A Pin Hole Camera

Snapshots  ConstructionWorking Principles  Observations  Demonstration  References/Links
 

Snapshots

Thanks to Kenny Hoff, here are a couple of snapshots of my pinhole camera.
 
Picture 1
Picture 2

The hole is at the centre of the metallic base in Picture 1. The light-proof casing is the black plastic which allows the object cylinder to slide.
 

Construction


I used a Pringles box as the basic 'body' of the camera. I experimented with a 'wax-paper' and the translucent Pringles cap for use as a screen, and found image quality to be much sharper with the plastic cap. This is probably due to the fact that the transparency of the wax-paper is not uniform, it easily gets wrinkled and is difficult to keep as a flat screen. I tried to get a zoom feature by varying the focal length of the camera. The pringles box was cut into two and the translucent cap was fixed to one end of the viewing cylinder, and a view-plane on the other. The other half of the pringles box with the metallic base was pierced to form the hole. The viewing cylinder was fixed in a thick cardboard casing, while the object cylinder was left free to slide. To prevent light from leaking through the sides of the object cylinder, a light-proof casing using trashbag plastic was attached to the sliding cylinder. (This also prevents the object cylinder from sliding out! :-). The insides of the cylinders were covered with thick black paper, to avoid distortions from light reflecting off the sides of the shiny inner surface of the pringles box.
 

Working Principles


The light enters the small pinhole in the front of the camera, and projects onto the plastc screen. The image the person sees projected onto the wax paper is inverted laterally and vertically.

Observations

Varying Focal length


A 'zoom' effect can be obtained by increasing the focal length (the distance of the screen from the pinhole). As the focal length was increased by sliding out the object cylinder, the image of the object became larger. However, the field of view decreases as the focal lenght increases. This is shown in the figure above - the orange object appears bigger as we increase the focal length, however, since the object is larger - less of it is visible on the same screen, hence the bigger blue object is not visible at f3.

Varying object distance

A similar effect is observed as the object is moved away to the pinhole. As we move the object further away, the field of view increases (more objects come in view) but the size of the object image also decreases.

Varying aperture

With a small pinhole the image formed is very dim, as the amount of light entering is small. However, if we increase the size of the aperture (hole), the image gets blurred. In case of an ideal pinhole, only 1 ray of light corresponding to a world point enters the camera forming a sharp image of the point. This also causes the image to be VERY dim. In the case of a real world hole, which has definite dimensions - multiple rays from a point enter the camera - forming multiple images.


Here, two images (I1 and I2) of point O are shown as an example. In reality, the rays from nearby points overlap causing the blur. For a detailed ( including the wave interferance fom a small hole) discussion of the aperture radius on the image, please go through Matt Young's page on pinhole cameras (first reference).
 

Demonstration

Initially I tested the camera on the image of a bright table lamp, during and after the construction phases. I was able to read the (inverted) label on the bulb, however the images of any object away was really dim. Later, I was able to get better images, by shining the lamp directly on to objects, with the shade occluding the bright bulb.

Here is a picture of a light bulb as formed on the screen. This was taken by holding up the camera towards a bulb in the ceiling in the reading room. Kenny put his digi cam against the view-hole and clicked. The image is pretty blurred as we did not focus the pinhole, and the digi cam might not have focussed clearly on the screen of the pinhole. The larger blob in the middle is the bulb, and the smaller blob is probably the specular reflection of the bulb off the shade.

    Picture 3.

I demonstrated the camera to my flatmate Abhishek. He knew about the pinhole camera priciple, and even helped me in its construction with some neat engineering ideas (using trashbag plastic to 'light-proof' the sliding cylinder arrangment).

For extra-credit I plan to demonstrate it to my 6 yr old niece. Her comments would be up soon after my visit to my cousins place.

References/Links

Credits