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  1. #141
    CPorter's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Every successful photograph I make shows theory. Isn't that the foundation of your defence of the Zone System? The quality of Ansel Adams' work is proof of the theory.
    Putting it in terms of theory is certainly acceptable, but the foundation of my defence of the Zone System lies in learning how valuable (to me) the process of visualization can be. My photographic "craft" is leaps and bounds from what is was, because of the Zone System, and being proficient in the craft of photography is the path to becoming better at visualization, IMHO.
    Last edited by CPorter; 01-23-2013 at 08:43 PM. Click to view previous post history.

  2. #142
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Are you reading the first or second version?

    Concerning exposure and film speed theory, can you outline some of the issues? Do you have trouble with how he frames the various historical methods and their underlying principles/assumptions? Fractional gradient? Or is it his method for an in-camera test for "effective EI"? Etc.?
    Michael,

    I have the second edition. So far, Iíve only been able to review some of Chapter 6: Film and the Making of a Negative, and scan through the rest. Havenít read this book in more years than Iíd like to admit. Still hate the formatting with the dense type, but now Iím even more impressed with the sheer volume of testing and detail.

    In order to help keep this from getting too long, Iíll touch on a few points I found interesting. The section on page 137 Geometric Variation in Illumination of the Film in the Camera nicely covers an important topic rarely touched on. He notes that the average light loss at an optical angle of 15 degrees off axis averages around 20% (12 degrees is the value the exposure meter standard utilizes for calculating K and q). Depending on the focal length and f/stop, the fall off can be easily Ĺ stop with over one stop not uncommon.

    While Henry doesnít touch on it, the first thing that came to my mind was how this information would effect in camera testing with a contacted step tablet. The portion of the step tablet / film where the degree of accuracy is most critical falls towards the outer edge of the image circle where the greatest proportion of light loss occurs.

    His summary of the history of film speed determination was good up to the fractional gradient part and most importantly the relationship between the fractional gradient and the current method. I believe he need to flesh out more about the concept of film speed and the reason why the fractional gradient was chosen. Even though Henry references the Nelson and Simmonís paper that introduced the Delta-X Criterion, which the current ISO speed method is based, he gives the impression that using a fixed density of 0.10 alone has a valid correlation to the fractional gradient method. I donít know if he just muddled this part or meant to imply the fractional gradient method and itís principles were abandoned for the fixed density method, but thatís what Iím left with.

    One page 142, Henry wonders why in the 1979 speed standard there was a statement in the forward, not considered the official part of the standard, that read, ďFor any specific developer, the sensitometric speed rating method of this standard includes a 1/3 camera stop underexposure safety factor.Ē He was rather puzzled at this and stated that ď1/3 stop overexposure was built in,Ē and why wasnít it in the standard?

    I think heís making more of this than is necessary. Between the fractional gradient standard and the 1960 ASA standard, the safety factor was reduce from around 2.35 to 1.2, as explained in Nelsonís Safety Factors in Camera Exposure, which is one of the two papers where the 1960 ASA speed standard was derived. Henry does talk about the 1.2 (1/3 stop) safety factor on the previous page. I believe the reason it was listed in the forward is because the safety factor is based on the statistically average scene luminance range which isnít a constant so the point where the shadow falls cannot be consistent and therefore neither can the safety factor, so such a statement couldnít appear in the body of the standard.

    These might seem like minor examples (and they are), but they help evaluate his knowledge of theory (and mine). I havenít gone through the entire book yet, but there are a number of things I really donít agree with and yes one of them is his preference for in camera testing, incorrect concept of metered exposure point, using the fixed density method to determine film speeds outside the ISO parameters, and more. Which we can go into in more detail later.

  3. #143

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    Thanks for the summary and confirming you're looking at the second edition. I only asked about the edition since in the second edition he notes that the first edition contained a number of errors in the film speed section.

    I'm far from being an expert on ISO, ANSI and the history of speed determination (including the various other methods going all the way back to H&D) so it was hard for me to evaluate Henry's text until he gets to in-camera testing etc. Light falloff and the possible effects on in-camera testing with contacted step wedges had also occurred to me when I first read Howard Bond's article on film reciprocity failure. In my own in-camera EI testing it had previously never even occurred to me to use a step tablet in the camera.

    Since I'm not all that knowledgeable when it comes to ISO speed testing, I found it hard in Henry's book to make what seems (to me) like a leap from the ISO discussion right back into an in-camera test using a grey card, placing it on Zone V, closing down 4 stops etc. to determine a working EI. There seems to be a disconnect there in the text. But I guess he just wanted to go through the proper theory first and then proceed to some more "real world" EI tests.

    One thing that still puzzles me. If one does that typical Zone System EI test with the outdoor grey card, is there a difference between metering the grey card and metering a white card?

  4. #144
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    Quote Originally Posted by Michael R 1974 View Post
    Light falloff and the possible effects on in-camera testing with contacted step wedges had also occurred to me when I first read Howard Bond's article on film reciprocity failure. In my own in-camera EI testing it had previously never even occurred to me to use a step tablet in the camera.
    I think this is a good example of my earlier topic. A photographer would more than likely consider the question of accuracy of an in camera step tablet test if they knew of off-axis light fall off.

    As for Henry's book, I'm glad there's someone who feels the same way.

    With grey vs white card question, I guess there could be a small question of the linearity of the exposure. The materials the cards are made of and how they interact with difference wavelengths, like infrared, and the exposure meter's photo cell can also make a difference; but I don't think there's anything fundamentally different with either as a choice of a testing target.

    Otherwise we've already discussed how as a choice of testing targets they shouldn't make a difference. They both represent the only exposure value in the test so there's nothing for them to influence.
    Last edited by Stephen Benskin; 01-24-2013 at 08:52 PM. Click to view previous post history.

  5. #145
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    Two related items from Henry. On page 184 he states "Kodak actually recommends a safety factor of an overexposure of 1/2 to 1 stop. When measuring exposure from a neutral gray card (18% reflectance, RD 0.745) Kodak recommends increasing the indicated exposure by 1/2 stop." Now I haven't read the two Kodak publications he references, I did notice he doesn't have any of the papers by Connelly in the bibliography. I'm a little surprised that all the effort with exposure meters, he never seems to have worked out the relationship between C and K. Also on page 145 he talks about the 0.10 speed point being 4 stops below Zone V (meter reading).

    Maybe this is comes from trying to balance the popular terminology with the scientific information. Maybe he didn't read the papers on exposure. There is a reference to Stimson's An Interpretation of Current Exposure Meter Technology, but not Connelly's Calibration Levels of Films and Exposure Devices which was critical to my understanding of exposure. Maybe exposure theory isn't an interest of Henry's.

  6. #146

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    The whole meter issue really bothers me. It's not Henry's fault. But I don't like problems I can't realistically solve, so this is an area where perhaps I was better off being ignorant of these things. Just reading the list of meter modifications (various filters, baffling) was depressing enough, but I wonder if the Minolta Spotmeter F I've trusted all these years really reads anything close to 1 degree.

    I'll have to re-read some of the book this weekend, but I'm glad I wasn't totally out in left field when I found Henry's jumping back and forth between exposure theory and Zone System testing somewhat confusing.

    For today I will leave you with another of my flare questions - I think I've asked this before but perhaps not in the same way so I apologize in advance for being repetitive. It's just that often with this stuff I think I get it and then the more I think about it I'm no longer sure:

    Example: In-camera test. Target is a white card illuminated by two flood bulbs. So the white card is very bright. Now, for simplicity (since exposure theory precision is not relevant in this example), assume we meter the card and want to give say 3 or 4 stops less exposure, to produce a fairly low negative density. Suppose this requires an exposure of 1/60s @ f/2. Make the exposure. Then shoot a second frame giving the same exposure by setting 1s @ f/22. For these two frames, is the flare factor the same or different? Assume only camera flare and lens flare, and that the lens diaphragm separates equal numbers of glass elements. So to summarize the situation a different way (just for clarity), the total units of image forming light reaching the film is the same in both exposures. In the first exposure the intensity of light entering the camera is much higher but for a short period of time. In the second exposure, the intensity of light entering the camera (and the rear half of the lens) is much lower but is present for a longer period of time. In my mind I'm thinking higher intensity causes more reflection off various surfaces (including the emulsion itself), so flare should increase even though the light is present for less time compared with the second exposure. Not sure. And it is difficult to test in practice due to possible shutter speed inaccuracies.

  7. #147
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    Suppose you dim the light with filters, to rule out the aperture shape/area change that might shape how large the internal reflections are.

    So put a clear gelatin filter for the first shot and a 2.1 ND filter for the second shot. That will give two shots that differ only in intensity x time.

    Here I believe you would find flare the same in both cases.

  8. #148

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    Really? Even camera flare (light bouncing around in the camera even with flat black surfaces, light reflecting off the emulsion adding to this)?

  9. #149
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    Quote Originally Posted by Michael R 1974 View Post
    In my mind I'm thinking higher intensity causes more reflection off various surfaces (including the emulsion itself), so flare should increase even though the light is present for less time compared with the second exposure. Not sure. And it is difficult to test in practice due to possible shutter speed inaccuracies.
    Michael, the higher intensity will of course create a higher reflective light level. But as they are part of the target, which is a uniform luminance source, it has nothing to effect. Think of flare as a bleeding of light from the brighter scene elements (average of 3 percent of the greatest scene luminance) adding to the darker exposure values. If the target is the only luminance value, what is it going to bleed into and how can it add to itself?

    Just reading the list of meter modifications (various filters, baffling) was depressing enough
    It's enough to make a person question whether knowable testing is possible with such devices.

    I'll try to re-read the exposure meter part of Henry's book so the material will be fresh for both of us. One thing to remember is sometimes the variables can create an accumulative effect and sometimes they can cancel each other out. If you have a chance, you might want to check out the thread Is the K-factor Relevant to me or should I Cancel it Out? Around the third page, I begin posting attachments outlining exposure including exposure meters and the variables in the equation for determining K.
    Last edited by Stephen Benskin; 01-25-2013 at 09:31 PM. Click to view previous post history.

  10. #150
    Bill Burk's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Really? Even camera flare (light bouncing around in the camera even with flat black surfaces, light reflecting off the emulsion adding to this)?
    Right. The light would bounce around just the same, only it would be proportionally dimmer.

    Now I use Zones loosely in this discussion. When I test film it's with a sensitometer. Then I work in Zones as I study my graphs because I find it easy to get my head around. I imagine where the different Zones will fall on the curve. I look at Zone I and lift that point to the Y-axis coordinate corresponding to about Zone II (I say about, because I look 0.4 LogE to the right). The difference is a certain number of Meter-Candle-Seconds which I take and "add" to each of the coordinates of my step wedge. Flare's impact quickly tapers off. But this is how I take a sensitometer test and make it agree with light meter marked with Zone System grays.

    In a sense I call the camera test you describe as a no-flare test. Even though the camera introduces flare. I say no-flare because the amount of flare at Zone V exposure is about what would take Zone I up to Zone II. BUT, in a camera test you stop down four stops and it no longer brings you from Zone I to Zone II. Now it only takes Zone -III to Zone -II (which is to say 'nada').



 

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