Re: human subjects

----- Original Message -----
From: "Joe Rollins" <jrollins@starband.net>

> GlacierI am trying to build a pinhole camera to use to take photos of human
> subjects. I need to get the time down to under 10 or 15 seconds. I shoot
> with Ilford MGIV. What focal length and diameter pinhole would be sugested.
> Joe Rollins

I am sorry to say, that this is a case for which a little bit of math comes in
handy.

As Thom said, you don't give too many details of your shooting, so I am going to
make some assumptions. I am going to assume you want to use an "optimum" size
of pinhole and that you will be shooting with a light level equivalent to f/16 @
1/6 secs, that BTW is the indicated exposure for shooting outdoors under open
skies/bright sunny day, otherwise known as "sunny16 rule". I am also assuming
you rate MGIV as ISO-6. Here is the analysis:

As per reciprocity table for MGIV (see it in my paper "Determining Pinhole Size
and Exposure " at http://members.rogers.com/penate/pinsize.htm ), for a 10 to 15
secs max exposure time you would need an uncorrected time of no greater than 8
secs. If existing light level calls for an exposure of f/16 @ 1/6 secs, we need
to find an equivalent pair of exposure values for when the exposure time is 8
secs. The answer is f/128 @ 8 secs. Once you know the f/stop your pinhole
camera must have, which is f/128, you can find the focal length for which when
using an "optimal" pinhole size, you'd get an f/stop of f/128, you do that with
this formula:

Focal length in millimeters = (f/stop x f/stop) / 738

In your case, f/stop = f/128, so we have:

Focal length in millimeters = ( 128 x 128 ) / 738

Focal length in millimeters = 16384 / 738

Focal length in millimeters = 22 mm

Further explanations upon request.

Guillermo
Received on Sun Apr 7 11:22:35 2002