light meter, the guessing is easier and more accurate, because you have a
good place to start.
3.
Before discussing how to interpret a light meter reading, let's first
talk a little about how light meters work.
There are two basic types,
photovoltaic and photoresistant.
a. Photovoltaic meters use a selenium cell, which converts light energy
directly into an electric current.
(It's the same kind of cell used to
power solar calculators.) The current is used to make a needle move. The
brighter the light, the stronger the current, and the stronger the current,
the farther the needle moves. This is an extremely straightforward way of
measuring light. Its chief advantage is that it needs no batteries, because
the electric current is generated by the very same light the meter is
measuring.
So this type of meter can be used anywhere, anytime; almost.
Photovoltaic cells are not very efficient at converting light into
electricity. In dim light, there just isn't sufficient current to move the
needle enough to make an accurate measurement. This can be compensated for
by making the selenium cell larger, but after a while it all becomes much
too big to be practical.
So selenium cell meters work only in relatively
bright lighting situations. Even so, average room light or office light is
usually enough to get a reliable reading with a good quality hand-held meter
of this type.
b. Photoresistant meters, on the other hand, use a source of
electricity to start with - usually a small battery.
The light-sensitive
element doesn't generate any electricity. Instead, it resists the electric
current from the battery.
As the light falling on the element becomes
brighter, the resistance drops, the battery's current flows more and more
easily, and the indicator needle moves farther and farther.
This has
several advantages. First, the cell's size is much less important. A very
tiny cell works just as well as a large one. Second, this method is very,
very sensitive.
Extremely low light levels can be accurately measured.
This has been put to good use inside many sophisticated camera, where the
cell can read the tiny portion of light that is reflected off the surface of
the film emulsion during exposure. In hand-held meters, indicated readings
of up to 30 minutes are sometimes offered. Of course, if the battery goes
dead, the meter is useless. And, just as with camera batteries, light meter
batteries go dead at the worst possible times and always when they get cold.
For most uses, the photoresistant type meter is preferred. But in extremely
cold weather or when batteries are scarce or unavailable, the selenium cell
meter still has its uses (fig 1-12).
4.
Once the needle has settled down, we need a way to read exactly how
far it has gone. In most cases, there is a simple scale, usually numbered,
painted on the background behind the needle. You read the number the needle
is pointing to and transfer it to a calculator dial to get an indicated
exposure.
In other meters, the needle is directly connected to the
calculator dial. Turning the dial causes the needle to move to the left or
right until the needle matches a mark on the background behind it. This is
convenient because when the needle has been set on the index marked
"nulled", the exposure indication is already set on the calculator dial, and
needs only to be read. Some sophisticated and expensive light meters have
done away with the calculator
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