Gary Miller (gmphotos@earthlink.net)
Thu, 29 Jul 1999 22:25:49 -0700
David;
The formula to calculate the f stop would be to divide the distance from the
pinhole to the film plane by the size of the pinhole. Now if you don't have
a good magnifying glass and a ruler, measuring the pinhole can be a bit
tricky. If you are going to make the pinholes with a needle try to get
numbered needles. Refer to the list below to give you approximate apertures
for these needles. Also, there is an optimum pinhole size for each focal
length listed. So for instance if your oatcan has a diameter of 4 inches,
you should use a number 13 needle and will get an f/stop of about 285. When
you do use the needle you should start by using just the tip in a piece of
aluminum or brass shim material. It is easier to make the hole bigger if
you have to. Remember to sand both sides of the piece of metal after making
the pinhole and blow through it or use canned air to make sure that it is
not clogged. The more round and smooth the hole, the better the results.
Most photo paper will be in the ASA range of 15-20. Knowing this will give
you an approximate starting exposure for your negative. But remember there
will be reciprocity. The easiest way is to just experiment. That is most
of the fun with the pinhole. Go for a relatively normal to dense looking
negative to start with for your sunprints. If you use RC paper you can
immerse it into hot water and peel off the paper backing. This will give
you your image on a much thinner base. The best paper to use for paper
negatives is single weight fiber paper, but it is getting hard to find.
Another quick trick when making the pinhole is to stick the back end of the
needle into a pencil eraser, then you can use the pencil to push the pin
into the metal and twist it gently back and forth.
Needle # Diameter of hole Focal length f/stop
15 .010 2" 199
13 .014 4" 285
12 .016 5" 312
10 .018 6.5" 360
9 .020 8"
400
8 .021 9"
422
7 .022 10" 445
6 .027 15" 546
Remember as the hole get larger the exposure will take longer because the
focal distance will be increasing. A hole larger than that recommended to a
particular focal length will start blurring the image sharpness. If you
choice not to be so precise, just poke a small hole directly into the
oatmeal box and go for it. Also remember that the shorter the pinhole to
film distance the wider the camera will be, but with infinite depth of
field, who cares. Because of the longer exposure times for telephotos
pinholes, and for sheer weirdness, most people head towards to wide to super
wide angle side. Good luck;
Gary Miller
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:40:39