The pinhole camera and the basics of camera building

This is the 2nd part of a six-part tutorial about photography basics. So far we’ve seen

• Some physics background related to photography

In the previous post on this series, we have seen that light incident on a rough object is reflected, also called difused, in many directions and this create so many unordered images that the eye mix them all up and we can’t distinguish them, like in a mirror reflection.

In this post we will see how to isolate and project images to understand the basics of a photographic camera. Principles of all cameras even the most modern digital SLRS and even webcams.

By any point of a room infinite rays of light are passing, but any one of these rays come from different source points of it, but if you could isolate just one ray, from one source passing through one specific point you could build an image.

Did you ever hear that only one line passes through two points? Well, this is true. So we must define two points and get the single ray of light that passes through it. This is the idea behind the black box.

A black box is something you can do at home and have a lot of fun out of it. It might seem childish but it is interest for grownups too.

A black box can be made of black thick paper. You cut and glue the paper in order to make a box that is much longer in one side. Well, It doesn’t have to be square though it can be as a regular telescope if you like.

One end of the box will be covered by the same black paper of the rest of the box. The other one, though, will be covered with translucent paper. In the very center of the dark end of the box, you will open a tiny hole the size of a pin. The device is depicted on the picture above.

A rough object, put in front of the black box, will reflect light in every possible direction, but the hole on the black box will only let one ray from each point to enter the black box, all the rest will be reflected. As a result, an image will be formed inside the box.

Because the box is black, all light that hits the side walls of the box will be absorbed, but the light that hits the end of the box will create an image on the translucent paper. Because the paper is translucent you’ll be able to see the image from the outside.

Believe it or not, this is the basics of how any photographic camera in the world words. In fact, this black box is named pinhole camera and could take a real picture if you put photographic paper instead of translucent paper, what you can actually do if you know how to use a photography lab.

We live on the ages of digital photography, so imagine a CCD instead of the photographic or translucent paper. You’d have a digital pinhole camera.

Focal length: 18mm

Focal length: 34mm

Focal length: 50mm

Focal length: 100mm

Focal length: 220mm

Focal length: 300mm

Now, what is the relationship between the length of the box and the size of the image? The longer the box is, the smaller the angle of the projected region will be and the greater will be the approximation and consequently the bigger the image.

At the same time, the longer the box, the more close to parallel the rays of light will be, hence, it lessens the sensation of perspective. What it happens is that the perspective is flattened.

On the other hand, the shorted the box gets, the greater the angle of the region and hence the more accentuated the perspective.

It is also true that a pinhole might be very small, but is not punctual, that is, it does has dimensions, hence it is not true that only one ray of light from each points goes through. The hole could not be punctual, of course, but it can be as small as we want, provided we have a pin thin enough to make it.

The bigger the hole, the more offset images of the object are created, and the more blurry the image becomes. On the other side, the smaller the hole, the less light enters and the more difficult is to see the image on the translucent paper or to impress either the photographic paper or the CCD.

If you take it to extremes, what would happen it you just remove the cap on the pinhole end? Looking to the translucent end you would see it lightened, with no recognizable image formed. In fact, images are forming there, but there are so many of them overlapping that they blend and you can only see white, just like when you watch a white wall.