[Leica] metering

[Jim Brick <jim@brick.org>]

Metering theory is very very simple. Film is manufactured with a certain
sensitivity. This sensitivity is related directly to photographing an 18%
gray card. Where did they get 18% gray? Easy. It's built-in in Rochester
New York. The sky there is always 18% gray. So it was a natural. :-)

Light meters are calibrated to give a reading that will render the image,
on film, 18% gray. So if you use a reflected meter to read a snow scene,
you get 18% gray snow. The reflected meter sees any subject in terms of 18%
gray. You have to know where to point your reflected meter. Wherever you
point it, the exposure is going to render the subject, on film, as 18% gray
density.

The incident meter never looks at your subject. Your subject can be white
or black, and the incident meter reading will be exactly the same. The
incident meter reads the light falling on the subject so it is not effected
by the color or density of the subject. The incident meter gives you the
18% gray reading automatically. You point the dome toward the camera lens,
with the same light falling on the meter as is falling on your scene. In
some cases, you have to stick the meter right in the middle of your subject
because the camera is in different light. In other cases (typically
landscape/cityscape/peoplescape) you can use the meter at the camera
position because the camera is in the same light as the scene. You can
favor highlights (transparency film) or favor shadows (negative film) by
adjusting the angle of the dome so more or less light hits the dome.

You, the photographer, must visualize what is important in your scene and
where, in the density scale, you want it to be recorded. And adjust your
exposure accordingly. This is the hard part.

For instance... read snow with a reflected meter. A reflected meter gives
you an exposure reading to render the subject pointed at as 18% gray. If
you want the snow to be white, you must open up two stops. Read the same
scene with an incident meter, no adjustment is necessary as the incident
meter never sees the subject. It thinks you are photographing an 18% gray
card so the exposure reading will render white as white.

The meter is a simple dumb device that sees everything in terms of 18%
gray. The typical Rochester NY sky.

Jim