This is a, usually homemade,
photographic camera that, to produce an image, uses, in place of the conventional
lens, a very small, sharp-edged hole in a thin opaque material, often brass sheet
or shim.
This hole produces an image upon the film plane that varies, in both scale and
perspective, according to the distance between the pin-lens and the sensitive
surface. If this distance is short the image produced is similar to that of a
wide-angle lens. Increasing the distance results in a telephoto, or long-focus,
lens effect.
Another quality that often draws photographers to the use of a pinhole camera
is the infinite depth of field displayed within the resultant image.
The principle of the pinhole camera is older than photography itself and was often
used in early camera obscura.
Pinhole camera's vary in construction from the simple cardboard 'shoe box' example
one associates with a school science class to professionally built scientific
apparatus made out of wood or metal. Conventional camera's, such as 35mm SLR or
5 x 4 sheet film monorails, can be adapted to take a pinhole lens, these often
produce excellent results.
The quality of the image produced by a pinhole camera depends upon the size, sharpness
and quality of the hole. This quality, up to a point, improves as the hole is
made smaller, but beyond a certain size it deteriorates rapidly. This is because
of diffraction. The combination of highest image quality and minimum diffraction
is described as the optimum pinhole diameter. The formula for determining this
can be found within these pages.
The ' shutter ' on a pinhole camera can vary from a simple hinged flap of wood
or card, placed in front of the pinhole, to a field camera shutter taken from
a broken lens. As the exposure times are typically in 10's of seconds, and even
minutes and hours, either method can be used with complete confidence.
It is even possible to use a camera made with multiple pinholes so as to create
photographs with rows of multiple images on one plate. The aim is to experiment
and, above all, have fun.
Making the pinhole.
It is worth taking
time and care over the making of the pinhole. It should be a clean, sharp-edged
hole, with no burr, and prepared from a material with as little depth as possible.
In schools silver, or aluminium, foil is often used. For a more robust pinhole
lens it is recommended that a material such as brass shim, or sheet, be used.
A piece of brass sheet, about 1/32 inch thick, is cut to fit the base or carrier.
Use a nail to ' TAP ' a dimple-like raise in the centre. Take care not to
make a hole in the sheet.
Use a piece of fine abrasive emery, or wet and dry, paper to smooth the raised
side of the dimple. Now, using a needle of suitable diameter, and working
from the depressed, or concave, side of the dimple, pierce a hole. Use the
fine abrasive paper to remove any burr, it may help to use a photographic
loupe or watchmaker's glass to check. Pass the needle through the hole again.
The final stage is to blacken the brass. The time-honoured method is to place
the brass over a candle flame. Check that no ash has blocked the hole.
For greater accuracy, it is possible to purchase pinholes, either ready made
or to order, cut by laser.
Formulae
for producing the optimum pinhole diameter.
In order
to minimize the effect of diffraction and therefore achieve an image of maximum
definition the diameter of the pinhole show be based upon the following formula:
d = square
root of (0.0016F)
Where d is
the pinhole diameter and F is the focal length.
To put it another way, for a given pinhole you should construct a camera that
has a focal length derived from the following formula:
F = 625(d
squared)
Where d is
the pinhole diameter and F is the focal length.
As stated, the quality of the pinhole image depends upon the size and sharpness
of the hole. The definition produced by any given pinhole is determined by
the size of the image patch corresponding to a point object. The diameter
of the image patch formed by the rays of light from a point source is given
by the following formula :
D = d ( u
+ v )/u
Where D = the
diameter of the image patch, d = diameter of the pinhole, u = the distance of
the object from the pinhole and v = the distance of the image from the pinhole.
Therefore if the distance of the object from the pinhole, ( u ), is greater
than the image from the pinhole, ( v ), even a large pinhole will give a reasonably
sharp image.
A pinhole of about 1/64 inch diameter will be found satisfactory for normal
photography with a pinhole to image distance of up to 6 inch.
Formulae
for determining the optimum exposure for any given pinhole diameter.
The exposure
depends upon the size of the pinhole and its distance from the film plate, i.e.
its f-value. This is calculated exactly as with any other lens. A 1/64 inch
pinhole 10 inch from the film plate will have an f-value of :
10 x 64/1
= f 640.
One factor
to take into account whilst determining the correct exposure is ' reciprocity
failure '.Photochemical theory states that, providing the exposure ( light
intensity x time ) remains constant, the photographic emulsion should respond
in a consistent manner. For example, an exposure of f11 at 1/500 second should
produce the same density of negative as an exposure of f5.6 at 1/2000 second.In
the field, photographic emulsions, when working at the extremes of exposure
i.e. very long or short exposure times, do not adhere to this law. This is
known as reciprocity failure.As exposures required, when practicing photography
via the pinhole camera, tend to be very long, reciprocity failure has to be
taken into account.In order to compensate for this effect it is possible to
use the table below.
Indicated
exposure x Compensation factor = Adjusted exposure
