Re: Building my first serious pinhole

> ----- Original Message -----
> From: trevor cunningham
>
> Hey all...been lurkin' for some time. But now I need a few questions
> answered. I've recently come into availability of B/W sheet film that is
> 13cm x 18cm (this is a feat in Cairo). I'm going to get a set of holders
> and will have a carpenter (my skills were heavily discouraged by my high
> school wood shop teacher!) build a box to suit. My goal is to achieve the
> effect of a 24mm lens on a 35mm camera and wish the image to cover my
entire
> piece of film with as little loss of image out of frame as possible. I've
> tried a pinhole calculator on the internet and am getting a focal length
of
> approximately 100mm to achieve such an image diameter...is this right? If
I
> want that really wide angle look, do I have to compromise my image size?
My
> intention is making contact negs for cyanotypes and gum bichromates. At a
> point, size becomes an issue.
>
> Enjoy...Trevor Cunningham

The focal length is 12.3cm or 123mm.

If you would like to have more than just the answer, keep reading, otherwise
stop here!! :-)

Cairo, Egyp, eh?
The ancient Egyptians were master builders and handled geometry, math and
angles really well, we got to honor them, with a more thorough answer:

There are mainly 2 ways to find the answer, first by pytagoras you find the
diagonal of the 35mm frame and the piece of 13x18 paper rectangle, or just
plain measure the diagonal in the paper and in the 35mm frame. Either way
you should come up with: 43.26mm for the 35mm frame and 22.2cm for the
paper. Just with these 2 diagonal you can have an idea of what you are
looking for, the number 22.2 is about half the number 43.26, if that is so,
the focal length of your pinhole camera should be a number that is about
half 24, the focal length should be about 12cm. If we want to be more
exact, let's see what relation there is between the diagonal and the focal
length for the 24mm lens in the 35mm format, if you divide the diagonal
43.26 by the focal length 24 you get a ratio of 1.8025, lets say just 1.8
this means that in order for your pinhole camera to have the same angle of
view that the 24mm FL in 35mm camera, the ratio between the diagonal of the
paper and its focal length should also be 1.8 so we have:

22.2 / X = 1.8 therefore

22.2 / 1.8 = X

X = 12.3cm see, very close to 12 as we said it would be.

12.3cm is equal to 123mm

Another way to do it is by knowing that the tangent of half the angle of
view is equal to half the diagonal divided by the focal length, the diagonal
of the 35mm frame is 43.26mm and the focal length is 24mm, so we have:

(43.26 / 2) / 24 = 0.9 = Tangent of half the angle of view

Half the angle of view = ArcTangent (0.9) = 41.98

Also, for the pinhole camera, the diagonal is 22.2cm and the focal length is
unknown (X), so we have:

(22.2 / 2) / X = 0.9 therefore

(22.2 / 2) / 0.9 = X = Focal length

Focal length = 12.3cm same as above

Half the angle of view is 41.98, the light fall off at the corners of the
paper if the pinhole is clean of burrs, round and made in thin material,
would be something like 1.7 stops, which contrary to what we may think is
not a lot for pinhole use, you should be OK with that focal length for your
pinhole camera. Someone else here said that the total angle of view of
pinholes is 125 degrees, fall off at the corners for such camera would be in
the order of 4.5 stops, and still considered OK for pinhole images. The
actual max angle of view of a pinhole camera depends on your tolerance to
fall off.

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Received on Wed Dec 1 07:28:05 2004