newtoyou wrote:
Would adjusting the pinhole to sensor or film distance make a difference in focus?
Bill
No. Pinholes do not have a focal distance per se--although in some ways they act as if they did!
(It's one of those "dolphins are not fish" things.)
For a given pinhole and format, if you change the pinhole-to-film distance, the biggest change is to
the angle-of-view. A short projection distance can create a very wide angle view. However, you
will have to change to a different diameter pinhole to get the sharpest image.
It's common to talk about a pinhole "focal distance" or "back focus", but this simply refers to the
maximally-sharp pinhole-to-film distance. Light is not being focused. At other distances, there is
no defocus. It's just not optimally sharp.
Lenses have a focal distance because two rays emitted by the same point in an object that follow
different paths and strike the lens at different places, are refracted so that they converge on the
same point in image. That's how the lens forms an image, while "gathering light" to do it.
Pinholes also make an image, but in a different way. A pinhole does not gather light. It simply blocks
all the rays except those passing though a very small aperture. The only way two rays emitted from the
same point can sneak though is if the angle between them is very small. The net effect of all such rays
is the circle-of-confusion that represents that object point.
For objects at infinity, the circle-of-confusion is simply the diameter of the pinhole. Simple!
Finally, pinholes have a hyperfocal distance (just as lenses do): any object from that distance to infinity
will be "acceptably sharp". With lenses, what's "acceptably sharp" is up to the photographer, and
usually depends on the resolution of the film or sensor, or on the viewing distance for the final image,
or some such practical consideration.
With pinholes there is a lot more diffraction, and making the circle-of-confusion (predicted by geometric
optics) smaller than the Airy disk buys you nothing. So as soon the object is far enough away so that the
circle-of-confusion is smaller than the Airy disk, it's always "acceptable". So that's the hyperfocal distance.
If you're willing to tolerate a larger circle-of-confusion, the the hyperforcal distance will be even less.
I perfer to define the hyperfocal distance of the pinhole as the distance for which one would expect an
circle-of-confusion the same size as the Airy disk (the center of the Airy pattern). Everything beyond
this distance is equally sharp (or if you prefer, equally unsharp).
So in practice pinhole cameras are somewhat similar to use to fixed-focus cameras--except that the hyperfocal
distance is much shorter (a few feet to a few inches).
BTW, this is not the way it's explained in most books, because in geometric optics there is a "camera model"
used for explanatory purposes, which excludes infinity. In theory, infinity (collimated rays) is a special case.
But in pinhole practice, most subjects effectively at infinity.
A pinhole has no aberrations or distortion. The only source of unsharpness is diffraction--which is a completely
different animal than defocus. If only there were some distance at which diffraction went away! But that airy
disk extends from the pinhole at a fixed angle, and the only way to reduce that angle is to use a larger pnhole
(which means a bigger circle-of-confusion) or a shorter wavelength of light.
If a blue filter makes the pinhole image sharper, then the pinhole is limited by diffraction (the right size or too small).
If it doesn't then the pinhole is too big.