Page 101 - The Wellington photographic handbook

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The stop value in enlarging differs completely from that en-
graved on the lens, and must be measured for the particular exposure
in hand.
If the lens is fitted with an Iris diaphragm, we may measure
once for all the diameter of the opening for the different stop num-
bers, by unscrewing the lenses and pushing into the iris, as far
as it will go, a piece of card cut into a V shape. We can then
make a list like the following :
F/8 — $ inch diameter.
F/ 1 1 — ^ inch diameter.
F/ 1 6 — \ inch diameter.
F/22 — Tr inch diameter.
2
F/32 — \ inch diameter.
The measurements will of course vary according to the focal
length of the lens employed. The above figures would be correct
for a lens of a focal length of 4 inches.
This list is kept at hand when enlarging. When all is ready
for exposure, the distance from the iris of the lens L to the easel
B is measured with a tape measure, and this distance divided
by the diameter of the stop. The answer is the real value of the
stop that is being used.
An example will make this clearer. Let us suppose that the
stop in use is that marked F/22 in the list above, and that the
distance from the iris to the easel is found on measurement to be
35 inches. As F/22 has a diameter of 2 inch, we divide 35
TT
by 2 That is to say, we multiply 35 by 11, which gives us 385,
TT .
and divide this by 2, getting 1 92. So that under those circumstances
we are using F/192 actually, although the stop is marked F/22.
This calculating may seem a bother, but it only takes a minute
or two, and will prevent the waste of a great deal of paper in trying
to guess at the exposure. Once having ascertained the true value
of the stop, and the correct exposure necessary with any given
negative, we have provided ourselves with some clear data which
will be useful later on. Using the same stop, and the same negative,
the exposure will always be strictly proportional to the time taken
for the meter paper to darken.
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