Re: [Leica] Zone V and greycard

["Dan Post" <dpost@triad.rr.com>]

I understood that! You're doing better, Erwin!
The Gray card measured reflective density is about R=.80 G=.80 and B=.80,
and when I worked in a photolab, that was the gray we attempted to match on
the morning test print.... but when all corrections were figured in, the
gray card in the test scene usually measure about .72 reflective density!
When it matched that- everything else fell into place!
Even in a one hour lab- I was able to 'match' print and reprints ( for
really picky customers! and pretty girls!) by measureing the original-
usually a white area of clothing or a flesh tone- the making a test print
and measuring the same areas of the test print- by comparing the relative
densities, it was easy to calculate the necessary corrections, and I became
quite good at it! Not bad for a one hour lab !
The arrangement and target tones can be targeted the same way in B&W and
using a densitomet- or even an enlarging meter to read the realtive density
of the negative! I photograph a test target that has a plain white matte
board, a black matte board, and a grey card- all 8x10 and mounted on a 16x20
matte board.
Measuring the negatives, taken at different EI, I find the one that has
about a .70 transmission density- above base fog, and it gives me a good
indicator of what I need to shoot the film at. When I want to find out if my
development is too vigorous or not, I use the indicated EI and shoot a roll
of film, divide it into several small pieces and develop at different times-
I then measure the density difference between the white card image and the
black card image, which gives me a density range- I pick the time that gives
me the best range printable on 2 1/2 grade paper. The gray card is very
useful for all sorts of things!
Dan

- ----- Original Message -----
From: "Erwin Puts" <imxputs@knoware.nl>
To: "L U G" <leica-users@mejac.palo-alto.ca.us>
Sent: Thursday, August 24, 2000 2:32 AM
Subject: [Leica] Zone V and greycard


> 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
>
>