Re: Pinhole...urgent
KVANT@nbivax.nbi.dk
Wed, 12 Apr 1995 13:20:18 +0200
Richard Cayne wrote: on April 6.
> Am trying to build a pinhole in a can 3.5" in diameter and
height of can approx. 8" high. Used .001 inch brass shimstock and
> #12(.013") sewing needle to create hole in brass shim stock.
> Calculated fstop to be in the 220 range. Shot 1 shot of a library
> with reading on 1/30sec at f90 for 400ASA HP5 Plus film for shadow
> reading and calculated exposure for the pinhole to be approx. 25 min.
> including reciprocity factors.
> Film was adequately exposed with shadow detail however details were
> not as sharp as they should have been and fiber like starnds were seen
It is not quite clear what you mean by "should have been".
I will suggest two reasons:
1) Your actual hole is significantly larger than the 0.013" tool. Or:
2) Your hole is actually .013". In this case wave-optics leads to a
diffraction pattern: the "Airy pattern" with a diameter of the first
circle of zero-intensity of 0.75 mm for green ligth -- larger if the
picture were taken with a tungsten light source. So you should expect a bit
more fuzziness than geometrical considerations suggest.
For a reference to the Airy pattern: see any undergraduate textbook on
optics. eg. Hecht, Optics, Schaums outline series, McGraw Hill, 1975.
An optimum hole size for a camera of your dimensions should be sought
in the vicinity of 0.5mm diameter and you should expect to be able to
resolve some 1.5 lines/mm on.
> on the negative. I assume that the fiber like stands were due to inadequate
> preparation of the hole, but I did take care. Please comment on the
> following:
> 1)_ Is the brass shim stock selected too thin?
> 2) The fiber like stands -- caused by what?
The fiber like strands are more puzzling. If they are black on the negative
and your can has a reflecting surface it might be reflections, focused
by the cylindrical surface, so called "caustics" and I suggest that you
line the inside of your can with rough black paper.
This is only a unqualified guess however.
> 3) How do you really tell if you have a clean hole?
> By the way my hole was done under a bi-microscope at
> 90x.
Thats the way!
> 4) Whn I attempted to make another hole I noticed when holding
> the lens up to a regular light that I though I saw fibers
> or a fuzziness through the lens although under 90x it seemed
> clear. What the hang is going on? How to make the right
> hole? Needle used by the way was a No!2 sewing (hand) and
> was perfectly round.
Forget about it! It is an extremely complicated problem to solve. And
you will need detailed information of the optical scene, including any
irregularities in your eye in order to adress it.
I have good experience with holes made by burning a red-hot
(or somewhat cooler) needle
through black paper, black plastic tape or black plastic foil used for
packing sensitive electronic equipment.
QUESTION: I wonder why it is so important
to find the relatively best solution to a
very bad ( on a absolute scale ) optical system. But i just joined the list
and missed the former discussion. Can anyone give me a reference to a file
i can reach with FTP where i can find this discussion??
The best pinholes are probably ones *without sharp edges*. The problem is that
sharp edges gives you this diffraction ring system around a rather sharp
peak in light intensity when you image a point source.
A technique known by astronomers and laser-physicists as "apodization"
meaning "cut off the feet"(of the intensity pattern), gives a somewhat larger
central spot which then falls off smoothly to zero. It can be implemented by
using a pinhole with so called Gaussian transmission.
Many years ago when I build lasers, a company in Canada made laser mirrors
with such a reflectivity profile. Maybee they make holes too? Information
should be sought in the laser buisiness shopping guides.
Mads Hammerich