RE: figuring pinhole exposures

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From: Christopher Lovenguth (chrisml@pacbell.net)
Date: 04/13/02-11:16:02 AM Z

This is what I use. I don't know who the author is but we made them in
school. What you do is make two bars with matt board or something durable
with lines going horizontal that match when the two pieces are put next to
each other. My 2 strips are 3/4" x 13 1/4". The lines are 1/4" apart
horizontally. These are the numbers you put on each bar when they lay next
to each other:

         Two strips next to each other
               |
1/20 | 4
------ | ------ < example of where the lines go.
1/15 5.6
1/12 6.3
1/10 7
1/8 8
1/6 9
1/5 10
1/4 11
1/3 12.5
2/5 14
1/5 16
3/5 18
4/5 20
1 22
1 1/3 25
1 2/3 28
2 32
2 3/4 35
3 1/4 40
4 45
5 50
6 1/2 56
8 64
10 70
13 78
16 90
20 100
26 112
32 128
40 142
52 160
64 180
1:20 200
1:40 225
2:08 256
2:40 285
3:20 320
4:16 360
5:10 395
6:30 445
8:32 512
10:30 570
13:30 640
17:04 720
22:45
28:26
34:08
45:30
56:53
1:08:16
Time F-Stop

The other thing I have written on the holder (more about that below) is
this:

Indicated Exposure in Seconds & Min:
Reciprocity compensation factor - X -:

1 sec X 1 1/4
5 sec X 1 1/2
15 sec X 2
45 sec X 2 1/2
2 min X 3
5 min X 4
10 min X 5
20 min X 6
40 min X 8
60 min X 10

Then when you meter you use the pieces like a slider ruler and match you
time and F-stop. Then all the other times and F-stops will line up. I made a
holder for mine that is just a backboard with two horizontal strips that
keep the two moving peaces with the Time and F-Stop held but able to move.
The back piece is 14" x 3.5" with strips 2" x 3.5" taped to the back about
1" from each edge. The whole thing is no more then 14" x 3.5" so I keep in
my 8X10 camera case. It has saved me many times not just for pinholes.

For some reason I have found this system less confusing then the typical
"wheel" version. I don't know why, maybe it is just that all you have to do
is look down a line to see all the facts. If size is an issue, then go with
the "wheel" version, which typically tends to be much smaller.

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