Pinhole camera construction should always start with a careful design. The reason for this is rather simple: if you follow some widely spread pinhole “science” and go with a Cola can approach (or matchbox or whatever container you fall for), you end up with something that may or may not produce an image, but more importantly, even if it does, the result may be quite discouraging. It goes without saying: pinhole photography is for real and unless your desire is to just see that oatmeal box produced image (than by all means knock yourself out), you should gain a good grasp of pinhole inner workings.
Pinhole principles should prove fairly easy to understand. There are just a few important terms every pinholer should recognize, differentiate and understand.
- pinhole: the light restraining opening of relatively small size (in place of a traditional lens), its optimal size is limited by two counteracting principles, one calling for it to be as small as possible (so light rays are as best focused as possible), the other restricting minimum size due to diffraction
- pinhole diameter (HD): actual cross dimension of the hole
- optimal pinhole diameter (OPD): best compromise between smallest possible hole for highest resolution vs. smallest required to fight off diffraction, both differentiate OPD by a factor of 1.225, diffraction principle derives the smaller of the two, there is no clear consensus on which is more appropriate, both appear to be equally favored, although the larger one seems to have taken a lead in semi-commercially available apertures
- film to hole distance (FHD): distance from film plane (curved or flat) to the plane of the pinhole itself, this term is often equalized with focal length (which technically is incorrect as there is no focal point associated with pinhole), here FHD will be used exclusively
- angle of view (AoV): what film actually sees or records from light rays passing through the hole, it is directly related to film hole distance (FHD) and hole diameter (HD)
- angle of coverage (AoC): what the hole itself is capable of covering, this is a function of HD, but especially the thickness of material the hole is made in, typical AoC is in the 125 degree range, but can go further (especially if a curved film plane is employed)
- light fall off: difference in film illumination between center of the frame vs. outer perimeter
- vignetting: result of light fall off showing as darkening of recorded scene as it approaches edges of film frame, it can be controlled fairly well with careful camera design, it is however appreciated by many photographers as being part of the pinhole “charm” and often desired, vignetting will always occur in cameras with AoV approaching AoC and is very severe when AoV < AoC down to a complete blackening of frame edges
- depth of field: is generally the measure of scene’s depth within which acceptable sharpness is present, while in theory there is only one plane of sharp focus, due to resolving power of human eye subjects in front and behind such a plane also appear sharp, pinhole photographs have an inherent extreme depth of field and while they are never as sharp as lens derived photographs, they do show a depth of sharpness that is astonishing (keep in mind, this is actually possible only with carefully designed and properly executed cameras)
- f-stop or aperture: measure of light gathering power of an optical design which in our case is a function of the film to hole distance (FHD) and the hole diameter (HD), in pinhole photography f-stops are typically much smaller, often reaching numbers north of 300 (the larger the number the less light coming through) than in lens designs which generally only reach 64 in some large format designs, 32 for medium films and 22 in 35 mm, the aperture has direct impact on exposure duration
- pinhole plane (PP): the plane within which the pinhole is mounted, typically it would be placed square to optical axis of the film plane, but as pinhole would often have it, it is sometimes altered for desired (or not) distortion and perspective wow factor
- film plane (FP): the plane within which the sensitized material is placed, film plane can be flat or curved (other shapes are possible, but I’ll leave that on the wow side); the flat plane has main advantage of not having straight line projection distortion (and by extension better perspective control of final image), but also introduces light fall off due to changes in film to hole distance from film center to outer edges (this in itself brings exposure challenges, later on that); the curved plane (in its standard approach) is based on a circle whose radius is equal to film to hole distance (FHD), thus removing light fall off on curved axis, the major drawback is projection issues (again, an attraction to some users) due to curved plane, point of view, and straight lines not being rendered as such for most viewers (later on that), to facilitate better control of projection issues some design elements can be featured, this however complicates overall design
- exposure: amount of light allowed to strike the sensitized material in the camera, this is controlled by sensitivity of recording material and duration for which light is allowed to reach it, such a duration is further a function of overall camera design, pinhole size, its distance from film plane, and most obviously strength of light available at the time of exposure (this topic will be later discussed in detail)
- parallax: offset of optical axes of imaging and viewing paths, when viewing takes place through imaging path (as in digital cameras, SLRs or most large format designs) there is no offset and image viewed is same as one recorded, in all other cases where imaging path is completely enclosed and previewing of scene must take place outside of it, the two do not coincide unless some compensation is used; in pinhole photography parallax is seldom discussed (even any sort of viewing appliance is often omitted), which it appears is mostly due to general disinterest in seeing any importance in this, the fact however is, that if we envision a pinhole photograph approached with similar to lens based precision, then we should aim to control every possible aspect of image making; knowing what the film sees is certainly one of them
- diffraction: tendency of light rays to bend as they pass an edge or obstruction, thus they do not continue on a straight path from origin to destination, this effect has a major impact on pinhole image perceived sharpness and is the limiting factor of pinhole’s minimum diameter
Above is a diagram with a simple design case study for a 120 film and three desired frame sizes for both flat & curved film plane. Some principles are clearly visible. You can also download a PDF version of it below.