Several types of material will make a good pinhole.
The types I have used with success are brass and aluminum, though I have
heard silver sheet is also a good material. Brass shim stock (sheet
brass) can be found in hardware stores, hobby shops and auto supply shops.
The best pinholes I have made from brass sheet were made in .003 inch thick
stock (.076 mm). This thickness gives me holes that cover quite well
at focal lengths down to about 3 inches (75 mm) on a 4 x 5 format.
A good source of aluminum sheet is the aluminum
they make soft drink cans from. It is readily available and the remainder
of the can can still be tossed into the recycling bin. The only problem
with soft drink can aluminum is that it is curled and needs to be straightened.
I accomplish this be rolling the X-acto knife handle over it from several
angles. It eventually straightens enough to use it. Another
source of good aluminum sheet is the bottoms of baking pans (the kind designed
for single use). Instead of tossing them into the recycling bin right
away, cut out several appropriate sized pieces for pinholes.
My method of making holes is a compilation of information
from several sources. It works for me. Before starting, I assemble
the materials, a piece of material about 1.5 x 1.5 inches (38 x 38 mm),
a sheet of sand paper or emery paper (fine grit, preferably mounted on
a block of wood), a sharp needle mounted in a piece of dowel or an X-acto
handle, a piece of cardboard bigger than the material, a loupe (x8 or x10)
or microscope if available and a way to measure the diameter of the hole.
I use a caliper that measures in increments of .001 inch that I got at
a discount hardware store for $15 US. My impression is that exact
dimensions are not as important as how circular the hole is and how smooth
it is.
Start by putting the material on the piece of cardboard
(it may be helpful to hold it in place with tape) and use the needle to
drill a hole in the center of the material. I hold the needle at
right angles to the material to get the roundest holes, and the large handle
of the dowel or X-acto handle really helps. Some folks punch the
hole and others drill it with the needle. I have always had good
results with drilling, since it makes nice round holes. Don't
use too much pressure or the needle will go too far and make too big a
hole. The cardboard helps here.
After you have made a hole (preferably too small),
it will have a burr sticking out on the other side from the one where the
needle went in. Use the sand paper to remove the burr by sanding
in a circular motion. The ultimate goal is as thin a material as
possible with a nice round hole in it. At this point, there will
be a lot of debris in the hole and if all went well there will be a small
hole with which to work. Use your caliper or whatever you are using
to check the diameter of the hole. The loupe will help you to see
the shape of the hole and make the measurement, and a microscope will make
it even easier if you have one.
Now you are ready for the next iteration.
Turn the material over to drill it from the other side for the next iteration.
Take your time and drill several times if necessary. Also, be very
careful not to make another hole next to the one you want. This is
one of my most difficult problems, since I have ruined more pinholes than
I can count this way. After you enlarge the hole from the other side,
sand off the burr in the same manner as before. Keep turning the
material over and enlarging the hole until it is the size you want.
My experience is that +/- .001 inch makes little or no difference in how
well the pinhole works. Before you use the pinhole, be sure to get
any debris out of the hole and check to make sure it is as close to perfectly
round as you can make it -- unless you are going for the effect you get
with an elliptical pinhole.