Page 102 - The Wellington photographic handbook

It is well to have a little projection P on the reflector, where
it will not interfere with the even lighting of the negative, and
always to put the exposure meter on that projection when measuring
If the reflector cannot be got at from the outside, which
the light.
will be the case if the work is being done in an upstairs room, the
window may be opened at the bottom and a door provided in the
blocking out board at D, so that the meter can be passed in and out.

Another example may help to make this clearer. Let us
suppose that one day using WELLINGTON Bromide paper
and f/ 192 we find that the exposure meter placed on P takes 28
seconds to darken to the standard tint, and that with a certain
negative an exposure of 6 minutes proved to be correct. A week
later, we want to make another enlargement of the same size from
the same negative (or from one identical in density and in colour)
and we test the light again and find that the meter new darkens
in 16 seconds. A simple proportion sum gives us the exposure.

Six minutes are 360 seconds. We multiply the former exposure
by the latter exposure-meter-time, and divide by the former ex-
posure-meter-time. That is to say, we multiply 360x16=5760,
and divide this by 28—5760-^-28 = 206 nearly. The correct exposure
under the altered conditions is therefore, 206 seconds, or, say,
3£ minutes.

Should the degree of enlargement be altered, or the size of
the stop, or both, it is easy to calculate the corresponding change
in exposure. We must first find out, as described above, the new
value of the stop. Then the exposure varies as the square of the
" F " value of the stop.
Let us continue the example. We have just seen that under
certain conditions we found that with stop f/192 the exposure was
3£ minutes. But we will suppose that we have altered the scale of
our enlargement, so that, on working it out, we find we are no
longer using f/192, but f/150, and we want to know the corresponding
exposure.
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