Alley in Pisa, Italy

by Lars Kotthoff

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I have found two challenges of creating a pinhole camera with short focal length (by focal length, I mean the distance from the pinhole to the film):

  1. Pinhole size: Short focal length means we need a small pinhole, which is hard to make.

  2. Vignetting: The light has to travel further to the edges of the film (compared to the center) which means there will be vignetting.

Are there any more effects I haven't understood? Does the fact that the light has to travel further to the edges of the film (compared to the center) cause any aberrations etc?

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Technically a camera without a lens can't have a focal length. I'm not sure what the equivalent is for a pinhole camera though. –  DJClayworth Nov 5 '13 at 17:39
    
The vignetting is not (directly) caused by the distance difference to the edge of the film, but due to the thickness of the punctured cover ("lens"). Indirectly, of course, having a big difference in the distance means that you have a large angle of view, which means that you need a thinner cover so extreme angled rays are not blocked by the hole sides. –  ysap Nov 5 '13 at 19:07
2  
@ysap Pinhole cameras suffer from "natural vignetting" (the cos^4 law) no matter how thin the material is. You get two powers of cos(theta) from the inverse square law (as the OP points out), one power from the light illuminating a larger film area off-axis, and one power from the hole "looking" smaller off-axis. cartage.org.lb/en/themes/arts/photography/photproces/… –  coneslayer Nov 5 '13 at 21:05
    
Interesting - I did not think about these effects. I think, however, that for ideal pinhole camera, with infinitesimal hole size, this should not be a problem (but other problems, like diffraction may arise). –  ysap Nov 6 '13 at 20:06
    
For maximum sharpness, you have to make a hole that is both small and circular. The edges of the images will suffer from both vignetting and reduced sharpness. I’ve made a 6×9 (really 56×84 mm) pinhole camera with 25.5 mm focal length. That’s about the shortest reasonable. Vignetting is strong but I can live with it. You could use a curved film surface to reduce the vignetting, at the expense of some distortion. –  Edgar Bonet Nov 13 '13 at 10:31

2 Answers 2

up vote 5 down vote accepted

You need a very thin wall for the pinhole, I'm not sure that it has to be smaller. The mechanism of a pin hole camera is very simple. It simply blocks light from anything other than a single point and the rays from that point get projected to the opposite side of the enclosure. If the walls of the pinhole are thick, then the light can't come from an extreme angle because it hits the inside of the hole rather than making it through, but as you make the wall thinner, it is harder to have it remain opaque.

There shouldn't be any aberrations I can think of because there are no actual optics to cause prisming. There might be some diffraction issues possibly due to the small hole, but that's all I can think of. This wouldn't be due to the traveling to the edge of the film though, it's still just light continuing on its original path from its original point and blocking the other light.

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The pinhole needs to be small in order to produce a sharp image, but not so small that diffraction becomes a problem. How it works - Size of the Pinhole talks about the size of the pinhole, the diffraction limit, and provides some sample images that illustrate the point.

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