Is there any sense in talking of focusing a pinhole?

[JAMESCF]

If you have a pinhole of say f250, is there any sense at all in speaking of its focus, will there be any visual change of the focus if it is extended beyond its focal length?

I am imagining that in designing a pinhole camera one might create a fixed focus point that is extended beyond what would be considered the pinhole’s focal length [?] to favour foreground subjects

A pinhole must have a focus range? If it is positioned to an extreme outside of this will the whole image be uniformly out of focus or will there be some subtle depth of field that could be artistically exploited?

(Forgive my patchy understanding of pinhole arithmetic if this is nonsense)

Thanks,
James

 

[Joanna Carter]

If you have a pinhole of say f250, is there any sense at all in speaking of its focus, will there be any visual change of the focus if it is extended beyond its focal length?

Quick example - a 72mm lens on a 5" x 4" camera, at f/64, gives you a hyperfocal distance of 2ft and a nearest acceptably sharp object at 1ft. So, I think at f/250, you can be fairly sure that everything will be "in focus", but might not appear totally sharp, due to diffraction and not depth of field.

 

[David M]

A specific pinhole will give optimum results at a specific distance from the film. There are calculators available on the web. You will almost certainly see little difference with modest deviations from this. The quality of the image is also affected by other factors, like the shape of the pinhole (which can be used creatively) and the smoothness and thickness of the pinhole’s edge.
However, there are different ideas about that word optimum.
It’s a tug o’ war between resolution (smaller hole) and diffraction (larger hole). So there will be a subjective element. Some people rather like fuzziness. One man’s meat and all that.
On top of all this, there’s the small matter of practical exposure times. With f-numbers in hundreds and reciprocity snarling in the background, this can be a real problem. Not insurmountable; people have made year-long exposures of the Sun’s trajectory.
I believe that for close distances the calculations differ, but I haven’t explored this. Although we conventionally speak of a pencil of light, it’s really a very narrow cone so when the pinhole-to-film distance and the subject-to-pinhole distance are similar, this becomes significant. For distant landscapes, the cone is narrower and thus more close to parallel.

 

[karlt]

If you change the focal length from the optimum that that size pinhole was designed for then you also affect its aperture.

90mm @ f250 = pinhole of 0.36mm

180mm with a pinhole of 0.36 = aperture of nearly f65

If I have my calculations right?

 

[David M]

90 x 0.36 = 32.4
180 x 0.36 = 64.8

As the pinhole moves away, it appears smaller to the film, just as it would appear to the eye.
Although it’s a handy expression, it’s not a focal length, simply pinhole-to-film-distance. A pinhole can’t focus. A lens can increase sharpness, but at the cost of increasing blur at other distances.
Using a pinhole is a juggling act between the factors involved. Aesthetically, a non-optimum image might be more satisfying, but this isn’t a promise.