From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Newsgroups: rec.photo
Subject: Re: pinhole camera
Date: 31 Dec 1992 07:25:06 GMT
Organization: Lawrence Berkeley Laboratory, California
NNTP-Posting-Host: 131.243.192.10

In article <BzwME7.8LF@fc.hp.com> pdn@fc.hp.com (Paul Nuber) writes:

>When building a pinhole camera, what is the optimum size for the pinhole?
>I thought I read once that it was about 1/200th of the focal length.

There was an article in the "Pinhole Journal" by a fellow named 
Connors who did picture quality tests with various pinhole 
diameters.  Connors found that the best resolution resulted when 
the diameter of the pinhole in inches was = SQRT(.000054 * "focal 
length" in inches).  As I recall, he did his tests with the 
subject at fairly ordinary distances.  As an example, for a "focal 
length" of 6", you have a diameter of SQRT(.000054*6) = .018 
inches.  If you like your formulae with bigger constants, you 
could also say diameter in inches = .007*SQRT("focal length" in 
inches).  
 
The bigger the film format, the better the angular image 
resolution.
 
Here's a table for the 3 focal lengths I use on my 4x5 with B&W 
Polaroid film:
 
Focal length   Focal length   hole diameter  drill size    f/no
 in inches        in mm         in inches
 
  3.375            86             .0135          #80        250
  6.000           152             .0180          #77        333
 10.666           271             .0240          #73        444
 
I like the 6 inch focal length best - I leave it on an old shutter 
on a lens board.  Many others like the shorter focal lengths.  As 
the astronomer said, an old felt hat is often the best 
shutter.  Don't worry too much about the focal length; just get 
the pinhole _approximately_ the correct focal distance from the 
film.  +/- 25% would't be noticeable, I'm sure.  
 
There are many methods of making pinholes; here's mine: I sandwich 
a piece of .001 inch thick brass shim stock between two 1/16 inch 
thick pieces of sheet aluminum and drill through with the drill 
size from the above table using a high speed drill motor body such 
as a Dremel tool or a Moto tool or a Sears' Lil' Crafty or 
whatever.  The aluminum backing pieces (the "bread" of the 
sandwich) keep the burrs to a minimum.  After the brass is 
drilled, I take off the remaining burrs with 600 grit abrasive 
paper.  Then I ink in the inside diameter of the hole with a black 
"Magic Marker" type of thing - whatever's black and sticks to 
brass and isn't clumpy so as to leave junk in the hole.  Just be 
sure to inspect your pinhole with a magnifier when it's done to 
make sure you have a good round hole - irregularities decrease 
image sharpness considerably.  Irregularities can be dirt 
particles inside the hole so I keep my pinholes in dust-free 
environs.  You blacken the interior of the pinhole to cut out 
reflections off the short cylinder which is the hole's inside 
diameter.
 
f/no is always just focal length divided by pinhole diameter, the  
equivalent of a lens' diameter of aperture.  So the f/no of the 6" 
focal length pinhole is 6/.018 = 250.
 
I really shouldn't be calling this length the "focal length" of 
the pinhole as focal length is a property of a lens, not a 
pinhole.  Pinholes are afocal, that's their charm, but they do 
have a distance from the film or image plane at which they give 
their best resolution.  Maybe I should call this "optimum length" 
instead of "focal length".
 
             JHBercovitz@lbl.gov    (John Bercovitz)