# What are the challenges of creating a pinhole camera with short focal length?

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?

• 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
• @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