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  1. #51
    markbarendt's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    What is the point of that? It is the same as setting your EI and reading the differences in f-numbers.
    If you always meter to the same point, say zone 3, to set exposure it gives a direct finished reading on the meter, no math required after taking the reading.

    Measuring SBR would essentially be unchanged.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  2. #52
    Stephen Benskin's Avatar
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    Quote Originally Posted by markbarendt View Post
    This actually brings up something that I have wondered about for a while.

    Why is it that we don't set spot meters to read the camera setting directly from the speed point?
    Off the top of my head, I don't think anybody wants to have to buy separate meters for b&w and color reversal. Exposure is more critical with color reversal film, so it won.
    Flare makes determining where the exposure would actually fall a nightmare. There's very little flare at the metered exposure point.
    Human vision compresses tones in the lower range. We are a lot more sensitive to tonal differences in the middle range.
    Many scenes don't have shadows deep enough to even reach the speed point exposure. You'd be metering at a point where there's no subject luminance.
    And flare again, because it is just that important.

  3. #53
    markbarendt's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Off the top of my head, I don't think anybody wants to have to buy separate meters for b&w and color reversal. Exposure is more critical with color reversal film, so it won.
    Who won makes sense.

    My point was not in having different meters, it was more just to be able to read the camera setting directly when we use our normal pegging point, whatever that might be.

    Quote Originally Posted by Stephen Benskin View Post
    Flare makes determining where the exposure would actually fall a nightmare. There's very little flare at the metered exposure point.
    This is truly interesting.

    Essentially you are saying that meters can get/find better readings in the midtones and highlights, and that they struggle with the lower light values, correct?

    Quote Originally Posted by Stephen Benskin View Post
    Human vision compresses tones in the lower range. We are a lot more sensitive to tonal differences in the middle range.
    Many scenes don't have shadows deep enough to even reach the speed point exposure. You'd be metering at a point where there's no subject luminance.
    And flare again, because it is just that important.
    And our own perception struggles at lower light levels.

    These two factors seem to provide a really strong argument for pegging something in the middle when we are deciding on settings rather than the shadows.

    Really, if we are striving for accuracy and repeatability in our techniques so that we can get what want rather having to take what we get, why would we meter a place where we and our tools have the hardest time providing reliable results?

    Seems like a recipe for building in errors.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  4. #54
    Stephen Benskin's Avatar
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    So, how do we know the speed point Ė metered exposure ratio is 1.20 log-H or 16x (15.8) for Zone System testing? Itís right there in the Zone System testing instructions. Meter a card and stop down four stops. Zone I is the speed point and it is at 0.10 over Fb+f. Four stops of exposure is 1.20 log-H.

    It should be obvious at this point that for a 125 speed film where the speed point has an exposure value of 0.0064 for Hm has to have an EI lower than 125 for the 1.20 log-H range to fit between the metered exposure and the speed point. What would that EI need to be?

    Exposure constant / (Hm * ratio) = EI
    or
    8 / (0.0064 * 15.8) = 79.2
    or
    EI 80

    A film with an ISO of 125 and a speed point Ė metered exposure ratio of 16 (1.20 log-H) will have an EI of 80. Of course, Zone System testing gets to this same conclusion differently.

    Traditional Zone System testing is done in camera which means people are far more limited in the exposure information available to them than if they used a sensitometric approach. A card is metered and the camera is stopped down four stops. (While the Zone System test uses only a single tone, the graphs illustrate how the the full exposure range would fall.) Below is an example of how the exposure will fall with a 125 speed film metered using an EI of 125.

    Click image for larger version. 

Name:	Speed Point - Metered Exposure Ratio - Zone System.jpg 
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    Zone I exposure will fall 0.20 log-H units below the 0.10 speed point. Exposure needs to be increased by 0.20 log-H in order to bring the Zone I exposure up from 0.0041 lxs to 0.0064 lxs. While the film speed remains at 125, the meterís EI is now at 80. This is what creates the change from the speed point - metered exposure ratio of 10x (1.0 log-H) to 16X (1.20 log-H).

    Click image for larger version. 

Name:	Speed Point - Metered Exposure Ratio - Zone System 2.jpg 
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    Doesnít the first Zone System testing example look a lot like the sensitometric exposure example? The only difference is the choice of scene luminance ranges. The Zone System uses 2.10 and tone reproduction uses 2.20. So the only difference between the two is the sensitometric example has the shadow fall 1/3 stop further down on the curve.

    Click image for larger version. 

Name:	Speed Point - Metered Exposure Ratio - Luminance Range 1.jpg 
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    The final step is where a lot of people get tripped up. Thereís a difference between the sceneís luminance range and the cameraís exposure range. While the original scene might have a luminance range of 2.20, by the time it gets to the film plane, the exposure range averages at least a stop smaller: 1.90. Sensitometric testing is done in non-flare conditions, so the interpretation of the sensitometric results has to incorporate flare. While the difference between the metered exposure point of the subject and the shadow has a difference of 4 1/3 stops, on the film plane it is only 3 1/3 stops. It's not a coincidence that the ISO standard has a speed point - metered exposure ratio of 1.0 (3 1/3 stops) and not 1.30 (4 1/3 stops).

    While the Zone System testing utilizes a camera, the flare is practically non-existent by the way the test is conducted, so it too can be considered non-flare testing conditions. So, just like what was done with the earlier sensitometric example, the exposure range needs to be adjusted to account for flare. To simplify thing, Iíve reduced the exposure range to 1.90 log-H.

    Click image for larger version. 

Name:	Speed Point - Metered Exposure Ratio - Zone System 3.jpg 
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    Itís interesting how similar this example is to the fractional gradient example, but it shouldnít surprising. Both the Zone System and 0.3G were developed in the early 1940s. At that time they had a good correlation between the two results. Take a look at their speed point Ė metered exposure ratios. While at first the fractional gradientís 1.50 might not seem to match the Zone Systemís 1.20, if you adjust for the 0.3G having a speed point approximately one stop (0.30 log-H) units to the left of the Zone Systemís speed point, they are the same.

    Click image for larger version. 

Name:	Factional Gradient Speed w Flare.jpg 
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    It wasnít until the 1960 standard reduced the safety factor and changed the speed point Ė metered exposure ratio which then caused the resulting speeds from the Zone System and the ANSI/ISO method to diverge.
    Last edited by Stephen Benskin; 01-13-2012 at 09:51 PM. Click to view previous post history.

  5. #55
    Bill Burk's Avatar
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    These are great diagrams, your final diagram fits exactly Zone System benchmarks - Zone 0 at 0.3G, Zone I at 0.1 speed point, Zone V at the metered point.

    You say the safety factor was removed in 1960? Does that change the final diagram, moving everything one stop to the left?

  6. #56
    Stephen Benskin's Avatar
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    Quote Originally Posted by markbarendt View Post
    This is truly interesting.

    Essentially you are saying that meters can get/find better readings in the midtones and highlights, and that they struggle with the lower light values, correct?
    Not better readings. Better confidence in exposure placement.

  7. #57
    Bill Burk's Avatar
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    I'd like to see three graphs marked like this that show

    What the Scene Reflects

    What the Meter Indicates

    What arrives at the Film Plane

    This model is wrong, because it shows what arrives at the film plane, including the one stop of flare in the shadow.

    So for example a Scene Zone 0 came up to Film Plane Zone I.

    But did a Scene Zone 0 come up to a Meter Zone I? The meter readings are not without flare. Whenever I aim a spotmeter at the darkness under a car for example, a lot of flare influences the meter reading.

    Click image for larger version. 

Name:	Fractional Gradient Zone Correlation.jpg 
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  8. #58
    Bill Burk's Avatar
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    Quote Originally Posted by markbarendt View Post
    This actually brings up something that I have wondered about for a while.

    Why is it that we don't set spot meters to read the camera setting directly from the speed point?

    The math isn't the issue since the formula can be adapted.
    The SEI Photometer does what you are thinking. There is a black spot and a white spot. If you put the black spot on ASA 4, the white spot is on ASA 400 - exactly 6 2/3 stops apart.

    You are expected to meter a shadow, and set the ASA to the black spot. Or for transparencies meter a highlight and set ASA to the white spot.

    But the value of using a Zone System sticker on a meter is that you can pick any recognizable value and spot it and place it on that Zone.

  9. #59
    Stephen Benskin's Avatar
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    Below is basically the same as the current 10X ratio example except that I've added on the camera image curve. I don't know if this helps clear things up.

    Click image for larger version. 

Name:	Exposure-Speed Relations - two quad.jpg 
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  10. #60
    markbarendt's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Not better readings. Better confidence in exposure placement.
    Your words are better than mine.

    The question/problem remains though, even when reframed with fresh words.

    Why would we (or a meter or film manufacturer) want to find our exposure settings based on points that should be trusted less?

    It seems to me that the old adage "shoot to the shadows and develop for the highlights" like most simplifications, can lead us a bit astray here. In this case by, on its surface at least, encouraging us to measure a reference point that should rightfully inspire less confidence of exposure success.

    It seems to me also that, the decision to use a mid-tone as the reference point/standard in concert with a tested constant (ISO or EI) to know where the speed point for a film falls when finding exposure makes real sense.

    This thought also seems to me to answer why the data sheets, from say Ilford, suggest differing EI's dependent on the development planned even though the true speed point changes much less.

    The manufacturers seem to be adjusting the constant to place the mid-tone point on the alternative curve at roughly the same density as on the normal curve, instead of pegging the speed point. So for example faces should print at nearly the same average brightness in each prescribed process. Is that a fair assessment?

    Still and yet it seems that the shadow point can be found more reliably by measuring a mid-tone, it simply requires a bit of familiarity with the tools at hand.
    Last edited by markbarendt; 01-14-2012 at 08:02 AM. Click to view previous post history.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

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