Site Updated
June, 2004

PINHOLE VARIABLES

There are two main variables with a pinhole camera:

pinhole to film plane distance

size (diameter) of the pinhole aperture

Since a pinhole does not focus light as a lens does, changing the pinhole to film plane distance (focal length) has little effect on image sharpness however, when the image is examined critically, it is found that there is an optimum pinhole diameter for any given pinhole-to-film-plane distance.  Increasing the size of the pinhole from optimum allows more light to pass through and decreases the exposure time but it also decreases image sharpness.   As a general rule the smaller the hole the sharper the image and the longer the exposure..    Decreasing the pinhole size does not necessarily increase image sharpness for when the size is decreased beyond the optimum for the specified distance diffraction causes a decrease in image sharpness.

a satisfactory pinhole camera is a compromise between
 acceptable image sharpness and acceptable exposure time

There are a number of OPTIMUM PINHOLE DIAMETER formulae in the literature - try this one:
                       ¹/₂₅ the square root of the distance between the pinhole
                                       aperture and the film plane (focal length)

Above formula applied to a 80 mm (3¹/₈ inch) focal length camera
                       ¹/₂₅ the square root of 80  mm = 0.35 mm.

The PINHOLE APERTURE can be made from any suitable material with any suitable hole drilling technique however, a carefully made aperture in stiff, thin metal will produce superior results.   Aluminium, brass or copper sheet metal (approx. 0.3 mm) is ideal for making pinhole apertures.  Aluminium pie/baking dish or soft drink cans are suitable (domestic aluminium foil is too thin/difficult to work with and is not recommended).   Brass shim used in the automotive/brake lining industry is ideal but more difficult to find - art & craft suppliers may have brass and/or copper sheet metal 0.2-0.4 mm thickness.

The pinhole aperture is usually made with a hand sewing needle as they are manufactured to a standard diameter

RELATIVE APERTURE
Derived from focal length (F/L) divided by the pinhole aperture e.g.

80 mm (31/8 inch) focal length of camera  80 0.5 = f160  #10 sewing needle diameter = 0.50

EXPOSURE
It is possible to predict pinhole camera exposures based on:

• f 16 rule
• reciprocity data

When the following variables are known:

• relative aperture
• approximate ISO of the light-sensitive materials.   

f-STOP PROGRESSION
Standard lenses provide a range of familiar f-stops e.g. f1.4 to f22

The key to understanding f-stop progression is the knowledge that each successive f-stop increases logarithmically by the square root of 2 (i.e. 1.4)
.
 f1.0    f1.4     f2.0    f2.8     f4.0    f5.6     f8.0    f11.0      f16.0    f22.0

Note the doubling/halving relationship between each alternate number:

 f1.0                f2.0                f4.0                 f8.0                    f16.0
           f1.4                 f2.8                f5.6               f11.0^*                   f22.0

With this knowledge we can extend this series of numbers to accommodate minute pinhole apertures and predict exposures in a variety of pinhole cameras.  It is necessary to commit to memory any two numbers in the series e.g. the first   2 numbers f1.0 and f1.4 (my preference) and extend the series thus:
f1   f1.4  f2   f2.8  f4   f5.6  f8  f11^* f16  f22  f32  f45 f64  f90 f125^* f180 f250 f360 f500 f720 f1000

 * by convention, f11.2 is rounded down to f11, and similarly, f128 to f125.  

INTERVAL OF TIME/SHUTTER SPEED:
Is a arithmetic progression - conventional cameras operate in the fractions of a second range i.e.  1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2

The interval of time for the shutter to remain open in pinhole work is more likely to be in the whole seconds range e.g. 1.2.4.8.16.30.60.120.240.480.560.1120.etc.                                                                           15
In pinhole practice, rounding interval of time numbers up or down will make the calculations easier, for example, amending 16 seconds to 15 simplifies the numerical progression: 15 . 30 . 60 . 120 . 240. 480 etc. which convert to whole minutes with ease.  

Allowing the shutter to remain open for twice the interval of time will double the amount of light entering the pinhole (in conventional photography this action is referred to a decreasing the shutter speed).   Halving the length of time the shutter remains open will halve the amount of light entering the aperture (referred to as increasing the shutter speed).