18% gray
Basic metering and exposure
This page addresses the way a calibrated system
responds to metering on a specific subject area (spot metering) and using the
resulting meter reading as the exposure. Light meters are calibrated to
reproduce subjects as middle grey. Middle grey is the printed value that
reflects 18% of the light striking it. At this website, the printed value
of middle gray is referred to as either Zone V, Zone 5, middle gray or
18% reflectance. The 18% reflectance value is approximated on the gray square.
Technically speaking, it is not strictly the case that
light meters are calibrated for precisely 18% grey reflectance.
Some manufacturers veer slightly from that standard. The amount of
departure from the 18% reflectance standard is known as the
K-Factor for a particular meter. This presents
one of several problems which can be overcome by careful testing and calibration
of a photographic system (camera, light meter, shutter, lens,
film/developer, enlarger, enlarging timer and photographic paper/developer
combinations).
Your light meter and camera are conspiring to reproduce everything
as middle gray. The meter has no way of knowing whether it is pointed
at a light area or a dark area. The Zone System uses this bit of understanding
to build a set of tools which allow you to soar into new regions of
photographic control.
So if you want to reproduce something as middle gray, there is
a chance it will work. First point your meter at the subject
you want to be middle gray, meter on it, then use the
indicated reading. That is the exact thing that the meter does for you;
it calculates a combination of F-stop and shutter speed, at a given film speed,
to reproduce the area it is reading as middle gray.
There are problems getting this to work in real life. Your shutter could be
off a bit, the f-stop could be off a bit, the meter could be off a bit, and
the film may not react to light at its exact stated film speed. This set of problems can be overcome by carefully testing and calibrating your system. More about calibration and testing later.