Sensitometry and the Plotter/Matcher, Part I

Backing up a bit from yesterday, I acknowledge that a number of people were mystified as to, well, what the heck I was talking about. A densitometer, more specifically a transmission densitometer, is simply a device for measuring densities—usually, for our purposes, on negatives.

As you know even if you've never shot one in your life, a negative is a tonally reversed image; dark things in real life (called "shadows" through the photographic process even if they aren't literally shadows) expose the negative very little, resulting in clear (transparent) or close-to-clear areas on the film; bright things in life (called "highlights") result in heavily exposed areas of reduced silver metal in the negative—the dark parts.

A negative. Photo by John Loengard. Extra points if you
recognize whose negative it is.

A transmission densitometer merely measures how much density there is in any given area of a negative. It's just a measuring device, like a scale or a thermometer or a ruler.

Of course, the fun doesn't end there. Because anything you can measure, you can plot. (Mathy types rub hands together, twirl ends of mustachios.)

What sensitometrists do—well, the activity we're mainly concerned with here, anyway—is to expose a film to various measured amounts of light, develop the film in a particular manner (i.e., with the developer you're interested in, at a controlled time and temperature), read the density, and plot the amount of exposure against the density it creates on an X-Y graph.

If you do this a bunch of times for a bunch of different exposures (again, with the same film and development protocol) and plot all the points on your graph, you can then draw a line between all the points. Voilà—the film curve.

 

Courtesy nfsa.gov.au

The parts of the curve are labeled in this nice illustration I found (thanks, Australia!), so I won't belabor them further. Except to note that "base + fog density" (commonly written fb+f and pronounced "film base plus fog") is simply the "background density" of an unexposed but developed and fixed piece of film. You can see the fb+f at the edges of the negative in John Loengard's photo above.

The curve was first devised by Ferdinand Hurter and Vero Charles Driffield, the fathers of sensitometry, in 1890, and was called the "H&D curve" in their honor for many years. Now it's more commonly known as a "characteristic curve" or "film curve." Sometimes you encounter the terms "D-LogE curve" or "Y'know, that charty thing for, like, the films or whatever" ('80s highschoolese*). All different names for the same thing.

Now, see that straight line section of the curve? How steep that is describes the contrast of the negative. Develop the test films for more time, the straight-line section gets steeper or more vertical. Develop for less time, and the slope gets milder. That's the origin of the term "Contrast Index" or CI, which was an method developed by C.E.K. Mees and Loyd [sic] Jones of Kodak Research Labs to describe contrast with a single number; there were even handy CI protractors for measuring the angles of curves. Mees and Jones, by the way, are responsible for our current system of B&W contrast management: developing to an average (or ballpark) CI and then fine-tuning contrast with paper grades. Different story for a different day.

More usual in more modern times has been to plot a "curve family"—several curves describing the same film in the same developer but developed for different amounts of time. Here's one of those, also clearly labeled if you'll look at it for five seconds. Note that as the development times go up, the curves get steeper.

Now, if you think about this, that represents an awful lot of test exposures if you're exposing one sheet (piece) of film at a time in, say, a 4x5-inch camera. So, naturally, a shortcut's been devised. What photographers do now to get this information relatively quickly is to use a pre-made piece of transparent material called a "step tablet" or "step wedge" that has a number of steps (usually 11 or 21) of known density already on it. One such is sold by Stouffer Industries. So what you do is expose your film to this step wedge, then read the densities on the film with...you got it, a densitometer.

A densitometer, as I mentioned yesterday, is very easy to use. You turn it on, put the film over the sensor, press the button or lower the sensor arm on to the film, and the reading comes up on the readout. That's all there is to it.

The other type of densitometer is a reflection densitometer, for reading densities on prints.

So what's sensitometry for?
Ctein said yesterday that he has a densitometer but has no use for it. That makes perfect sense to me, because he's mainly a color photographer. Consider this passage from Ansel Adams's fine book The Negative, in its various iterations over the years possibly the most inspirational technical book ever written (cited as such by literally generations of photographers):

Many consider my photographs to be in the "realistic" category. Actually, what reality they have is in their optical-image accuracy; their values are definitely "departures from reality." The viewer may accept them as realistic because the visual effect may be plausible, but if it were possible to make direct visual comparison with the subjects, the differences would be startling.

—Ansel Adams, The Negative, from the Introduction

A densitometer to my mind is really only useful in creative photography in helping to manage the "departures from reality" of the tones in black and white photographs. And, for the most part, they're used by people who can develop film one picture at a time, which means sheet film, which means large-format photographers, who use view cameras.

How any B&W photographer departs from the norm in terms of tonal values is an indivisible part of that photographer's style, from the high contrast of a Ralph Gibson to the low contrast of a Henry Wessel. It's one thing that makes B&W photography so different from color photography: every good black and white photograph is an interpretation.

Ralph Gibson, Priest Collar

Henry Wessel, Albuquerque, N.M.

Note that I've named two photographers who've probably never used a densitometer in their lives. That's entirely by design, because most photographers don't...and don't need to.

Yr. Hmbl Ed. hopes that all the above has been relatively pain-free. Even more fun stuff to come, so please stay tuned.

Mike

*Working with high school students can cause one to crave nouns.

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Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.

Featured Comment by Dennis Mook: "This brings back memories of the mid-1970s when I was learning the Zone System by reading books by Picker, White/Zakia & Lorenz, Adams, and others. I was a forensic photographer at the time and was fortunate to have all of the resources of the police department's darkroom at my disposal. I dutifully exposed many, many rolls of 35mm Tri-X (our standard crime scene film), Plus-X, and Panatomic-X as well as sheet after sheet of 4x5 Tri-X (my personal work) according to procedures laid out by my literary teachers. I then used a transmission densitometer to plot film base + fog and establish characteristic curves for these films with several developers. I found myself totally caught up in the technical side of photographic chemistry at the expense of getting out and making aesthically pleasing photographs! Did all this experimentation make me a better photographer? Maybe in some way as it allowed to understand the very simple 'expose for the shadows and develop for the highlights' axiom, albeit at the cost of a lot of time. I believe the one other thing it taught me was to be very disciplined in my approach to making my art which paid off in my photography as well as printmaking."

Comments

[Rob Atkins]

The negative in Loengard's photo is: Imogen Cunningham, Magnolia Blossom, 1925. His photo is included in his book Celebrating the Negative, a book I treasure. It's a virtual course in photography: to see how the prints we all know so well look as negatives is truly enlightening. Harry Callahan's Aix-en-Provence, for example - the image comes to life in the printing. The negative holds so much more information, yet it's Callahan's interpretation of the negative that creates the magic.

I sent John Loengard an email last year telling him of what high regard I hold his book and received very a gracious reply. I recommend Celebrating the Negative to, well, anyone who loves photography.

[Allan]

This with densitometry seem to be difficult for the big boys. In the eighties and early nineties Fuji wrote the numbers wrongly underneath the diagram. They wrote 3.0, 3.5, 2.0, 2.5, 1.0, 1.5, completely unaware that the it was logarithmic values and should have been -3.5, -3.0, -2.5, -2.0, -1.5, -1.0. After I commented it form them they simply dropped 3.5, 2.5 and 1.5 from the diagrams.

Kodak loved to show diagrams on their data sheets for their films. Once I played with their diagrams and data. The inclination of one curve would be a dot on a curve in another diagram. To my surprised the dot didn't even come close to the curve! I showed it to several others that confirmed I made the correct calculations.

Ilford opted for the simple way and not include any diagrams.

The most accurate data sheet came from Agfa, I never found any errors.

[Clifford Gwinn]

Imogen Cunningham

[Richard Man]

I am sure this is one of the standard reply phrases in TOP:

"It's article like this that makes me keep coming back."

No, I have not used a transmissive densitometer before (although I did use a reflective one to plot the curves for digital negative work) and probably will not use it ever as I am mostly only shooting 35mm film and I have a hybrid workflow that works pretty well, but this is just one of those well written, short but not too short explanatory articles.

Keep it coming.

[Dale]

I believe that negative was made by Imogen Cunningham... yes?

[Arne Cröll]

I think that negative is Imogen Cunningham's "Tower of Jewels (Magnolia Blossom)".

[Mikkelsen]

I do all my post film-development processing and printing digitally these days and have no use for a densitometer either. I do however adjust my film development parameters based on issues encountered during film scanning and this may relate to density management. Wonderful capture of this caring and elegant presentation of Imogen Cunningham's negative for "Magnolia Blossom".

[Dennis Allshouse]

Surely these days Microsoft Excel would be up to the numerical and plotting side of the process.

[Carl Weese]

So, everybody knows it's by Imogen, it seems. I also have a densitometer I haven't used in years, partly because it doesn't measure UV light and so is useless to me since it can't read the density of my pyro-developed negatives as the UV-sensitive Pt/Pd paper will see them. As a point of interest, it can be used with ordinary HQ-developed negatives in the following way. Measure and record the densities of the step wedge. Print the step wedge for least time needed for full black in the print. Now, measure highlights, shadows, and other important areas of a new real-world negative with your densitometer, refer to the print of the step wedge, and you see what tones will result at standard exposure.

In practice, what I actually do (and teach) is write exposure and formula on the border of each Pt/Pd print. To print a new negative, I look for a similar picture that I've printed before and see what exposure/formula was used, then get the negative out of the files and look at it on the (big) light box next to the new negative to see how they differ. Mileage may vary, but I find this much more practical than use of a densitometer.

[Iain]

“Nowadays, the whole guild of the sciences is occupied in understanding the canvas and the paint but not the picture; one can say, indeed, that only he who has a clear view of the picture of life and existence can employ the individual sciences without harm to himself, for without such a regulatory total picture they are threads that nowhere come to an end and only render our life more confused and labyrinthine.” Friedrich Nietzsche

[robert]

I got to use a densitometer once, at the Maine Photographic Workshops where we were schooled by Tillman Crane in the art of black and white--for several years thereafter I had hand developing down "cold" or should I say 68 degrees.

You can apply the rules of the zone system to roll film, and understanding how development affects negs is really important, and it makes me sad today that I go into pro labs with my film and the shake and bake it so hard that I get unprintable (scanable) negs.

With all of this sudden resurgence of film love out there in internets I have to sigh deeply--for the most part none of the processing/printing/Fuji Frontiering is holding a candle to what film can do--it's all garbage processing and mucky results.

And yet it survives and seems to thrive which I guess is a good thing.

[Robert Poe]

Maybe I'm way off here, but can a reflective densitometer be used to test the relative D-max of various inkjet printing papers? There are many other factors in selecting a paper, but if I'm printing B&W, D-max is an important consideration.
...bob

[JPH]

Very interesting article, thank you for writing it, explaining what a densitometer was... I had heard of them before, and knew they were a darkroom tool, but having never used one, had never really known what they were used for... which also led me to be lost in yesterday's entry of yours... so again, thank you for taking the time to educate some of us on it all.

Also (off-topic), thank you very much for using as an example, Ralph Gibson's Priest's Collar photo. Throughout the years, I have harvested photographs that have caught my eye, have admired and liked, and/or have wanted to have to use for inspiration... all from here and there, wherever I have found them on the world wide web, downloaded and have them in an 'External Photographers' section in my iPhoto to peruse and enjoy.

Currently, my count is 771 of them (two of them are yours, by the way - 'Unscared Of The Dark', and 'Rockcreek Park'), and have always made sure that I got the name of the Photographer, and the title of the image, if I could... there were sometimes when the info was never available for some reason. Also, when I very first started doing it, I was negligent in making sure I acquired that information.

I think Mr. Gibson's image was one of those early ones. Seeing it here though, I recognized it instantly. So, was able to go in and duly note that wonderful image with his name and title.

Coincidentally, something like that occurred just last week too, with the passing of Eve Arnold, in checking out her work, I discovered she was the Photographer of one in my External Photographer collection, 'Bar Girl in Brothel, Havana, Cuba'... though, I already did have her name credited to the photo, as well as the photo's name.

Okay, anyway, just wanted to thank you again, for both the informative article, and the little side thing of Mr. Gibson's photo.

[Paul]

Imogen Cunningham. If I were a little younger, and she was a little more alive-er...

Seriously, I think she is the most talented of all the west coast photographers of her era. More blasphemy.

[JH]

In fact, I'll bet most of us have densitometers in our possession; they are called "scanners." However, instead of providing densitometry measurements, they reproduce the density of the negative on a screen mapped to produce an image.
Should someone want to write the software, you can get the same data as a densitometer. It may not have the same dynamic range, but the function is similar.

[JH]

After I posted my comment, I did a search for "densitometry software for scanners" and this popped up: http://www.polari.com/densitom.htm
It's been done numerous times.

[Walter Glover]

My experiences with density meters were very similar to those of Dennis Mook. There are ongoing benefits from the discipline and understanding but at considerable cost in time and materials, to say nothing of shifting the purpose of photography momentarily from the content to the medium. I have several acquaintances for whom this momentary shift endures as a life-long obsession.

Walter

[Mike Johnston]

"I have several acquaintances for whom this momentary shift endures as a life-long obsession."

There are a lot of diversions along the way, aren't there Walter? Some of them terminal. For me testing cameras all those years was not good for my work. But the testing paid and the art photography did not, so that's that then.

Mike

[Mike Johnston]

"(two of them are yours, by the way - 'Unscared Of The Dark', and 'Rockcreek Park')"

Thanks, JPH, I'm complimented.

Mike

[Don Bryant]

Thanks for mentioning Wessel, he is one of my favorites.

Here is a little video on Wessel that shows his (non technical) approach to printing in the darkroom. You have to admire his organization.

http://www.kqed.org/arts/programs/spark/profile.jsp?essid=17900

[Kenneth Tanaka]

Indeed it is Imogen Cunningham's Magnolia Blossom. It's a very pretty print.

[Roger Moore]

It's one thing that makes B&W photography so different from color photography: every good black and white photograph is an interpretation.

I don't think that distinguishes color from black and white photography at all. Every photograph is an interpretation. Though color photographers may not have as much control over developing their negatives as black and white photographers do, they still have a lot of room for interpretation through their choice of film and printing method. Color digital photographers obviously have even more room for interpretation.

[Gary Nylander]

I've never used a densitometer either, but I have studied the zone system and have simplified it to work for my own way of seeing.

[scott kirkpatrick]

While today is digital and positive color, not old school B/W, I still hear zone system language used. But there is an important difference -- digital imaging is linear. One photon leaves behind one electron. Electrically changing the threshold (or ISO) shifts the curves to the left without changing their slope. Increasing development gives a fan of increasing slopes (midtone contrast). While I have never seen curves like the H&D Kodak charts for a digital imager, I would guess they look like slide 5 in www.cs.huji.ac.il/~kirk/Electronic_Imaging.ppt . Have you seen this discussed somewhere?

scott

[Pekka Kiirala]

Densitometer is a must when developing E-6 films. My first job in photography was E-6 technician and I developed and measured test strips at least three times a day - with Machbeth densitometer.
Misunderstanding is, that color reversal process produces same and exact colors every time. Actually colors shift and process needs adjustments during a day. Without test strips and densitometer technician would be flying blindfolded. Of coarse best reference was developed films and sometimes plots were well inside limits but real transparencies had color bias.
I also looked after our B&W process, but never used densitometer because process is much more constant and photographers did not bother to expose negative film so accurately than reversal film.

[Mani Sitaraman]

Carl, there appears to be a pyro (i.e. UV) densitometer on the market.

http://www.darkroomautomation.com/pem.htm

It would likely be superfluous for someone with skills as masterful (and masterly) as yours, but have you come across it? Is it any good?

[Ian Goodrick]

Not used a densitometer for many years. I was introduced to them at college in the late 70's, as it was part of the technical side of the course.
The only time I used one in anger was while employed as a staffer at a photographic department at a large local government office here in the UK. We were using a Kodak Versamat processor and I was given the task of raising the quality of our B+W output, as I was the only photographer who had been to college and new how to use the densitometer I ended up with the job. This was no great task, each morning when the machine was up to temperature, a pre-exposed test strip was processed and a curve plotted. if it was out of tolerance the developer was adjusted, another strip processed and plotted if that was ok that was it for the day.
I had not thought about this for years but now I can even recall the smell of the processing room I was working in.
Ah memories!

[Ian]

I am lucky to have access to a densitometer and in the last year have used it to get a grip on my processing times for 5x4 sheet film. Being a physics type it worked for me.

The key thing is if you understand what it tells you, it takes all the guesswork and uncertainty out of it - even if most of the people on the forums seem to like the experimentation.

Muddy shadows? - there in the (long) toe of the curve. This explains why Bruce Barnbaum advocates zone 4 shadow placement - TriX has a long toe - (even though he decries sensitometry). FP4 in D76 does not need it.

My second point is when you have it sorted out for your film / dev combination, put the densitometer in the cupboard and take some photos. Don't be a geek.

[James B]

I had no idea of whose negative that was, nor had ever heard of Imogen Cunningham. Even worse, looking at the negative and trying to inverse the tones in my brain, I was able to convince myself it was a photograph of a model taken from behind her, with tied-back blonde hair in a pony tail with some form of decorative hair slide, naked shoulders, and quite skinny.

Gulp. It's a flower. I'm going to need to go back to Photography 101, or Ken Rockwell. Or blind myself as my eyes are clearly useless.

[Ed Cornachio]

Ansel Adams had two secret Zones, known only to himself.

[ctein]

Dear Allan,

Wow, did you jog an old memory.

The Fuji data sheets were not in error; they used an obscure (my opinion) system of engineering notation I encountered back in high school. It dates from the time when calculations were done by slide rules or calculators. Who were PEOPLE. (Although an annoying percentage of engineers considered them only as tools of convenience they called "gals." Ah, for the good old days... not such much.)

Numbers with negative exponents were written oddly. The series 1, 0.8, 0.5, 0.2, 0.1, 0.05, 0.01... = 1x10^0, 8x10^-1, 5x10^-1, 2x10^-1, 10^-1, 5x10^-2, 10^2... got written out as 0,1.8,1.5,1.2,1, 2.5, 2 ... The number before the decimal being the power of ten and the number afterwards being the multiplier. I *think* an overscore (maybe it was an underscore?) was used to indicate it was a negative exponent. Why not use the minus sign in front to indicate a negative? Because then there was the possibility to confuse a negative exponent with a subtraction operation.

Supposedly this made engineering calculations easier. I never got that-- it did use a slide rule but I WAS a calculator (one of three things Ansel Adams and I have in common)!**

As an example, 0.5 x 0.8 would be (bar)1.5 x (bar)1.8 = (bar)1.4

Much clearer, of course, right? Uh huh. Sure.

I never found a systematic error with Kodak data, but editors in tech pubs being human, the wrong graph would occasionally appear with the wrong product. There was also a great deal of confusion when Kodak switch to fixed-point-slope and contrast-index systems of measuring film contrast instead of gamma. If you consistently found a mismatch between graphs, it's possible you were misreading a c-i graph as a gamma graph.

pax / Ctein
==========================================
-- Ctein's Online Gallery http://ctein.com
-- Digital Restorations http://photo-repair.com
==========================================

**(the other two are a scrinchy sorta voice and a truly fine beard)

[Karl]

Like many photographers in the day, I developed an "eyeball" system based upon the the fundamental control elements: one film; one developer; one enlarger, one paper, a fixed and repeatable proofing time/aperture/enlarger head height;and a ring-around to establish the actual film speed of my combination.

Beyond this -- and of course this may well be personal hoodoo -- I think that densitometers can evolve you in the wrong direction: the techno-clinical rather than the emotional.

Just sayin'.

[Noons]

And for the top prize, the only thing missing is the very simple, but often forgotten:
"Density range of a film is NOT the same as dynamic sensitivity range"!
I can't count the number of times I've seen online discussions where some purported "expert" claims to be able to deduce the lighting range of a film from the density range curve.
Film with a narrow density range can react to a wide dynamic lighting range and record it as a gradation of tones. The reverse is also true.
I call this tonal compression, for lack of a better term anywhere.

[Karl]

Looking at the photo of the negative, looks like he could have given it another stop of exposure -1. Shadows are blocked :-)

[ctein]

Dear Noons,

The way I explain it to people, very simply (and slightly imprecisely) is, "Exposure range is what goes in. Density range is what comes out. The ratio of the two is not fixed, and it's called (overall)'contrast'."

Seems to work for most folks.

pax / Ctein

[Wayne]

Thank you very much for this excellent piece. You have explained, very well, something I have always been curious about. I do prefer B&W photos and have previously encountered references to "density." Your effort here will make future viewing and reading more enjoyable.

I definitely prefer this type of writing to general whining about weather and elevator music. Wait, now I am whining. See how ugly it is? Imagine how disconcerting a whining giant must be to those around him.