Is there a concept of "focus" and thereby defocus for a pinhole camera? If yes, then what is it? If it is assumed that only one ray of light enters in through the pinhole from every point on the scene, that would mean that every point is in focus?
But is there a definition for focus of just one ray?
When we view a photograph made by a pin-hole camera, we are viewing an image comprised of countless circles. These are projected on film by the pin-hole. Their size is a function of the diameter of the pin-hole. The circles are called “circles of confusion” because they juxtapose each other; thus their boundaries are indistinct.
It is the size of these circles that determine if the image will be perceived as “sharp”. If the observer sees disks, the observer will perceive the image as being fuzzy. If the circles are too small to be seen as disks, the observer will perceive the image is being in good focus.
The size of the pin-hole is the key. If too small, twin demons of interference and diffraction induce a fuzzy image. Also, if too small, the exposure time becomes too long. We enlarge the pin-hole to gain image brightness, and this enlarges the circles of confusion. Now we must abandon the pin-hole and substitute a lens.
What size circles of confusion? A disk viewed from 3000 diameters distance appears as a point. Thus a 1 inch diameter coin viewed from 3000 inches is perceived as a point without dimension. That’s 250 feet. That’s too stringent for photography because of the contrast of our media and viewing conditions. So we define the circle size as 3.4 minutes of arc, which works out to 1/100 of an inch in diameter viewed from 10 inches, or 2/100 of an inch viewed from 20 inches (reading distance). Converted to metric, it’s 0.5mm in diameter viewed from 500mm.
Depth of field for a pinhole camera is theoretically infinite. There is a formula to determine optimum pinhole size for any given focal length, the distance between pinhole and film. In practice we choose a hole size small enough to give good sharpness, big enough (and accurately round enough) not to produce diffraction effects.
My two cents on this topic.
There are two similar concepts. Projection and focus. I will make a step-by-step list.
1. A scene can be viewed when it receives light and the light is reflected. Depending on the materials light can be reflected in a lot of directions (dispersed) when it hits a diffusive material.
2. When you can isolate some of those rays making them pass through a hole and forget about the rest, and when the light inside the room is dark enough, you get a projection on the opposite side. Some rays from specific sources on the scene A will hit different zones of the wall B, C, D
3. If the hole is small enough the rays are projected on a specific point on the wall because the others are left out.
4. When using a lens, you take those additional rays C, D and bend them using refraction so now they also hit the intended point on the wall B.
So far so good.
In photography a focused point is a point of the projected image, but only when the most amount of rays coming from a specific point from the scene (A) are hitting the same point (B).
When using a lens you refract them so they hit that point. On a pinhole camera, the only way to do that is not to allow those divergent rays to enter the room in the first place. The narrower the hole the more selective you are, the more focused the image you have.
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Not quite but there's an analogue of focus in pinhole photography. In lensed cameras the design allows to achieve very good focus (small size of the blur disk on film or sensor) but only near the focus plane, thus the size of the blur disks are dependent on the distance to camera. This is a direct consequence of the fact a lens focuses light on the film or sensor, thus the mechanism of focusing is inapplicable to a pinhole camera.
However, there is an analogue of focus in pinhole photography, and it is the result of the size of the blur disk varying depending on the distance of the subject to the camera. In a pinhole camera the size of the blur disk is determined by two types of blurs: geometric and diffraction. Generally speaking, diffraction blurs all objects the same amount irrespective of object camera-distance, while geometric blur is dependent on subject distance. This turns out to be important only for closeup work (subject distance < 10x focal length), so most pinhole designs disregard this effect and assume you are shooting more far-away objects. Note, that even considering this you cannot get a greater blur on more distant objects (as in a lensed camera) than on closer objects, just a more uniform blur across a greater subject-to-camera distance.
If you want to research this topic further search for the Prober-Wellman equation; see this article by Paul Prober and Bill Wellman.
