comparing characteristic curves and the meaning of separation

Hi all,

I'm reading "the negative" from Ansel Adams. On page 90 , where characteristic curves are compared, he talks about the SEPARATION of values.

 

"Most films today have very long straight-line regions, but if a

shoulder is present, you should observe at what zone the separation

of values begins to diminish." A. Adams

 

What does he mean by that?

 

Thank you

He means the region where contrast diminishes.

 

The slope of an H&D curve indicates the contrast of the negative. The steeper the slope, the higher the contrast.

 

Modern films and developers rarely show a 'shoulder' where the highlight contrast begins to compress. Not until you get into regions of extreme exposure anyway.

 

You can see a gentle rolloff of contrast in these Neopan 400 curves; starting about 6.5 stops above the speed-point.

If you scan film you can make the tone curve whatever you like, within reason. To add an artificial shoulder, or to correct one.

 

If wet printing, the film curve has to be concatenated with the curve of the paper. So the film's curve alone doesn't tell you the whole story.

Maybe these greyscales will help to make things clearer.

The top scale shows a normal 8 stop range with a one stop difference between steps.

Next from top shows the sort of compressed highlights you get from film with a shoulder to its curve.

Below that is the scale as seen on 'pushed' film - or N+1 development if you prefer - the tones are expanded and overall more contrasty.

And bottom is an N-1 developed greyscale where the contrast is reduced and all the tones are compressed.

Thx a lot Rodeo_Joe.

I will take some time to fully grasp the info you provided. But I will come back on it.

I'm reading " The Zone System Craftbook" by John Charles Woods. Hope things will clarify. I 'm new to the zone system.

IMO, it's easier to understand sensitometry first, then read up on the zone system. That way you get the correct technical understanding. This is free and good- https://www.kodak.com/content/products-brochures/Film/Basic-Photographic-Sensitometry-Workbook.pdf

Yes, for many the curve doesn't curve at all at the top.

They could run it farther out, until there is at least a little bit of curve.

Graphing film to see how it responds to exposure and development was pioneered by Ferdinand Hurter and Charles Driffield who published 1890. These gentleman are the founders of what is called the H&D curve. Scientist of that era relied on the slide rule, there was no computers or calculators. The slide rule is based on substation of addition and subtraction of logarithms (log notation) to multiply and divide (to name a few applications). The H&D graph is thus based on graph paper with logarithmic ruling. Ansel Adams applied the H&D characteristic curve when he and Fred Archer devised and published the “Zone System”.

 

 

It is proper form, film is procession exposed in ½ stop increments and the yield is maximum black thru minimum density, usually 21 induvial levels of exposure. The backing (density) of each step is measured using a transmission densitometer and then grafted on logarithmic ruled paper. The log ruling is required because after the thirteen or fourteen step the exposing energy is 8,192 times the first. Using ordinary linear graph paper would make the graph yards long. In other words logarithmic graphing base 10 makes the graph practical and I think, elegant.

 

 

The real increment of the graph is the f-stop. This is a 2x change in exposing energy. In other words, up the exposure 1 f-stop doubles the light energy, 1 f-stop down, half’s the exposing energy. The key value here is 2 (double or half). In log notion the value 2 is written is 10^0.30 (ten elevated to the 0.3 power). This is key to understating the plot because 1 f-stop = 30 units of density, 2 f-stops = 60 density units, 3 f-stops = 90d, 4 f-stop = 1.20d (each f-stop = 0.30 density delta because 2 in ordinary numbers = 10^.30 in log base 10 notion.

 

 

OK, keep in mind, 30 density units = 1 f-stop (this is a truism): But --- Thru only on the straight line of the H&D graph (region of film latitude). Also true only if the straight line goes upward at a 45° angle. Now the contrast of the photo material is measured by this angle. We take a protractor and measure the angle of this straight line. Then we use a trigonometry table and find the tangent of this angle (TAN). A 45° angle has a TAN of 1. If the TAN is 1, the angle is 45° and 1 f-stop has a delta of 30 units of density. The TAN of this straight line is, in the jargon of photography “GAMMA”.

 

Now comes the massage: A straight line slope angle, GAMMA 1 has been proven to be too contrasty for pictorial photography. Typically a negative that prints on grade 2 photo papers is considered to have “normal” contrast. For technical reason beyond the scope of this expiation, the slope of the straight line of a pictorial film is likely set to a GAMMA of 0.8 = 38 ½ °. Now the negative fits nicely on #2 grade paper however 1 f-stop no longer = 30 density units. This GAMMA .8 slope means 30 x 0.8 = 24. If you are following me, a GAMMA of .8 translates to 1 f-stop delta = 24 density units (not 30).

 

 

Bottom line: We adjust GAMMA via developer choice and time and temperature. When we uses these adjusters, we modify the delta ((count of the density units per 1 f-stop increment.

 

If this is confusing to you, I have been studying this stuff for 50+ years and its remains gobbledygook !

 

As to the H&D graph:

 

We divide this plot into regions: The region of under-exposure (toe). Region of film latitude (slight-line). The region of over-exposure (shoulder). The curve switches to a downward direction (region of solorization).

 

One key to understand the implications of this curve is to measure, with protractor, the angle of the stright line. Most pictoral films will measure about 36° (slope angle). For the moment assume 45°.

 

We find the angle and using a trig table, fine the Tan of this angle. The Tan of a 45° = 1. Note, this is a log (logarithmic) graft. Also note, the f-stop is the common unit of exposure. This unit increments 2X i.e. each f-stop increment constitutes a doubling or halfling of exposing energy. In log units base 10, a doubling of halfling = a delta of 10^0.30. In other words, if we up the exposure 1 f-stop, we expect the graphed density to clime 0.30 denity units. Conversely, if we lower the exposure 1 f-stop, we expect the density to decrease 0.30. Now this happens only on the stright line, and only if the slope angle is 45°.

 

By the way, the Tan of the straight line is called “Gamma” Thus we aret talking a film with a Gamma of 1.

 

Now pictorial films are deemed too contrast if they have a Gamma of 1. Mainly this is due to the the fact that a grade 2 printing paper is considered “normal” as to contrast. Also, in log notations base 10 a Gamma of 1 = a stright line angle of 45°. Since log 0.30 = 2 this means a 1 f=top delta translates to a density change of 30 units of density = 1 f-stop.

 

Because pictoral films are adjusted to yield “normal” contest on grade 2 paper, The typical pictorial film has its straight line depressed to a Gamma = 0.80 (an angle of 38°). This translates to: 1 f-stop alters the density of Gamma .8 film = 0.30 X .8 = 0.24 density units.

 

In other words the typical pictorial film responds to exposure change with a delta of 24 density units.

 

Edited January 8, 2021 by James G. Dainis

Several typos paper grade 2 not 1 is "normal" contrast.