example,

Near Snail Lake, MN
As it turns out, there is a lot of science in photography. It is much more than I can discuss in a single entry — there is the chemistry of film, optics, exposure, not to mention the artistic concepts such as composition, mood, etc… Besides, I’m not much of an expert yet so I’m not really qualified to discuss most of that (yet!).
So let’s start out by considering the worlds simplest camera, the Pinhole camera. Once we understand that camera and its limitations, we can move on to understand why people use other kinds of cameras most of the time.
A pinhole camera is quite simple. It consists of a box with a large piece of photographic paper mounted at one end, and a very tiny hole that can be covered up at the other end.

A Pinhole Camera
We all know that light moves in a straight line. By using a tiny pinhole, each point on the film only sees a tiny piece of the outside world. I’ve drawn to small grey lines showing how the light from the tree passes through the pinhole to a corresponding point on the film. One incidental result of this is that the image is upside down on the film.
If you make the box longer, the image will get bigger. If you don’t believe me, draw a new pinhole to the right, and trace from the back of the box to see how much of the tree you’ll see on the film. You should see less of the tree — so the image is “zoomed in”. This is one fundamental concept of photography : The longer your focal length, the more zoomed in you are. In fact, when photographers talk about lenses, the talk about the focal length. For a standard camera, a typical lens might have a focal length of 30mm. My friend Jessie’s favorite lens is a 50mm, which is a little bit more zoomed in. A “fish eye” lens might be 15mm. A telephoto lens is in the range of 75 to 300 (or more) mm. 300mm is about a foot, which starts to be a pretty awkward length of lens hang off the front of your camera.
This raises an interesting side question : Why do you need zoom, can’t you just move closer to the subject? For some things that is true, a similar effect can be achieved by moving the camera closer to the subject. But in some cases, the use of zoom provides an important tool in how an image is composed. Consider a scene with a tree and the moon on the corner of the block. You are the photographer, and you stand in the intersection using different focal lengths on your pinhole camera. For this example, I’ll assume that you adjust the position of the camera so that the tree is the same size in each picture.

The same moon and tree picture taken with a very short focal length, medium focal length and long focal length
What you notice is a striking difference in composition. With the very short focal length (e.g. “zoomed all the way out” according to our “focal length / zoom” rule) distant objects (such as the moon) appear very far away. Also, things at the perifery of your vision are more prominent in the picture, such as the street extending away to your right and left. With a very long focal length (e.g. “zoomed all the way in”), distant objects appear larger relative to close objects. Also, objects in the perfery are absent as being “zoomed in” restricts your field of view.
There is a corollary to the “focal length / zoom” rule. As the focal length gets longer, less light makes it to the film. Imagine that the tree is covered with ten christmas lights. If the whole tree is in frame, then ten christmas tree lights worth of light is making it to the film. If you zoom in to show only light, then one tenth as much light is making it to the film. Just because that light looks bigger doesn’t mean that it gets any brighter.
This brings us to the final issue I wanted to talk about: exposure. Exposure means how much light makes it to the film. Exposing film is a chemical reaction, and a specific amount of light is required to make the reaction work. Too much light will make the picture look washed out, or “over exposed”. Too little light will make the picture dark, or “under exposed”. A pinhole camera has four ways to adjust exposure. You can use a shorter focal length to get more light, but at the cost making distant objects look tiny (as discussed above). You can expose the film for a longer length of time (it is not uncommon for pinhole cameras to have exposures lasting minutes or even hours). Long exposures make it hard to take pictures of anything that moves, however. (Early cameras were really little better than pinhole cameras, and took several minutes to expose. This is why pictures from that time are so serious — it is not possible to hold a smile for several minutes without moving, so people held a relaxed pose that they could stay in for several minutes.) You can put a filter in front of the pinhole to reduce the amount of light getting in. You can also alter the chemistry of the film (this is called the ISO of the film) to change how reactive it is to light.
The long exposure time of a pinhole camera is the primary reason that people invented lenses. I’ll save a discussion of lenses for another entry, however.