smieglitz said:
Stephen,
"...my "system" for film testing is to use the camera and lens combo I normally reach for, and do tests in camera not in the darkroom. While I know this is not as accurate as using an enlarger to control variables, my basic idea is that flare is inherent and part of my exposure inside the camera. Since film tests are the result of in-camera exposures, flare is included as a normal variable and dealt with in development."
If, for example, I choose to do zone system testing using a specific lens and camera and an average target (gray card), isn't the flare factor automatically taken into account? I mean, as long as I don't change the equipment and I photograph the same types of scenes always (after doing a pictorial test following the gray card density and CI calibration trials), as long as everything comes out in a way I can predict, why should I be concerned with all the computations and graphs regarding flare? Isn't that already visually represented in my personal data?
I don't mean to be dense here (no pun intended), but I see what you are discussing as highly theoretical, great laboratory science, but not of very practical use to me. In a nutshell, I'm after a personal EI, not an ISO measurement of film speed.
FWIW, in practice I've found a CI of about .62 worked better for me with TMY when I printed on Ektalure. (I really haven't made an enlargement or silverprint in a couple years since I moved to LF & ULF and Kodak stopped making every product I liked.) I liked my negative zone I to be about 0.12 above fbf and zone V to be about 0.70 above fbf. IIRC, my paper range was around 1.45 for that scale. I've observed that my negatives tend to be more dense and contrasty than others' but they work well for me.
As an aside, I'm finding your refences to Gustav Fechner and Psychophysics interesting and plan to check the citations you've listed in that regard. Also, I'm not sure anyone actually gave you the answer you were looking for in your thread on the Sunny 16 rule, but I believe that is derived from the "Exposure Formula." An old version of that formula (via AA in "The Negative") takes the reciprocal of the luminance of an object in candles per square foot as the shutter speed while the "key stop" is the square root of the ASA speed. Somewhere Ansel gives the luminance of the clear north sky 45 degrees from the horizon as 256 foot-candles. If one was using ASA 64 film the exposure would be 1/256 second at f/8 for that "average" subject. This would equate to an exposure of 1/ASA @ f/16, here 1/64sec @ f/16 by the Reciprocity Law. I've never read that derivation anywhere, but I believe that is where the Sunny 16 Rule originates.
Joe
Wow, Joe, I hope you don't mind if I just touch on some of the subjects you've raised in your rather long and interesting post. The quote you give was the subject of the recent article in PHOTO Techniques. In a nut shell, I found that while there is some flare involved in shooting a gray card, it is only a single tone and since 80% of veiling flare comes from the subject, the card isn't sufficient enough to approximate the influence of flare in general use. I even tried a using a card with a checker board of tones, but it was near impossible to find the right tonal distribution. Ultimately, I found it best to use a technique that eliminated as much flare from the testing as possible and add the flare in later. I actually had lots of data, but it had to be cut from the final draft because of length. The original manuscript was 7000 words and PT likes the average article to be no longer than 2000 words.
The average shadow falls around 1.28 log units below the meter reading exposure. As you know, ISO film speed is measured at 1.0 log units below the meter reading exposure point, and the Zone System stops the lens down 1.2 log units. The point where the shadow falls is approximately the same point as the fractional gradient speed point. While I'm thinking about it. The speed point isn't necessarily the point where you desire the shadow exposure to fall. It is mostly just a place in which to determine speed. Anyway, the one to 1 1/4 stops of flare bring the shadow up to just over the speed point creating a small safety factor. Of course, this is with normal flare. Higher flare conditions will produce a higher film speeds and lower flare condition will produce a lower film speeds. So, those concerned with "nailing" their personal film speed are mostly chasing their tail. I'm not saying don't test. It's that some people don't know when to stop.
Now the problem I had with the ZS method of film speed testing is that it doesn't account for the addition of flare on the film speed. So, stopping down four stops is almost the same as the 4 1/3 stop difference assumed for the shadow in the standards, but the ZS stops there. People usually end up considering their film speed to be 2/3 stop slower than it is. Not that there is any wrong with a slight safety factor. I just think it's better to know you have a slight safety factor and not mistakenly thinking it is the actual film speed.
An interesting observation that I couldn't cover as well as I wanted to in the article starts with the ZS originating in the 40s. At the time, the factional gradient speed method had approximately a stop safety factor. Most of the time, people using the Zone System found their personal speeds closely matched the ASA speeds. In 1960, the safety factor was dropped. Film speeds effectively doubled, but the Zone System didn't change. What does this mean? It means the results from ZS testing give speeds more like the pre 1960 standards then the current standards. So, if you ever read anybody complaining that there was too big of a safety factor prior to 1960 and that it had a detrimental affect on film quality, think about all the high level of quality all the ZS people have achieve. I think this is another example where theory can be beneficial to practical photography. Actually tone reproduction and exposure are both considered theories, just like gravity is still a theory. Technically, you are always using theory.
The thing about theory is that it is only theory until you need to use it. The variance in film speed is theory, but with it, you are aware the degree of tolerance in exposure and the degree of accuracy. You, yourself are also using theory in your testing with a camera example. It does come in handy.
Ultimately, all the testing comes down to understanding the materials, and if you have a negative that does what you want it to do, it doesn't matter what technique you used to test with or if you tested at all. Who cares if your negatives have a higher contrast then other people. It's a good negative if it works for you. Testing just tends to simplify, clarify, and to better define. It's a helpful tool - not an end.
About psychophysics. I personally feel it is at the very foundation of photography. Without it there wouldn't be a standard of quality. After all, quality in a photographic image is subjective.
One of the reasons why Lyod Jones' work has stood the test of time is because he was the first to use psychophysics in connection with photography. Remember at all those different film speed methods before he came along? Before Jones, a particular methodology was chosen based mostly on assumptions. Hurter and Driffield put a lot of emphasis on the linearity of the straight-line portion. The Germans thought that by developing the film to gamma infinity, they would establish a constant contrast from which to determine the film speed. And the reasons went on. Jones had people pick the best looking prints. He then looked at the exposure and development information from those prints, established a procedure from the information and then compared the new procedure along with all the other film speed methods, then compared the results derived from the different methods to the technical information from the judged quality prints. The method that came closest to achieving the same quality in the most number of situations represented the best film speed method. BTW, Jones has a couple of papers specifically on psychophysics and photography if you're interested.
You remember the Sunny 16 mess? That was yet another example of me miscommunicating my intensions. You are really close with the 256, but it isn't footcandles (and you need to add K to the value). It is footlamberts (divide footcandles by pi), and it isn't exactly the 45 degree explanation and definitely doesn't have anything to do with Adams. This is a really interesting subject. What makes it special for me is that for any exposure, your exposure meter wants to give the film 8 meter candle seconds of exposure at one second. The value of 8 thus becomes a constant represented as P. P = 8. For Sunny 16 (exposure equation has f/stop as A^2, so 16^2 = 256, giving unity between the exposure and f/stop. All that is left is to divide P by the shutter speed and when the shutter speed and film speed equal unity, the resulting exposure equals the exposure at the meter calibration point or 8/film speed (8/Hg or P/Hg). Notice how P is 10x greater than the constant used to calculate B&W film speed (0.8/Hm). Sorry, this is a rather bad and brief explanation of Sunny 16. What I find really cool, though, is the interconnection of all the equations and values and how it can all be reduced down to two numbers. Something for another day and thread.
I hope I covered most of your questions. Let me know if there are any gaps or omissions. Also, I've attached an attempt at an example of the exposure equation and its relation to P and Sunny 16 (somewhat).
