Real world differences in pinhole sized apertures

Question

I'm interested in experimenting with a pinhole camera and am considering the Ilford Obscura or Titan.

The focal lengths are slightly different and not of concern for me. What I'm really interested in knowing is if the slight difference in aperture, f/290 for the Obscura and f/206 for the Titan, make much difference?

Obviously, exposure times will vary, but we're already talking long exposures. At this small an aperture, are there differences in diffraction or any other image quality difference?

calocedrus · Accepted Answer

You can calculate an approximation of the size of the actual diffraction spot (or Airy disk) as a function of your pinhole f-number and wavelength of light:

D = 2 x 1.22 x lambda x F/#

Across the visible spectrum (from blue at about 400nm to red at about 600nm), you get:

For the Obscura at f/290: D(blue) ~ 0.28mm to D(red) ~ 0.42mm
For the Titan at f/206: D(blue) ~ 0.2mm to D(red) ~ 0.30mm

So you get less diffraction with the Titan and the influence of light wavelength on the spot size is smaller with the Titan.

But then enters in consideration the film you will use: if its grain (depending on its sensitivity) is larger than any of the above calculated spot size, then there will be no difference between the Obscura and the Titan.

Alaska Man · Answer

The size of the pinhole aperture is directly related to the focal length ( I.E. the distance from the pinhole to the film plane dictates the aperture needed.)

There is an optimum size of the pinhole to give the best image quality. I would surmise if both pinholes are optimized for their respective focal distances then the difference in quality is negligible, especially since the images created by a pinhole are not the crispest and sharpest of images.

quality and preciseness of the manufacturing process in making the pinholes will have a considerable affect.

Jindra Lacko · Answer

Both Titan and Obscura seem priced a tad high for what they deliver (essentially a black box with a small hole in front). In the pinhole community (and it is a lively community!) there is trend for DYI cameras.
But with the Ilford pinholes you get what you pay for; the diameter of the pinholes -  i.e. the f value - is both reliable (not always the case with a home made pinhole) and optimized for the focal length used. They display only little vigneting in corners (so they are not for you if you are after severely vignetted pinhole look).
Both cameras will perform well, and there might even be a marginal advantage in the more narrow aperture - longer exposure times are easier to control by hand (it is easier to measure e.g. 5 seconds than 1.7 second).
As for image quality: pinhole images do not enlarge particularly well, which is the point for shooting in 4×5".
One word of caution: with both cameras you will be in danger zone of reciprocity failure, so keep that in mind when calculating exposure.

Real world differences in pinhole sized apertures

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