The Weston Master family of exposure meters Page 23
Step 2: After the meter has regarded the scene and delivered a
"reading" of the measured illuminance, the user turns dial X until an
arrow on it points to the meter reading on an adjacent scale on the
dial Y. The way dial X is moved varies over the different models. On
this model, dial X has little tabs to use to move it. We see here that
this setting has been to a meter scale ("light") value of "just a little
over 4". For our purposes here, we can consider the numbers on the
meter scale to be arbitrary.
Step 3: Adjacent rings on dials Y and Z carry, respectively, a set of
f-numbers and a set of shutter speeds. Any adjacent pair of f-number
and shutter speed will give the photographic exposure "recommended"
by the meter, based on the measured scene luminance and the set
exposure index. The photographer chooses a pair that best fit the
needs of the shot. For example, he may have in mind a certain
f-number, to get a certain depth-of-field behavior, or a certain shutter
speed, to accommodate a moving subject.
For example, in the figure I call, attention to the almost coincidence of
"2
8
" (the format used in this model to designate f/2.8) and "5"
(meaning a shutter speed of 1/5 second). This means that one
"perfect" way to implement the photographic exposure
recommendation of the meter is to use a 1/5 second shutter speed
and an f-number just a little greater than f/2.8. (Of course, the
photographer is probably actually limited to certain discrete shutter
speeds and f-numbers, so some compromise may have to be made in
doing this.)
B.2 The U-A-(B)-C-O markings
All the exposure meters in described here incorporate a system I
describe as the "U-A-(B)-C-O" markings (certainly a peculiar name!).
We begin the story by recalling that, in Step 2 above, the
photographer sets the arrow on dial X to the meter reading on dial Y.
Suppose because of some situation (a difficult lighting situation, or the
desire for some special artistic effect, the photographer wishes to use
a photographic exposure that is greater or less than that the meter
would ordinarily recommend, say for example an increase of "one
stop" (a doubling of the exposure) or perhaps a decrease of "one
stop" (a halving of the exposure).
One way to help the photographer do this would be to provide two
alternate arrows on dial X, such that if one is used (instead of the
“regular” arrow) to point to the meter reading, the calculator is in
effect set to half the meter reading (and the meter would thus give
twice then "normal" exposure recommendation), and if the other is
used to point to the meter reading, the calculator is in effect set to