[pure-silver] Re: History... Clarification

  • From: Jean-David Beyer <jeandavid8@xxxxxxxxxxx>
  • To: pure-silver@xxxxxxxxxxxxx
  • Date: Mon, 19 Oct 2009 17:38:38 -0400

Ray Rogers wrote:
The stuff I answered in public were in reply to the questions asked in
public.

You are correct by virtue of the fact that I expressed the same interest
in both locations. But by and large a substantial amount of what appeared
to be a private answer reappears in your public response... which is fine
but somewhat schocking, as I wasn't expecting it.

Me:
I don't really grasp the physics behind why the diameter of images are
related to the intensty of the light, unless it involves light
"bleed"...

You: "I would not call it that...

If you consider the geometry of those zillions of pinhole cameras,
consider just one of them. The light that passes through will be brightest at the center and fall off to the edges. Now the brightness depends on the amount of light coming through the negative and the lens.
So with normal film, that is what you would see. But if you use high
contrast litho film, all brightness over a certain amount will develop
and those less than that will not. And the brightness is the brightness
of that circle (in the case of a round aperture in the lens)."


So under normal exposure condtions, rather than "light bleed" its more
like "light fall-off"?

Yes. The pinhole sees the aperture of the lens. If you draw the rays, you will see that all the light hits the middle and it falls off towards the edges, just as any lens does.

Your explanation does makes it much clearer!

What difference does round vs square apetures make?

Well, I use a round aperture because that is what my Compon-S f/5.6 180mm lens has. But square apertures are better (the H.D curve has a better shape). People also used other shapes for different reasons.

Note that usually, the screen was put in a camera and it photographed a positive image. For that a square or nearly square aperture is best. I photograph a negative image, and this actually works better, for me at least. And I do not have a square aperture I could easily use.

As an experiment, I made a crossline screen with 1/10 inch wide drafting tape with 1/10 inch spaces in between. It actually worked, although it was too course for routine work. 50 lines per inch is about the minimum anyone would want to use. With that screen, you could actually see that those holes were a pinhole camera. You could easily see the image of the negative, you could see the falloff, etc. Very interesting.

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