[Leica] 18% metering often wrong

[Mike Johnston <70007.3477@compuserve.com>]

I can see that we're going to have to do an article on metering. There are
certainly a lot of misconceptions about it floating around.

18% metering is not accurate metering. It is an approximation--even if your
meter is calibrated to read middle gray, and you are "giving it" middle gray to
meter (such as a gray card, or the grass, or the back of your hand, etc.) 

Why? 

Picture it this way. Say your film will record 7 stops--visualize these as a
line of 7 gray blocks in a row, with middle gray in the middle, three lighter
tones approaching white on the "highlight" (right) side and three darker tones
approaching black on the "shadow" (left) side.

The problem is that you must match this to a scene. The scene has a range of
brightnesses as well, which, for the purposes of our illustration, you can also
visualize as a "stepped" series of grays--a line of gray blocks, black on one
end, white on the other. 

If you come upon a scene with a 7-stop range of subject luminances, you're
fine. The row of gray "blocks" from the scene line up perfectly with the line
of gray "blocks" that represents the film's range. The problem arises when
you're _not_ dealing with 7-stop scenes. 

First, let's use as an example a scene that has, say, an 11-stop SBR or subject
brightness range (this would be called a "contrasty subject"). If you simply
find a middle gray area in that scene, meter that accurately, and expose your
film, you can see what you've done if you "graph" it by lining up the two rows
of gray blocks one above the other, with the middle (middle gray) blocks
matched up. Picture it in your mind. Is it clear that you will still fail to
record on your film nearly two stops of range on either end of your film's
density range? This means that some of the shadows will be empty black and some
of the highlights will be blank white (blocked up). 

Now imagine a scene with a 3-stop SBR. Again, you find the middle gray in the
scene and accurately meter it with your 18% meter. The trouble is, now the
scene is of lower range than the film. So you have effectively "overexposed"
the film, because you have given the lowest tone in the scene sufficient
exposure to render it not as the first block on your film's density range, but
as the _third_ gray tone in your film's density range--one block to the left of
middle gray. 

The basis of the classic Zone System (which is certainly not "extremely
excellent" but contains a number of inherent errors), is to correct for this
basically flawed situation by separating the shadow and highlight readings. 

In the first case, with an 11-stop SBR and a 7-stop film DR, you would need to
meter the darkest tone in the scene and "place it" congruent with the darkest
patch of gray in your film's DR. This would require, in our example here,
nearly 2 stops more exposure than your 18% gray-card reading suggested. Then,
you would have exposed the film correctly, such that shadow detail would be
recorded. But your highlights would be way off the scale--3 to 4 stops over and
above the film's ability to record them. So what do you do? You curtail
development to bring the highlight values back down into the film's range. This
is called "contraction." 

This is the basis of the expression (coined, I believe, by Loyd Jones), "Expose
for the shadows, develop for the highlights." It is also the reason why any
system which does not allow for individualized development of individual
exposures is _not_ "The Zone System."   

In the second case, with a 3-stop SBR and a 7-stop film DR, you would need to
do exactly the same thing--meter the darkest tone in the scene and "place it"
congruent with the darkest patch of gray in your film's DR. The difference is
that this time, this results in _less_ exposure than your 18% gray-card reading
suggested. But now, the highlights would be registered on the film as a muddy
middle gray! To compensate for this, you increase development to move the
highlights back up to where the film wants them to be. This is "expansion."

In the case of a 7-stop subject and a 7-stop film DR, metering a middle gray
with a meter calibrated for middle gray should yield an accurate, near-ideal
result. But whenever the subject brightness range varies from this ideal, a
middle gray reading becomes more or less of an approximation. (This is also the
somewhat woeful situation that slide-film shooters are stuck with, because they
cannot adjust the DR of their films. They are saved somewhat by the fact that
color can substitute for contrast in aesthetic effect--as an example, consider
that you can get away with a picture that has a large _green_ area that has 18%
reflectance next to a large _red_ area that has 18% reflectance, because the
colors themselves "contrast" with each other, but you cannot get away with the
same trick in black-and-white, because the whole frame would be an
undifferentiated middle gray.)

The system that C.E. Kenneth Mees and Loyd [sic--that's how he spelled it]
Jones devised in the 1930s is the one we still use today--the film assumes an
SBR of 7 stops, which they discovered to be the average, and the range of paper
grades approximates the expansion and contraction of two stops of SBR either
way, for correction after the fact of most (not all!) of the typical departures
from average. It's a servicable system for rollfilm shooting, assuming averaged
metering and averaged development. It works pretty well most of the time.

The Zone System is a whole 'nuther level more accurate. The inaccuracy of the
classic Zone System is simply that when you change the development of the film
to expand or contract its range, you also change its speed point, or the place
on the characteristic curve where shadow detail threshhold is reached. Plus,
the adjustments--the "N numbers"--are not linked to anything real--they're
simply arbitrary adjustments which hopefully the photographer will apply
consistently, and learn. So the Zone System, while also serviceable, is
actually a fairly crude approximation itself. Astute individuals will realize
that Ansel Adams was already an experienced photographer by the time he
formulated it, and astute photographers who employ the Zone System will have
realized that it still requires the application of experience and judgement.  

Another weakeness of the Zone System is that it relies on reflected light
meters, which are subject both to flare and to area-averaging. These reflected
light meters are _much_ more intuitive for most photographers to understand,
and hence to use--which is why they're preferred, rather than because of any
technical superiority--but incident light meters are technically superior
assuming the photographer has the knowledge and the skill to use them...a big
assumption.

If you know how the materials really behave, you can expose any scene and
develop any film and know in advance exactly what grade of paper you're going
to print the negative on. Phil Davis's "Beyond the Zone System" is the most
accurate practical exposure and development system in use today, because the
speed point is automatically adjusted for development and the expansion and
contraction is stepless and tied directly to the paper you're intending to
print the negative on.

As I quoted Phil in an earlier post, any exposure meter reading which is not
based on a shadow reading is nothing but an approximation. It will be correct
some of the time, and it will be incorrect (to a greater or lesser degree) some
of the time. I hope this has helped explain why.

- --Mike
Copyright 1998 by Michael C. Johnston