A Pin Hole Camera
Principles Observations Demonstration
Thanks to Kenny Hoff, here are a couple of snapshots of my pinhole camera.
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.
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.
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.
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.
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).
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.
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
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.
Kenny Hoff for his Kodak DC210
digital cam (the res of the captured images is pretty nice - I've reduced
them to 25% here).
Abhishek Chandra for his suggestions