Re: confusion re: pinholga

----- Original Message -----
From: "Shannon Stoney" <shannonstoney@frontiernet.net>

You seem to be interested in numbers, most people in this list aren't, nor
they need to make good images. Nevertheless, I could send the "long answer"
with numbers/formulas if you ask for it (meaning if you want it or need it),
otherwise, bellow is the somewhat short answer to your questions:

> The pinholga exposure guide on the website
> (http://www.holgamods.com/pinholga/exposure/exposure.html) says to
> multiply the f/64 reading by 12 to get the exposure time (and add time as
> needed for reciprocity failure). But the Pinhole Designer software
> (recommended on the Pinholga site and downloaded by me) says that this
> pinhole diameter is f/199 and that you should multiply f/64 by a factor
> of 10. It also says that this pinhole diameter is optimal for a focal
> length of 20mm, not 40mm, as stated on the holgamods site.
>
> So I am wondering: are my images going to be sort of fuzzy because the
> pinhole is not the optimal size? And, should I multiply by 10 or by 12?

If diffraction didn't exist a pinhole would cast an image point about the
same size as the pinhole diameter, this is equated to the geometric
resolution of the pinhole. With that in mind and knowing diffraction do
exists: an optimum Pinhole size is a compromise between a small pinhole
that causes lots of diffraction but that has better geometric resolution and
a larger pinhole that has less diffraction but also less geometric
resolution. The pinhole size that Holga uses seems to be smaller than
optimum, this will cause more resolution loss due to diffraction and at the
same time better geometric resolution due to the smaller pinhole. If good
resolution is what interest you, I think is better to err using a smaller
pinhole than optimal than a bigger one, as IMO the geometric resolution has
faster impact on the resolution of the image than some extra diffraction due
to smaller pinhole. Should you multiply by 10 or 12? There are lots of
factors affecting exposure "accuracy": focal length may not be 40mm as
stated, pinhole size may not be exactly what Holgamods says it is,
reciprocity corrections aren't perfect, your film ISO is exact and correct
only for the manufacturer labs processing, your meter (if using one) maybe
inaccurate, shutter is manually actioned, light conditions may change during
a long exposure, etc. So again, Should you multiply by 12 or 10? It really
doesn't matter, the difference between multiplying by 12 or 10 is just about
1/4 of a stop, which is negligible on itself and even more so when there are
other factors, mentioned above, that also affect the "exposure accuracy" you
seem to look for.
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Received on Wed May 23 08:54:54 2007