## Focus concept for a pinhole camera?

When we look at a photo that was taken with a pinhole camera, we see an image that is made up of countless circles. These are projected onto film through the hole. Their size is a function of the diameter of the hole. The circles are called "confusion circles" because they face each other; hence their boundaries are indistinct.

The size of these circles determines whether the image is perceived as “sharp”. When the viewer sees panes, the viewer perceives the image as blurred. If the circles are too small to be viewed as slices, the viewer will perceive that the image is well focused.

The size of the hole is key. If they are too small, two demons of interference and diffraction induce a blurred image. If it's too small, the exposure time will be too long. We're enlarging the hole to make the image more bright, and this adds to the confusion. Now we need to get out of the hole and replace a lens.

What size circles of confusion? A disk from a distance of 3000 diameters is shown as a point. Thus, a 1 inch diameter coin is perceived as a point of no dimension when viewed from 3000 inches. That's 250 feet. This is too strict for photography due to the contrast of our media and viewing conditions. So we define the circle size as 3.4 minutes of arc, which is 1/100 of an inch by 10 inches or 2/100 of an inch of 20 inches (reading distance). Converted to metric, it has a diameter of 0.5 mm, seen from 500 mm.

### Michael Clark

@Floris Do you take photos for the viewing pleasure of hawks?

### Michael Clark

@Floris There is nothing in this last paragraph that is not generally accepted in photo theory. Requesting a quote for the 3000: 1 ratio corresponds to requesting a quote for the "Sunny 16 rule" or the "1 / focal length rule".

### Alan Marcus

Reference CB Neblette 1965 "Photographic Lenses. Arthur Cox 1974 Photographic Optics.

### Carl Witthoft

You use "interference" and "diffraction" there somewhat loosely. Furthermore, the exposure time is irrelevant for the present question. And if you want to make claims about circles of confusion versus area, please provide the equations with a reference. @MichaelClark, whether or not they are actually "generally accepted" has nothing to do with responsible technical writing.

### Laurence Payne

@MichaelClark, Stack Exchange is possibly the best-peer-reviewed medium where not only can you find your answers but your questions torn to pieces, and the obsession with staying close to the subject is only seen on an Asian college course exceeded!