Re: Building my first serious pinhole

I'll offer you my informal two-cents' worth. I have been using the equivalent of 16mm on
4x5, 8x10, and 7x17. This started out for me with 4x5, using a pinhole at bellows of
about 48 mm. I've also built a 7x17 calculated to operate at a distance of about 6-7",
which again is about the equivalent of 16mm. Finally, I have used a pinhole on my 8x10
at a length of about 110mm, about the same, give or take.

I have to discuss this in inches. Sorry. A pinhole's angle of view is about 125 degrees.
To cover this, you will need a "focal length" of Image Circle divided by 3.5. So a 5x7"
needs an image circle of at least 8.6". Therefore, to cover it (and little more), will require
a "focal length" of 2.5" or about 63mm. I'm not sure off-hand what the 5x7 equivalent of
a 24mm lens (135) would be, but 100mm of bellows certainly should be plenty enough to
cover your film. In theory, you could even go shorter than than, I think, and still cover.

Now I am using wider-angle than you are. I push mine to the shortest tolerable bellows
length, which, as I said, is the 35mm equivalent of around 16-18mm. At that angle, the
fall-off at edges is quite severe - at least several stops from center to edge. This makes
pretty dramatic results, though. And if you are going to do alt-processes, then you will be
surprised at how much tolerance your emulsion gives you. Anyhow, I don't think your
100mm bellows is all THAT much for 5x7. If you are uneasy about it, I suggest that you
just make a shoebox pinhole that's 100mm long. It would take half an hour, and require
no expertise whatsoever. Just tape a sheet of film, calculate your exposure, and try it.

Frankly, I think your bigger problem, if precision is your goal, is going to be the size of
your pinhole. Any pinhole at all will work for any focal length you want; but there is such
a thing as optimum bellows for a given diameter. The blurb that accompanied my drilled
pinhole set says that the optimum size for 87.95mm of bellows is an aperture of .351mm,
and the optimum for 118.23mm is aperture size .406mm. So for 100mm, you will need
an aperture size between those two. I could be wrong, but I think the size won't have
any effect on coverage. That's a relation of the bellows length and the film's diameter.
But a hole size other than optimum is going to give you fuzzy results.

So in other words, it seems to me that your 100mm length ought to cover 5x7 with ease.
You may not even suffer all that much fall-off. Then again, if you do, you might find you
like it. Pinhole is not about playing Ansel Adams, it's more about the world of the surreal.
As for resolution, that is going to depend on the hole size.

Mike

On 30 Nov 2004 at 23:06, trevor cunningham wrote:

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 optimist believes this is the best of all possible worlds.
 The pessimist fears it's true" - J Robert Oppenheimer

http://www.geocities.com/tr_cunningham

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Received on Wed Dec 1 00:31:10 2004