From: Katharine Thayer (kthayer@pacifier.com)
Date: 12/19/02-04:31:53 AM Z
Shannon Stoney wrote:
>
> >
>
> > At 2:23 PM -0800 12/17/02, Shannon Stoney wrote:
> >>Katherine wrote:
> >>
> >> One interim solution I've come up with
> >>> is to fix a pinhole to a lensboard and stretch the bellows out as far as
> >>> it will go, which is around 30".
> >>
> >>That's interesting. So, in effect that makes a pinhole telephoto aperture?
> >>
> >>--shannon
>
> Sam wrote:
> >
> > No, Shannon. Not a telephoto. Just a long lens. A telephoto lens of
> > 30" focal length will physically measure maybe just 10".
> >
> > If you load your 8x10 holder with a piece of 35mm film and put it
> > behind your 12" lens, you'll get a narrow field of view, but the 12"
> > lens would not have transformed into a telephoto lens.
>
> But, what if you change the pinhole aperture to have a focal length that
> matches the bellows extension of 30"? I'm looking at the chart in Eric
> Renner's book on p. 124 where he gives different pinhole sizes for different
> focal lengths. My pinhole camera is six inches deep, takes 8x10 film and
> has an aperture of f352. That's the "correct" pinhole aperture for
> sharpness at six inches focal length. So it's wide angle and it's in focus.
> If you wanted it to be telephoto and in focus, couldn't you increase the
> bellows length to say 30" as Katherine says, and make an aperture that would
> cause the image to be in focus, ie the "correct" aperture for a 30" focal
> length? (Eric's chart would say 1.0267 mm, or f750.) Wouldn't that be like
> making a pinhole telephoto image, in the same way that my camera makes a
> wide angle image? I wonder if the image would look like a telephoto image,
> sort of, taken with a lens.
>
> --shannon
I'm coming back into this discussion after it's sort of over, but it
seems to me that all the intervening arguments about the technical
difference between a telephoto lens and a long focal length lens have
confused the issue unnecessarily, and that perhaps a summary is in
order. Shannon wanted to know if you can get the same image through a
pinhole aperture a certain distance from the film as you would with a
lens of the same focal length. The answer is yes, and it's really quite
irrelevant whether the lens you're using for comparison is a telephoto
lens or a long focal length lens; if it's a 30 inch lens, the image on
the film made by a pinhole 30 inches from the film taken from the same
place will include the same elements in the same aspect as the lens
will.
In Shannon's comment above, it seems focus and sharpness may be
confused. As opposed to a lens, which focuses at a set distance, a
pinhole has infinite depth of field and so everything from foreground to
horizon will be in perfect focus. The sharpness of the image is
determined by the pinhole aperture and its relationship to the distance
of the pinhole from the film. I'm confused by Joe's assertion that in
pinhole, the terms wide angle etc refer to the thickness of the shim. I
have to disagree; in my experience the terms wide angle, normal, etc
refer to the focal length just as they do in lens photography. If the
focal length is approximately the same as the diagonal of the film, then
it's a "normal" length pinhole camera; if it's shorter than the
diagonal, then it's a wide-angle pinhole camera, and it's longer than
the diagonal, then it's a longer-length pinhole. Some pinholers would
call that longer-length pinhole camera a telephoto pinhole, but we sure
wouldn't want to do that here. But the thickness of the shim doesn't
affect the aspect ratio, does it Joe? This doesn't make sense to me. I
have found in fact that a thicker shim, or one that's not properly
sanded, doesn't change the area covered but it sure does cause some
light-refraction and reflection problems.
Katharine Thayer
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