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 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.
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.
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.
*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."