ISO film speed point relative to in-camera exposure

Discussion in 'B&W: Film, Paper, Chemistry' started by Anon Ymous, Dec 29, 2018.

  1. Anon Ymous

    Anon Ymous Member
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    Hello all...

    According to the ISO standard of film speed, the film must meet certain criteria. The characteristic curve must pass from two specific points. The first is a point where the density is 0,1 above film base plus fog. The second point is one that has received 4,33 stops more exposure (ΔE = 1,3) and must have a density 0,8 above the previous point. Based on the exposure that the first point has received one can calculate the ISO film speed with the appropriate equation. That's all fine, but requires some apparatus that most of us don't have. So, the question is, where are these (exposure) points relative to the exposure that one would get from a camera using auto exposure and shooting a gray card?
     
  2. Bill Burk

    Bill Burk Member
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    I've found in my tests that the camera (or meter) wants to put 10 times the light on the film as that light that was put on the film where the speed point (0.1 above base+fog) was found.
     
  3. ic-racer

    ic-racer Member

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    By definition, my in-camera exposure (adjusted by my personal exposure index) satisfies the ASA criteria with added safety factor of one stop compared to box speed. Simply put, with my equipment it frequently is the case that T-max 400 exposed to EI 200 gives me 0.1 log d for zone 1.

    This may not answer you question because, any target can be placed on any zone. So, as you state, "shooting a gray card" is the same as shooting a target of any reflective value in my test.

    Perhaps Bill answers what you intended to question better.

    My spread sheet calculates all those values automatically based on equations obtained from the scientific literature on the topic of film speed. If I do any more work on the spreadsheet it will be to add the important "Delta-X" calculation also. This spreadsheet superimposes the ASA triangle, but 'fitting' the curve to that triangle is by trial and error. This dataset fits very well. BTW, The "W-Speed" estimate of 0.3G is achieved independent of any 'fitting' to the ASA triangle.
    13min & ASA.jpg
     
    Last edited: Dec 29, 2018
  4. Bill Burk

    Bill Burk Member
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    Here's the graph of the test. The ASA speed point would fall directly under 100 on my upper scale when developed to the ASA parameters. You can see that ten times that would fall directly under 10 on my upper scale.

    The result of my test shows the camera put a little less than 10 times ( about 1/6 stop "less" ).

    I doubt that the actual aim is 1/6 stop below, I talked it over with Stephen Benskin who assures me it's 10 times... But it wouldn't bother me in the least to find out there was a slight 1/6 skew towards less exposure.

    Note that speed point in this specific curve is higher than ASA (it falls under 80 instead of 100). That is the result of developing to less than 0.62 contrast, and is irrelevant. The camera exposure placement is expected to fall under the 10 no matter how I develop the film.

    http://beefalobill.com/imgs/tmxaim.jpg
     
  5. Bill Burk

    Bill Burk Member
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    ic-racer,

    If you shot a gray card your density would be about 0.72 because the camera would try to put the exposure around the 2.11 mark on your exposure scale.

    On my test the density is 0.59, less than yours would be because my specific roll was developed less than yours.
     
  6. RalphLambrecht

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    that depends very much on how your camera metering system was designed. You are talking about the Zone-point. It usually receives4 stops above Zone I of course and typically has a a transmissiondensity of 0.72.
     

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    Anon Ymous

    Anon Ymous Member
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    Thanks for the replies, happy new year everyone.
     
  8. ic-racer

    ic-racer Member

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    A reflected meter would give the same exposure to film when pointed at any density card; white, black or anything inbetween. In-camera exposure index testing (the way I do it) requires only a uniform target. It can be any density ( though not any color). I don't know of any ASA or ISO standard for gray card density on film. There are examples in the "Zone System" literature of what the zones might be on film, but they are pretty user specific.
     
  9. ic-racer

    ic-racer Member

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    There were some epic threads on using a sensitometer, but applying those values to the field can be done in many ways. And I have tried many ways. My current method is below:

    Using the sensitometer, I find a processing time/temp that gives that fits the ASA triangle (like the film shown in the post above). Then I put that same type of film in a camera and make zone 1 exposures and process the film the same way. Film frames reading 0.1 log d would have been exposed at my preferred exposure index. This is not perfect or even maybe the 'best' way to do it, but with contemporary B&W films, the resulting exposure index gives me enough under-exposure safety for most conditions I encounter. Plus this is a super easy test.

    My spreadsheet for calculating W-speed, ASA/ISO speed, etc is for testing FILM. To test a meter/camera combination I use the above mentioned procedure.
     
  10. Bill Burk

    Bill Burk Member
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    ic-racer,

    I know the camera will want to make anything you meter come out middle gray, whether it’s white gray or black. What I meant was if you had a metered target in your picture its density would have been 0.72 because your contrast is to ASA parameters.

    Where my density at the meter point was lower because I developed to less contrast.

    This was my setup where I demonstrate the relationship of meter point to sensitometry (with an EG&G strip on the same roll to provide the curve).

    https://www.photrio.com/forum/threa...ed-to-match-18-gray-card.136266/#post-1781941
     
  11. ic-racer

    ic-racer Member

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    Basing exposure on a middle value, as you have shown does work. In fact I used a somewhat similar system with a ten-degree average meter for 25 years. Though, last year I got a 1-degree spot and now I am basing exposure on a low value which is the method favored in this paper: The Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure.

     
  12. ic-racer

    ic-racer Member

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  13. Bill Burk

    Bill Burk Member
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    But Anon Ymous,

    You should not design a test that verifies the 0.72 density at metered point.

    All the tests traditionally use a very low density change (for example 0.1 above base+fog) as the speed point because it changes very little with experimental differences. If you tried to key exposure index from 0.72 density you will have some problems that won’t be easy to figure out.
     
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  15. ic-racer

    ic-racer Member

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    Well stated.
     
  16. OP
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    Anon Ymous

    Anon Ymous Member
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    I don't, let me give you an example of what I had in mind and did. You may know that the US patent 5853964 has a formula (example 1, page 5) that is a close approximation of Xtol. I did some substitutions/modifications for things I don't have and made a homebrew formula, which required a small pH adjustment (pH 8,1 vs the 8,2 target). I gave my developer a try with a roll of 135 TMY2. I bracketed some shots of a gray card with my OM2S at -5, -3, -1, +1, +3 and +5 stops. I developed for the recommended time (9:15 at 20°C) and took density readings with my scanner, a Minolta Scan Dual III. These readings aren't necessarily spot on, but are the best I can have. The reading for the denser shot (+5 stops) is the most iffy and there was quite large variation in the readings I got, which I had to average. With this information, I wanted to know how close to the ISO standard I am, but I obviously don't know the exact amount of exposure the film got in lux-sec. Nevertheless, here's what the characteristic curve looks like:

    TMY2 Homebrew Xtol CC.png

    The 0 mark on the X axis is at -5 stops. It is indistinguishable from film base + fog. At X=0,3, what would be a 4 stop underexposure, we get at 0,1 above fb+f and the normal exposure would be at X=1,5. It seems to be a little overdeveloped considering the ISO criteria (red line). The curve looks quite upswept, which was rather surprising. Then I remembered that Fotoimport have some film - developer curves on-line and those for TMY2 can be seen here. The curve there also seems to suggest that this combination gives an upswept curve. As you may have guessed, I was wondering what the ISO speed would be in my case, although the contrast is a little higher than what it should.
     
    Last edited: Jan 2, 2019
  17. koraks

    koraks Member

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    I happen to have a Minolta Scan Dual IV, which is likely to be quite similar to your III. I have only used it with Minolta's own software, so perhaps you have a different methodology. But it seemed to me that the Minolta scanning software does whatever post processing necessary to get a sort of 'average' result from whatever negative or positive you feed into the scanner. I wouldn't know where to begin to get an absolute reading using this scanner. With a flatbed and a step wedge, sure. Or even using an enlarger and a step wedge. But with this scanner? I wouldn't know. How did you solve this issue?
     
  18. Nodda Duma

    Nodda Duma Subscriber

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    I’ve just started reading this thread with interest. I’ve become intimately familiar with the underlying math, as I work to characterize my dry plates.

    One aspect I’ve worked on is determining absolute (not relative) speed of my emulsion, since there’s obviously no prior characterization data. It gets fun because it’s compounded by the fact my emulsion isn’t panchromatic, but fortunately radiometry is one of my strong suits from my optics background and that helps. That complication aside, the math says to calculate log H based on a measure of lux - seconds.

    In the D-log H charts I generate, the horizontal axis increases from left to right because I’m showing density vs log exposure. ic-racer and Bill, your values decrease from left to right so what is your horizontal axis? Is it log H for reversal film?

    edit: Answered my own question .. you're showing negative log values, but without the minus sign.
     
    Last edited: Jan 2, 2019
  19. OP
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    Anon Ymous

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    I use Vuescan, it gives density readings of any point if you have done a preview.
     
  20. koraks

    koraks Member

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    I see; is it reliable? Last time I tried Vuescan was many many years ago and I didn't like it back then, and haven't had the need to try it again over the past 10 years or so.
     
  21. OP
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    Anon Ymous

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    I can't really say if it's very reliable or not, I don't really have a point of reference, but seems reasonable for not very dense parts. I like it more than the bundled software, I only need a raw scan anyway and do the rest elsewhere.
     
  22. bernard_L

    bernard_L Member

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    Interesting statement from Bill Burk. 10 times is 10/0.3=3.33 stops. These are stops above Zone 1. So the camera wants to place the proverbial gray card (and it does NOT matter whether it's 18%, 12% or..., the camera cannot read the forum disputes) on zone 1+3.33=4.33 (irrelevant coincidence with the number 4.33 in the OP).
    I had always assumed that a properly calibrated camera/meter would place a uniform scene on zone 5. Keep learning. Maybe that is why many photographers (incl. myself) derate TX-400 to 250: minus 2/3 EI, just what is missing from 4.33 to 5.
     
  23. Bill Burk

    Bill Burk Member
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    Oh my X-Axis (3.60 on the left 0.00 on the right) is attenuation... so it's simply the density of the step wedge in the sensitometer.

    On my graphs with scale at the top, that gives absolute log meter candle seconds.
     
  24. Bill Burk

    Bill Burk Member
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    You are really close to having the ISO parameters. But, since your developer is the variable you want to test, I'd do a control test with D-76 and try to hit the ISO parameters with that (a more standard developer). Later, when you hit the ISO parameters with your home-made developer, you can compare the x-axis differences between the standard developer and your developer.

    From that you can determine whether your developer gives more or less speed. Chances are the speed difference will be negligible.
     
  25. Stephen Benskin

    Stephen Benskin Member

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    Hey Bill,

    Do you still have that page up with the papers? Most of the key papers on exposure are there.

    I recommend starting with Calibration Levels of Film and Exposure Devices. Safety Factors in Camera Exposure also is highly recommended. Years ago, I put together something that connects many of the dots. It's called Defining K. The ISO B&W film speed standard isn't as straight forward as it appears. If you want to understand the reasoning behind the ISO criteria, then you need to learn about the Delta-X criterion. Quick fun fact, Point n in the ISO contrast parameters (Δ1.30 over and Δ0.80 up) isn't the metered exposure point, and point m, while being where the film speed is calculated, it doesn't represent minimal acceptable exposure and can technically be considered more of an EI than speed point.

    Finally, in practice, don't be so concerned about determining your own film speed. In Safety Factors, the paper discusses the reason the speed equation is 0.80 / Hm instead of 1.0 / Hm to compensate for apparent 1/3 of a stop speed increase due to the change in the spectral quality of the testing sensitometers from simulated sunlight to simulated daylight which basically increases the amount of blue light. Personal testing without a calibrated sensitometer has too many uncontrollable variables to have any real and repeatable precision. Film speed with general purpose developers is pretty much at the ISO speed. The metering tastes and methods of the photographer have a greater influence on exposure placement than small differences in film speed. Focus more on contrast.
     
  26. Bill Burk

    Bill Burk Member
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