Pinhole cameras

[thebbqguy]

Is it unreasonable to enlarge a "good" 4x5 pinhole image to 18 x 18 for printing?

[Scyg]

Define "unreasonable". I've enlarged 35mm pinholes to 8x10, which would be a far greater enlargement factor. They certainly didn't look like 8x10 contact prints, but that wasn't the point of doing it.

[Jim Jones]

The sharpness of a pinhole image is limited by
1) the diameter of the pinhole - the image will have a blur spot size of at least the pinhole diameter
2) diffraction, which makes a spot size proportional to the f-number. f-number = focal length/aperture diameter. . . .

An interesting phenomena occurs when a pinhole diameter is near the crossover size for minimum diffraction limited and geometry limited image blur. At that point, diffraction actually enhances sharpness, and the image of a 1951 USAF test chart will resolve line pairs finer than either diffraction or geometry predict. This is explained better here: http://inside.mines.edu/~mmyoung/PHCamera.pdf. This is most true for images on or near the pinhole axis. From near the edges of pinhole images, the pinhole appears elliptical, not round, and this introduces astigmatism as well as increased diffraction limitation. This can be seen in the chart below:

[Scyg]

With very wide angle cameras it is also worth considering the ratio of the pinhole's diameter to its thickness - you're effectively passing light through a tube, and the longer the tube, the more constrained the light path towards the edges of the image. For my panoramic cameras I ended up using .0005" (that's half a thousandth of an inch) stainless steel shim stock, which significantly improved performance at the edges.

[Greg]

An interesting phenomena occurs when a pinhole diameter is near the crossover size for minimum diffraction limited and geometry limited image blur. At that point, diffraction actually enhances sharpness, and the image of a 1951 USAF test chart will resolve line pairs finer than either diffraction or geometry predict. This is explained better here: http://inside.mines.edu/~mmyoung/PHCamera.pdf. This is most true for images on or near the pinhole axis. From near the edges of pinhole images, the pinhole appears elliptical, not round, and this introduces astigmatism as well as increased diffraction limitation. This can be seen in the chart below:

Thanks your for post. For my pinhole cameras I usually make around 20 pinholes and test them at the focal length that I am going to use them at with different frames on a roll of 35mm film. Inevitably, even though they differ by the slightest in diameter, one pinhole always produces an image that is superior to the others. That is the pinhole that I use on my LF or ULF pinhole camera. Instead of a USAF test, no longer have one, I use a stop sign about 40-50 feet away.

[Jim Jones]

Greg -- You may find this link useful: https://jimdoty.com/learn/lenses/usa...usaf_test.html.