# Concept of focus for a pinhole camera?

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?

• In effect, "f" is very large so the depth-of-field is also very large. Commented Mar 15, 2017 at 23:50
• Be careful with terminology! In camera-talk, 'f' is focal length. In a pinhole camera it will be a few inches, the distance from pinhole to film. The aperture will be a small fraction of f, so what we loosely call the 'f stop' will be a fraction with a large denominator. Commented Mar 16, 2017 at 0:14
• I think is erroneous to assume that there is just " one ray of light " entering a pin hole. I am attempting to wrap my head around this quora.com/Do-we-know-the-size-of-a-single-ray-of-light Also a pinhole camera can have a focal distance (distance from pinhole to film plane ) of a fraction of an inch to many feet. Commented Mar 16, 2017 at 10:19
• @LaurencePayne which is a good reason to demand capital-F for F-stop :-) Commented Mar 16, 2017 at 11:28
• Of course there isn't just one ray of light. That's the theoretical ideal case, in which every point on the picture would be pin-sharp. In a real pin-hole camera the hole has a finite size, the picture is blurred. But uniformly blurred, there's no plane of focus. The ideal case would have diffraction issues, and require a VERY long exposure of course! Commented Mar 16, 2017 at 11:39

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.

• @Floris Are you making photographs for the viewing pleasure of hawks? Commented Mar 16, 2017 at 5:05
• @Floris There's nothing in that last paragraph that isn't generally accepted in photographic theory. Requiring a citation for the 3000:1 ratio is like requesting a citation for the "sunny 16 rule" or the "1/focal length rule." Commented Mar 16, 2017 at 5:07
• Reference C.B. Neblette 1965 "Photographic Lenses. Arthur Cox 1974 Photographic Optics. Commented Mar 16, 2017 at 6:12
• You're using "interference" and "diffraction" a bit loosely there. Further, exposure time is irrelevant to the question at hand. And if you're going to make claims about blur circles vs. range, please post the equations with a reference. @MichaelClark whether or not they are in fact "generally accepted" has nothing to do with responsible tech writing. Commented Mar 16, 2017 at 11:30
• @MichaelClark, Stack Exchange is possibly the most assiduously peer-reviewed medium in existence, where you can find not only your answers but your questions ripped to pieces, and the obsession on staying narrowly 'on topic' is rivalled only by an Asian college course! Commented Mar 16, 2017 at 11:51

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.