John,
Before I got your second message, I wrote, but didn't send:
I'm glad that idea seems to help, but I can claim no personal credit.
When Bill Gottesman mentioned the technique in an email message I
didn't believe him at first. He, and others, showed me the light, and
convinced me that sunlight shining through a hole held parallel to a
sheet of paper would cast a round spot.
BUT see below...
Problem solved!
Mac Oglesby just solved the image ellipse problem that I had with the
pinhole sharpener when the sun is low. I had been holding my sharpener
perpendicular to the sun's rays which produces an elongated image of an
ellipse around sunrise and sunset. He said that if I hold the sharpener
parallel to the ground, that the image becomes round. I didn't believe him
and thought it impossible, but both Edley and I just tested it and Mac's
right! Although as Edley noted to me, the image is dimmer, but that's no
problem.(Using some tilting of the planes because its midday here).
Thanks Mac!
John
John L. Carmichael Jr.
Sundial Sculptures
925 E. Foothills Dr.
Tucson Arizona 85718
USA
Tel: 520-696-1709
Email: [EMAIL PROTECTED]
Website: <http://www.sundialsculptures.com>
-
Now, I would respond differently, for I think Rob Vugteveen is
probably correct. I assumed that, since a small hole in an index card
gives a round spot if held parallel to a projection screen, that that
would continue to be the case no matter what the distance between the
hole and the screen might be.
Curiously enough, about 5:00 this afternoon I was outside,
experimenting with an eighth inch hole. I was surprised that the
image I saw on a white sheet of paper (I could see NO usable image at
all on the black pavement) was apparently elliptical, not round, when
the hole was held far enough from the screen to form an image. The
image WAS round if the hole was fairly close to the screen. This
agrees with Rob's observations. Sorry to have passed along my
misunderstanding. I imagine that the truth of the matter was clear to
Bill and the others. And now we know, too!
As I played with the sun and the hole, I had difficulty deciding
exactly where, as I moved the hole, the image of the sun was
precisely half covered by the shadow (of my roof, in this case). The
possible error looked as though it might be as much as an inch.
This is turning into a very enlightening project.
Mac
Mac
I just got this letter from a person at Flandrau who will be helping us. He
tried to do as you suggested but got different results. What's your take on
his letter. (see below).
John
John L. Carmichael Jr.
Sundial Sculptures
925 E. Foothills Dr.
Tucson Arizona 85718
USA
Tel: 520-696-1709
Email: [EMAIL PROTECTED]
Website: <http://www.sundialsculptures.com>
----- Original Message -----
From: "Rob Vugteveen" <[EMAIL PROTECTED]>
To: "John Carmichael" <[EMAIL PROTECTED]>
Sent: Thursday, June 13, 2002 2:03 PM
Subject: Laws of Pinhole Optics suspended?
> John,
>
> Mac Ogelsby's solution of aligning the plane of the pinhole sharpener to
be
> parallel with the plane of the ground is not really valid. Sorry, but
Mac's
> WRONG!
>
> I tried it here using a 20-foot image throw on a variety of sloped
surfaces.
> The solar image produced by a 1/4-inch hole in an 8.5x11 inch card does
not
> change aspect ratio as I tilt, tip, and otherwise manipulate the plane
that
> the pinhole sits in. It only changes the image intensity because the cross
> sectional area of the pinhole is changed (like changing the f-stop).
>
> You can demonstrate why this is so by drawing this out geometrically. A
true
> "pinhole image" occurs when the diameter of the pinhole is insignificant
> compared to the distance between the image and the pinhole. In that
> geometry, the pinhole is considered a point source, and the shape of the
> pinhole has no bearing on the shape or size of the image. Even the slight
> variation in distance between the "front" edge of a tipped pinhole and the
> "back" of it has no effect on the aspect ratio of the cast image.
>
> However, if you hold the pinhole sharpener too close to the ground, you
end
> up casting an image of the hole instead of getting a true pinhole image of
> the solar disk. Then you actually would have seen the aspect ratio of the
> cast image change as you tip the card.
>
> Here's how to tell the difference: If you have a true pinhole image and
the
> shadow of the style is on your left, then the image of the sun will be
> occluded from the RIGHT as you move the sharpener back and forth across
the
> edge of the shadow, because the pinhole inverts the image.
>
> But if you are imaging a shadow of the hole itself (because the hole is
too
> close to the ground) then the shadow of the circle is on the left. That
> situation won't help you one bit in marking the lines you want.
>
> As you use the shadow sharpener, there will still be some ambiguity in
> location because you are subjectively determining where the image of the
> solar disk is "halved." I'm thinking about this problem...
>
>
> What do you think?
>
> -Rob
>
>
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