This article is intended to describe the theory and construction considerations behind the pin-hole camera and camera obscura.
The pinhole camera is a device used to illustrate the behaviour of light. It may be as simple as a box with a hole one end and a translucent screen on the opposing wall. It may have a variable aperture or a photosensitive film for preserving images. It may even be in the form of a whole room as in the camera obscura.
The construction of a pinhole camera for a class practical will depend on what materials are to hand. Suitable cylinders (of any cross sectional geometry) should be able to be easily held by students. It is suggested that these have a length to width ratio greater than one but less than five. Too short and the image will not be clear enough especially around the edges, too long and the image will be too dim and of limited field of view. Square section tubing has the advantage of being easier to form ends in paper or foil.
The use of a ground glass screen will mean greater care in handling, also is more expensive. Tracing paper is cheaper and easier, but is less able to tolerate pressure if outlines are to be traced on it. Medium gauge, preferably black, card can be cut and folded over the end or aluminium foil. Both of these can actually use a pin to form the hole. The hole can be made using a fine drill in a solid cap for example of thickish plastic. It will be shown later that there is a need for a sharp edge to the hole.
A light tight fitting is essential if photographic work is to be undertaken. The film or paper needs to be held securely and flat.
A bright object is required. On a sunny day outside buildings are ideal. Traditionally "carbon filament" lamps are used, although clear (not pearl) 40W bulbs may also be used if none are to hand. CFL (energy saver) lamps may also be used but the image is less defined. There is no reason why a 12V 21W-36W SBC bulb cannot be used for individual use. Pearled bulbs have a strange property that the filament can often be seen through the frosting, (although this is more usual in lenses).
The pin-hole camera relies on the "rectilinear propagation of light". That is light travelling in straight lines.
In the diagram the ray from the top of the tree can be seen going through the hole, this is extended to the screen. Similarly the ray from the base passes from though the hole and reaches the screen to form an image. This image is upside down (inverted).
The size of the hole is important. Too large and the image is blurred or direct rays take over, too small and there are diffraction effects to be considered. An optical pin or dissection pin is ideal.
The Camera Practical
This practical involves using a pinhole camera as a viewer. Some sources refer to the pinhole camera, used only to view, as a pinhole viewer. "Camera" is Latin for "room" and has nothing to do with photography (of Greek derivation).
The camera is made according to materials available. The ends should be made as smooth as possible (neat creases on square section for example). These may be held in place with tape, or rubber bands or other method more tailored to the equipment supplied.
If foil or card is used to create the hole, an optics pin is used to make a neat hole at the approximate centre. More substantial materials will require pre-drilling before the distributing to the class.
A suitable object (target) is then required. This is, inormally a bright, high contrast object. On a sunny day outside buildings are ideal. Traditionally "carbon filament" lamps are used, although clear (not pearl) 40W bulbs may also be used if none are to hand. CFL (energy saver) lamps may also be used but the image is less defined. There is no reason why a 12V 21W-36W SBC bulb cannot be used for individual use. Pearled bulbs have a strange property that the filament can often be seen through the frosting, (although this is more usually seen when investigating lenses).
The practical can be extended to develop the ideas on the nature of light.
If you consider a second hole in the front what would you expect to see?
A second hole would be expected to behave in a similar way to a single hole. Indeed it does, and two images are apparent on the screen. If the holes are far enough apart two distict images are seen. This result should be noted as more wavelike behaviour will be illustrated later using a lens.
Multiple holes will produce multiple images.
The size of the hole will make a difference. Clean holes are required: A hole punch or cork borer against a soft support (like cork!) will allow larger holes to be made easily. In the diagram, the rays coming from the top of the object (indicated as red) and passing through the hole at the top will form an image at higher on the screen than rays passing close to the base (blue). More light enters the camera so the image is brighter, however, the image is more blurred.
Note this is a simplified ray diagram, there will be rays at all points inside the aperture hole so lateral spread also forms.
If a more robust camera is made the aperture mechanism from an old camera may be deployed to facilitate the changing of hole size and explanation to a class.
A similar phenomenon occurs with ragged edges (blurring the end image).
Probably best illustrated on an optics bench.
A lens is used to form an inverted image on a screen. The position is noted . A simple pin hole when placed to form an image. This is replaced by a multiple holed slide creating many images. Reintroducing a slide will, as a surprise to many, produce a single image.
This is due to the wave nature of light passing through a lens.
Photography using a pin-hole camera
- It may be a useful cross-curriculum activity, linking Physics, Chemistry, Design & Technology, Art and History.
- It is suggested that commercial solutions, film and papers are used initially as this will reduce potential errors until the method is established. At this point substitutes may be made. The "proper" chemicals are not too expensive, especially for a class practical.
- The box for the camera needs to be light tight, thus lends itself to such a joint venture. Ordinary aluminium (cooking) foil does not make good apertures, but catering containers may be suitable.
- A method to transport film, when used, is required, although this may be best acheived using salvaged parts from a camera.
- Do ask your local photography shop for assistance. 1) they are likely to be staffed with knowledgeable people, 2) are keen to start newcomers to this field, 3)may have cheap parts and 4)with the advent of CCD (digital) technology are a disappearing resource.
(Photography may require a further wiki article but CLEAPSS Laboratory handbook (section 11.7) or web references such as: Ilford's Guide to black and white printing, Ilford's salt printing and A collection of pohotographic chemicals can be used as starting points.)
The information here is based on the discussion submissions of "beaky".
If a teacher is to demonstrate with colour film, a purpose made camera with a mechanism to manually wind the film is used. 120mm ( Medium format?) colour film needs to be loaded in complete darkness. This may be done with the help of a light tight bag. The images are then exposed for a few seconds depending on the hole size and film speed.
The pupils use photographic paper which can be loaded under a safe light (and this allows behaviour to be monitored). This produces a "negative" which can be turned into a proper print either by contact printing (see Ilford link) or scanning and inverting on a computer.
Exposure times - assuming the pinhole is made by a standard optical pin and the box is in the order of 10 cm deep try 30 seconds on a sunny day. You will need to find the optimum time by trial and error for each camera. When trying a new exposure time double or halve it rather than trying small changes.
Paper/film - just use black and white print film (Ilford or Jessops multigrade).
The 120mm film option is recommended over 35mm as 35mm is too small. The use of E6 processing (for transparancies is not included as this tends to be used with 35mm films), however this link is informative on the matter.[ http://www.yarki.net/E6/]
There is always the possibility of producing plates on glass. If this really gets you going!
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