[Leica] 18%, middle gray, and the "K factor"

[oliverbryk at comcast.net (Oliver Bryk)]

In the 1981 edition of "The Negative," Chapter 3 "Exposure," Section
"Metering Exposure," Ansel Adams writes:

[quote]...A calibrated middle gray value exists in the Kodak 18 percent
reflectance neutral gray card...Knowing that the meter is calibrated to
reproduce this value, we must remember that making a reading from any
[italics] single luminance surface in the subject and using that reading to
determine exposure will cause that surface to be reproduced as middle gray
in the final print...If we place the gray card within a scene and take a
meter reading from it, we are assured that the meter is measuring a middle
reflectance value, and we can avoid the pitfalls of a single averaged
reading of the entire subject...[end quote]

In the Section "Exposure Correction," Ansel writes about the K factor:

[quote] If pressed, the manufacturers of some exposure meters will
acknowledge that they depart from standard calibration of their meters by
incorporating a "K factor." This factor is supposed to give a higher
percentage of acceptable images under average conditions than a meter
calibrated exactly to an 18 percent reflectance. The practical effect of the
K factor is that if we make a careful reading from a middle gray surface and
expose as indicated, the result will not be exactly a middle gray!

[quote] Some years ago I conducted a series of nearly a thousand trials, and
found that when a meter is held at the lens and pointed along the lens axis
toward the subject,the resulting averaged exposure reading had to be
increased in about 85 percent of the cases to meet my demands for shadow
density. This effect does not directly relate to the functioning of the
meter itself, but has to do with the natural disposition of light and shadow
in most subject material. The manufacturers, apparently assuming that meters
are most often used for average readings of the entire subject area, take
this effect into account by using the K factor. With nearly all meters, this
factor is equivalent to giving a one-third stop increase [italics] in
exposure. Although preferable to work with what I consider to be the true
characteristics of the light and films. Intelligent use of the meter
eliminates the need for such artificial aids as the K factor. In the tests
described in Appendix 1 [Film Testing Procedures] we offset the effect of
the K factor by an adjustment of the film speed. [end quote]

In the Zone System, taking a meter reading from the 18% reflectance gray
card and calling that Zone V is (only) the starting point.

Oliver