From: Diana H. Bloomfield (dlhbloom@mindspring.com)
Date: 04/12/02-05:17:27 PM Z
Hi Shannon,
I don't know anything about that formula. Looks complicated, and I
don't think it allows for reciprocity failure.
But to answer your other questions . . . the f-stop for a pinhole camera
is the focal length divided by the diameter of the pinhole. For
example, if your camera has a pinhole to film plane distance of 5.5
inches, and your pinhole diameter is .0018 inches (that is the size of a
#10 hand sewing needle), this ratio is 5.5 divided by .0018, which is
300. So 300 will be your f number.
There are optimal pinhole sizes for a given distance (from pinhole to
film plane), though you don't have to use those. Here's the Pinhole
Visions site that includes a pinhole calculation guide for optimal
pinhole sizes: http://www.pinholevisions.org/resources/cameraTable.php
There's a lot of good information on that site, including a discussion
list you can join, so check it out.
There are folks out there who have worked out exposure calcuations--all
based on pinhole size, film/paper being used, and allowing for
reciprocity failure. Personally, I get used to whatever camera I'm
using and simply look at the weather and guess. Once I understand how
my camera works under certain conditions, I can pretty much determine
correct exposures. However, lately I've been using much larger (more
expensive) film, so I'm trying to be much more exact. There is a great
little guide, called the Black Cat Exposure Guide. You can find it on
the web. It's cheap and light and works great.
Hope this helps.
--Diana
shannon stoney wrote:
> I recently read a formula for figuring pinhole exposures that I can't
> figure out. It goes as follows:
>
> -Set your light meter to an f stop and call it A.
> -The meter will give you an exposure time that you call S.
> -Figure your pinhole aperture and call it B.
> -Now find the correct exposure time with the formula and call it X
> -S B 2/A 2=X
>
> My questions are:
>
> 1. what number do you use for the pinhole aperture? the fraction of
> an inch that it is (like say 1/64) or its measurement in decimals
> (0.0156) or the f stop that it is (420)? I am thinking probably the
> latter.
>
> 2. IS SB2 the numerator and A2 the denominator? If so, why is there
> a two in the numerator and the denominator? Don't they cancel each
> other out? How do you figure this formula, in practical terms?
>
> 3. Is there a pinhole list?
>
> --shannon
> --
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