Sorry to hear about the kidney stone. *ouch* me@myplace.to wrote: > > This may be because you are accustomed to seeing horrible results from > badly out of focus pinholes. Oops, the morphine hasn't worn off yet ;-) A pinhole can't be in or out of focus. However, for each distance from the film plane there is an optimal size of pinhole. Smaller or larger than this optimum and the image will not be at its sharpest. Yeah, I know this *sounds* like focus, but it's not. This is a result of failing to achieve the best balance between the size of the circle projected on the film plane by a point light source and the disk produced by the difraction of the light as it passes through the hole. Quite clearly this also involves object distance as well as the distance from the pinhole to the film plane, but you rarely need to worry about this unless you're making pinhole lenses for closeup work. Incidentally, many of the web sites you refer to are not applicable if you're using other than panchromatic film. Most of them use the wavelength of green light to determine the best size. On either side of this the calculations will yield a slightly larger or smaller pinhole. Thus, if you're using multigrade paper as "film" you should use a different size pinhole. And if you're using orthochromatic film, another size pinhole. This also applies to the colour of the subject. If you have a predominantly blue subject and a predominantly red object (and panchromatic film) you'll find that the optimal pinhole for each (all other things being equal) differs quite significantly. People who don't know any better, or people who are making casual coments may be able to get away with calling this a "focal length", but it isn't. http://www.stanford.edu/~cpatton/ip-ph.html Yes, I get all of my photography advice from the Hopkins Marine Station :-) > This ignores the fact the formula for finding the focal length for a given pinhole is > diameter of pinhole times itself times 750 No, that determines the optimal size for a given "focal length". In pinhole photography the distance from the pinhole to the film plane is the "focal length". > This shows just how sharp a pinhole shot can be if it is exposed at the > proper focal length for the size of the pinhole. When you move a pinhole > back and forth you are NOT changing the focal length, you are just moving > it in and out of focus. You're Sooooo close. The first statement is correct. The second should more properly be: "When you move a pinhole back and forth you are changing the focal length, NOT moving it in and out of focus -- but at the new focal length the pinhole size may not be optimal" This is similar to you telling me that if I can demonstrate that a lens has anoptimal f/ stop for the sharpest image, that I am changing focal length when I change f/ stop. The same is exactly true of a pinhole. It has no focal length -- the term focal length simply refers to the distance from the film plane. However it too has an optimal f/stop, which is essentially the only variable -- aperture. > You could also just search on <pinhole focal length> or <pinhole theory> > in Google and spend a few moments perusing the results. Just remember not to rearrange the normal relationship between cause and effect. Steve
