In my last post (many many months ago) I explained my own education about Zone System and how this empowered my visualization skills and technical control to achieve a desired image quality. Being a Zone System practitioner many years in the analog medium, however, makes for translating the same concepts and practices to digital workflows fairly straightforward. Yet, for those beginning to learn ZS confusion abounds since the photographers and bloggers writing about its application to digital photography typically lack a theoretical and practical understanding of sensitometry, the chassis upon which ZS is built. Sensitometry, the study of light-sensitive materials, provides not just mere information about how a medium responds to light, but also provides predictive power of how light intensity captured from a scene will be depicted as brightness in the entire imaging chain from light in the real world to the human visual system. Zone System is a an easier way to understand sensitometry, but I think it gets short-shrift by photographers who think it outdated, or fail to grasp its concepts. In the next two posts I would like to address a recurring question that leads to confusion - why are there 11 Zones and where did this number come from?
The Zone System scale. If you want a quick rundown of ZS and the visual definitions of these Zones I recommend looking at Norman Koren's site here. Some photographers dispute having 0 and X since they seem largely irrelevant but in Part IV I can explain the importance for leaving them on any ZS scale.
While arguing about something as simplistic as the total number of Zones may seem petty I find amateur photographers can be confused about why there are 11 despite the fact that there are fundamental reasons as to why this number was chosen and why it works for a wide range of materials. The relationship between Zones and photographic materials seems simple, if each Zone represents a stop then many photographers conclude their film or digital sensor should be able to capture 11 stops of light. I have neither the time nor space to inventory all of the incidences I have encountered but I have already written about one in particular where Joe Marine on the No Film School site claimed that a false color exposure tool in the Red camera is a Zone System with 16 Zones to match the 16 stop dynamic range of the Red sensor. Perhaps a much more polished presentation of this same analogy is on Martin Bailey's blog and podcast where he claims:
"I actually have a measured range of 11.9 stops on my Canon EOS 5Ds R, and DxO Mark have it at 12.4, so we're at around 12 stops of dynamic range in digital terms. This is the full range from full black to pure white, and I consider almost that entire range to be useful, so it's a bit wider than Adam's definition..."
Here Martin Bailey is also equating the entire range of Zones (with each a one stop change in exposure) to the dynamic range of his camera. Now Martin Bailey does not say this explicitly but many other sources move forward with the idea that Adam's 11 Zones were determined by analog technology, which has been surpassed by digital, and therefore Zone System is either obsolete or needs modification. Sometimes the claims for digital's superiority leads people to connect unrelated concepts between two mediums which further compounds the confusion such as this claim:
I think the zone system is still important to understand in the digital world, although we now have 256 zones rather than just 10.
By this logic black and white film has either a bit depth of 3.3, or an infinite number of Zones since it is, after all, analog.
Regardless, the argument I frequently encounter runs something like this:
a. Zone System was designed for film, which has a limited dynamic range, and so could only at most record about 11 stops of exposure.
b. The modern digital sensor can record 12, 13, 14+ stops of exposure.
Therefore: Zone System is obsolete, or it should be modified to contain more Zones.
So where is the fallacy? In this post I want to discuss the dynamic range of film in order to disprove the first premise of the argument. In a subsequent post I will explain what part of the photographic process determined there be 11 Zones, and how knowledge of a few simple facts provides information that is revelatory for anyone in the visual arts.
A common misinterpretation of the history of Zone System by photographers and bloggers whether intentionally or not is that the number of Zones originated from the analog medium's dynamic range, and in particular that of negative film. If this is the case we should observe an 11 stop dynamic range from Kodak's sensitometry charts for any given negative stock. Just to set down definitions keep in mind that dynamic range is the range from the smallest to the greatest recordable signal by a medium. For digital cameras this is the amount of light that is recorded between the noise floor up to full well capacity. In analog this is the amount of light just above Base+Fog density on the negative up to where the curve tops out into the shoulder.
I intentionally selected Tri-X 400 in D-76 developer since I know this to be a film and developer combination used by Ansel Adams. Below are the current sensitometry charts and I used the 6 min curve because there seems to be wide spread agreement this is "normal" development.
From Kodak Tech Pub F-4017. Marks in pencil are inserted by me using methods I learned in Beyond the Zone System by Phil Davis. I should point out that this method of finding a film's DR is incorrect because the shoulder of the curve has not been provided in its entirety by Kodak.
By this analysis I obtained a DR of 10 1/3 stops which fits the conclusions typically thrust upon film that it only has a 10-11 stop DR and therefore this is what Zone System is based on. However, this DR analysis, which I've seen performed again and again by those who don't really work extensively with sensitometry, is flawed. Notice how the curve of Tri-X abruptly stops at a Log Exposure of 0.34 and density of 1.7. This is not because the film can no longer record further exposure as density, but because Kodak stopped graphing the curve. The reason for this is because film curves can be charted for quite a long range; a range quite far beyond what is needed for print or projection stocks. Kodak is basically supplying you only with the info needed for basic photographic purposes.
I cannot find a fully mapped curve (I have made them for my own chosen negatives as you can see in DotZS Part II), but I did locate a depiction of a full film curve in Langford's Advanced Photography in a section on how extremely high levels of exposure produce the phenomenon of solarization. For your interest I calculated the DR of this unidentified film to obtain approximately 13 stops.
From M.J. Langford's Advanced Photography 1974, page 169
So, the only way I can calculate the DR of Tri-X is to use a french curve to best fit the shoulder and further extend where the line cuts off. I extrapolated the following DR:
Using a french curve to extend the shoulder of a curve is a practice recommended in Phil Davis' excellent book Beyond the Zone System. The best method of finding the curve of the shoulder would be to perform one's own sensitometry test and create a new chart with a densitometer.
Before crying foul that I manipulated data I would also recommend looking at Kodak Vision 5219 motion picture stock. For this sensitometry curve Kodak updated their thinking on how to graph these curves by extending the x-axis for a greater range of exposure as well as showing much marking the x-axis in camera stops. Once again you can see a rather large DR is covered by this film and one could argue that the shoulder is still cut short.
From Kodak Tech Pub H-1-5219t.
If film possesses the same dynamic range as many modern digital sensors then it is incorrect to claim that Adam's system of Zones is based on the DR of the capture medium. Both film and digital possess a range of recordable exposures beyond the 11 Zones of the Zone System.
Did Ansel Adams make a mistake? Have we been using Zone System incorrectly this entire time? No, the reason for 11 Zones has a very logical origin, and one that is surprising to many photographers who know little about the nature of human vision, and how the final print, projected transparency, or electronic display must depict tonality in order for the image to appear "correct" to our eyes.
More to follow in Death of the Zone System part IV.