[Leica] Zone V and greycard

["Erwin Puts" <imxputs@knoware.nl>]

Some basics first: objects do reflect different amounts of light energy.
Assume we use the 18% value as a starting point. Twice as much light energy
(the equivalent of one aperture stop) is 36%, the next step is 72% and the
next would be 144%, but that is impossible as the maximum reflectance
obviously is close to 100%. The range on the other side is:9%, 4.5%, 2.25%,
1.12%.
The full range is 1.12-2.25-4.5-9-18-36-72-99 or in stops: 7. As the average
scene is assumed to have a 18% reflectance, the whitest highlight then is
2.5 stops away from the middle value and the deepest black is 4.5 stops
away. The logic behind the old rule: measure the hadows and nderexpose by 3
stops, or mesure the highlights and overexpose 2 stops, finds its logic
here. The true middle value would be around 9%.
Why then is 18% called the midgrey value. Because the eye is no densitometer
or exposure meter and reacts logarithmally to changing levels of eflectance.
The grey value, associated with 18% reflectance is by many people identified
as a medium grey (it originates in the printing industry), even if it in
relaity represensts a darker grey.
Now zones: Adams used the Zone system as a way to find corectly exposed
negatives as the density range goes. So he used another approach. As all
film/exposure systems are adjusted to the 18% yardstick, it mens that a
correct exposure will place the greycard value in the middle of the straight
portion of the characteritsic curve. The density value there on a well
exposed/develped negative is D=0.75. Density alues are logarithmically
scaled and so one stop more or less has a value of D=0.3. So now for density
values: Grey card is 0.75, one stop more is 1.05, the next 1.35. Etc. You
can make this rnage for yourself. Film densities can rnage from 0.1 to 3.0
and higher. So a range from 0.0-0.3-0.6-0.9-1.2-1.5-1.8-2.1-2.4 is no
problem for a film to record. As films often use a Gamma of 0.7 the aperture
steps are not 0.3 but 0.7x0.3=0.21. Now starting from 0.75 as the base
point, we have a range
0.0-0.12-0.33-0.54-0.75-0.96-1.17-1.38-1.59.
Adams simply used the grey value as his middle Zone V and added equally
spaced steps on either side, which is almost permissable as film
densitomerty goes. His shortcoming is that he assumed the density range of
the film to match the range of reflectance values in nauture. That is not
true and therefore Adams had to torture the chararteristic curve into shape
wth his N-x and N+x development scheme to stretch the density range in the
highlights.  If you take a picture of a greycard, expose as is and then make
the famous +/- 5 stops to get the stepped strip of grey values, you will see
that density measurements will give you blocked highlights above two or
three stops. The rest is wasted. The classical Adams trick is to overexpose
and underdevelop to  stretch the range of the highlight denstities on the
film.
I use the Zone system myself, so I am not in disagreement with Adams
approach. But you need to understand that his method is an
exposure/development technique to fit the variable density range into the
variable reflectance range as it suits him.

Erwin