Re: Pinhole Enlarger Questions

----- Original Message -----
From: "CJ Rumpolo" <rumpyhw@yahoo.com>
To: <pinhole-discussion@spitbite.org>
Sent: Friday, August 20, 2004 10:55 AM
Subject: [pinhole-discussion] Pinhole Enlarger Questions

> Hi, I have been mentally playing around with an idea to make a pinhole
> Enlarger. ..... some stuff deleted here

> I decided that on a 4x5 enlarger a 150mm lens is the normal lens. So I
made
> up a foam core box that has two sections. The top section is 150mm in
height
> the second section is 300mm in height.

I will try to give you a somewhat "over-kill" answer that may or may not
answer all your questions. Before someone mentions it, yes, you don't need
to know all the following stuff to enlarge a negative using a pinhole.

As I like to say, pinhole photography is not different than glass lens one,
and enlarging is not an exception. When you enlarge 4x5 to 8x10, the
trajectory of the projected image (from the negative to the paper) will
describe 2 pyramids joined by one of their vertices (this is where the
pinhole would be), but because the proportions of the base of both pyramids
are the same, we can consider just one side of each pyramid for this
analysis. Let's pick the top triangle with base = 5 (this one is inverted,
base at the top) and the bottom one with base = 10. When you draw such
triangle you could easily visualize that the angles of these 2 triangles are
equal, if so, the proportions of all the measurements of the 2 triangles are
such that the bottom triangle is twice as big as the bottom one, this tell
us that the distance from the base of the bottom triangle (paper) to the
pinhole has to be twice the distance from the pinhole to the base of the top
triangle (negative/film). If "P" is the distance paper to pinhole and "N"
is the distance pinhole to negative, we can then write:

P = 2N

But you knew this, that is why the measurements of the enlarger you are
planning has distances to pinhole of 150mm and 300mm.

And the above should be all you need to enlarge your negative, just add a
pinhole of any size and voila! you got yourself an enlarger.

You want to "complicate" matters?: If you start with the Gaussian form of
the lens equation (do a google search on "lens faq" if not familiar with
it), and if "F" is the focal length of the lens (pinhole in your case, in
other words, the focal length distance your pinhole is "optimum" for,
according to whatever flavor of optimum formula you prefer...if any!), then
we have the following lens equation:

1/N + 1/P = 1/F

N and P have the same meaning as earlier in this email.

But we already said that P = 2N in our enlarger capable only of enlarging
twice the original size (original size having a maximum of 4x5" size). So
we can write:

1/N + 1/2N = 1/F therefore:

F = 2N / 3

If N = 150 and P = 300 , BTW, this measurements comply with the formula P
=2N, then we have:

F = 2N / 3
F = 2*150 / 3
F = 100

You can see that because what in glass lens photography is know as bellows
factor, the actual "infinity" focal length of the pinhole should be 100mm,
in other words, if you want an optimum pinhole in your enlarger, it'd have
to be made for a focal length of 100mm not 300mm. BTW, you selected 150mm
because that's the normal lens for 4x5, but in an enlarger, the distance
lens to negative is not the distance associated to the focal length of the
lens, but the distance lens to paper.

You really don't need to know the f/stop, but if you want to know it, it'd
be given by the division of the distance pinhole to paper or distance "P"
divided by the diameter of the pinhole. As I said before, you don't need to
know all of this, especially if you will not be using the so called
"optimum" pinhole for the focal length.

Corrections welcomed.

_______________________________________________
Post to the list as PLAIN TEXT only - no HTML
pinhole-discussion mailing list
pinhole-discussion@spitbite.org
FAQ at http://spitbite.org/pinhole-discussion/list.html
Received on Fri Aug 20 18:30:50 2004