Assignment #1, Rubric attributes: 1,2,3,7,9,11,12
(Abelardo Morell: “Times Square in Hotel Room, 1997″)
In this project you will build & use a pinhole camera. You will process the (paper) negatives, & print from the negatives.
- Suitability of the container
- Light-tightness of the camera
- Ease of operation
- Diameter of pinhole
- Roundness & cleanliness of the pinhole
- Light-tightness of the shutter
- Determination of the focal length
- Determination of the f/stop
(Andrew Reed: digital pinhole image)
A pinhole camera is a very simple camera with no lens and a single very small aperture. Simply explained, it is a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box.
Generally, light strikes an object from an infinite number of directions. If light falling on an object is divided down to individual rays, and we apply the principle that:
the angle of incidence = the angle of reflection
we can see that light source “A” is reflected from the subject at too great an angle to pass through the pinhole aperture but light sources “B” and “C” will pass through. Every point on the subject that faces the pinhole will have only one angle (with respect to a plane tangent to the subject’s surface) that will reflect and pass through the pinhole.
We can see above that since the pinhole is not small enough to pass a single ray of light, additional angles are admitted and thus create an array of overlapping circles of light. This causes pinhole images to lack critical sharpness. We may wish to reduce the pinhole diameter to increase sharpness but we soon come to a point where diffraction occurs and sharpness starts to reduce. An additional result of aperture reduction is the need for substantially increased exposure times.
There have been many formulas proposed over the years designed to calculate the optimum diameter for the pinhole. Large holes admit more light and therefore shorter exposures but create overlapping circles of light causing blurred images. Generally, the smaller the hole, the sharper the image but too small a hole causes diffraction. Therefore the optimum diameter is the smallest hole that can be used for a particular focal length before diffraction occurs.
British Nobel Prize winner Lord Rayleigh (John William Strutt, 1842–1919) worked on pinhole diameter formulas for ten years and published his work in Nature (1891). Lord Rayleigh’s formula is still one of the formulas used today.
Lord Rayleigh’s formula for subject distances above 1 meter may be written as follows:
d = 1.9 x ( λ x f ) 0.5
where d = pinhole diameter, λ = wavelength of light and f = focal length or distance from pinhole to light-sensitive material. For the wavelength of light different average values may be substituted. Often the value of the yellow-green spectrum is used, i.e. 0.00055 mm. Substituting and simplifying we get:
d = .0446 x f 0.5
[all units are mm]
A pinhole camera’s shutter is usually manually operated because of the lengthy exposure times, and consists of a flap of some light-proof material to cover and uncover the pinhole. Typical exposures, using photographic paper for film in bright sunlight, typically range from 15 seconds to 60 seconds or more, depending on the sensitivity of the paper used and the effective aperture of the pinhole.
Pinhole camera construction
Basically, a pinhole camera consists of a light-tight box with a pinhole in one end, and a piece of film or photographic paper attached into the other end. A flap of cardboard with a tape hinge can be used as a shutter.
We will drill the pinhole using a sewing needle a piece of thin aluminum sheet. The optimum diameter for the hole will be calculated from the focal length of the box chosen. This piece is then taped to the inside of the light tight box behind a hole cut through the box.
An oatmeal box can be made into an excellent pinhole camera. Pinhole cameras can also be constructed by replacing the lens assembly in a conventional camera with a pinhole. In particular, compact 35 mm cameras whose lens and focusing assembly has been damaged can be reused as pinhole cameras—maintaining the use of the shutter and film winding mechanisms. As a result of the enormous increase in f-number while maintaining the same exposure time, one must use a fast film in direct sunshine.
(Scott Speck: Catoctin Furnace)
Calculating the f-number & required exposure
The f-number of the camera may be calculated by dividing the distance from the pinhole to the imaging plane (the focal length) by the diameter of the pinhole.
focal length÷aperture diameter
For example, a camera with a 0.02 inch (0.5 mm) diameter pinhole, and a 2 inch (50 mm) focal length would have an f-number of 2/0.02 (50/0.5), or 100 (f/100 in conventional notation).
Due to the large f-number of a pinhole camera, exposures will often encounter reciprocity failure. Once exposure time has exceeded about 1 second for film or 30 seconds for paper, one must compensate for the breakdown in linear response of the film/paper to intensity of illumination by using longer exposures.
Other special features can be built into pinhole cameras such as the ability to take double images, by using multiple pinholes, or the ability to take pictures in cylindrical or spherical perspective by curving the film plane.
These characteristics could be used for creative purposes. Once considered as an obsolete technique from the early days of photography, pinhole photography is from time to time a trend in artistic photography.