# 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. Mar 15 '17 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. Mar 16 '17 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. Mar 16 '17 at 10:19
• @LaurencePayne which is a good reason to demand capital-F for F-stop :-) Mar 16 '17 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! Mar 16 '17 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? Mar 16 '17 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." Mar 16 '17 at 5:07
• Reference C.B. Neblette 1965 "Photographic Lenses. Arthur Cox 1974 Photographic Optics. Mar 16 '17 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. Mar 16 '17 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! Mar 16 '17 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.

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: (if the link will work try: http://www.huecandela.com/hue-x/pin-pdf/Prober-%20Wellman.pdf)