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  1. #21
    Stephen Benskin's Avatar
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    Quote Originally Posted by Kirk Keyes View Post
    For B&W negative films, arithmetic ISO speed is determined by:
    ISO Speed = 0.8 / (Exposure st speed point)
    The speed index result is rounded off to the nearest 1/3rd f-stop.

    For color reversal films, the the exposure needed to get to the shoulder point (S), which is about 90% maximum density is determined. Then the exposure needed to get to the toe (T) which is 0.2 density above the minumum density is measured. These two exposures are averaged, that is: ((S+T)/2), and this value is the Speed Point. The reciprocal of the exposure at the Speed Point is then multiplied by 10 and the result is the arithmetic Speed Number.
    Thanks Kirk,

    For B&W it's 0.8 / Hm
    For Color Reversal it's 10 / Hm
    Connelly uses HR for color reversal films to avoid confusion. Iíll be doing the same.

    Hm and HR is the exposure in meter candle seconds at the measured point for the respective film speeds. The reason why it needs to be a sensitometric exposure is to produce known values to work with. Few people have this capability, but itís possible to determine the necessary values for Hm and HR for any film speed by dividing the aim film speed into the constant.

    100 speed - .8/100 = 0.0080 mcs 10/100 = 0.10 mcs
    125 speed - .8/125 = 0.0064 mcs 10/125 = 0.80 mcs
    200 speed - .8/200 = 0.0040 mcs 10/200 = 0.05 mcs
    400 speed - .8/400 = 0.0020 mcs 10/400 = 0.025 mcs

    For a black and white negative with a speed of 125, an exposure of 0.0064 mcs at the film plane will produce a negative density of 0.10 over Fb+f when all the other conditions of the standard are met. With a transparency an exposure of 0.80 mcs with a 125 speed film will fall half way between the points HS and HT.

    But how can the same film speed produce two different exposure values? As you can see from the examples of the two graphs from the speed standards, the speed points for black and white negative films and color reversal films fall at very different points on the characteristic curve. This is accomplished by having different values for the film speed constants. For black and white negative film itís 0.80 and for color reversal film, itís 10.0.

    Sometime in the 70s or 80s, the constant for color reversal film changed from 8.0 to 10.0 while the method of determining the speed point didnít. The film speed determined from the same point on the film curve went from 8/0.064 = 125 to 10/0.064 = 156. Changing the value of the film speed constant changed the resulting film speed value. In the case of color reversal film, the film speed increased by 1/3 stop which decreases the film exposure by 1/3 stop.

    Why change the constant and not the speed point? Why not just shift the speed point to the right by 0.10 logs? One of the purposes in film speed testing is to define the limiting points on the film curve that are critical to the perception of quality in the finished image. For color reversal film, it is the two defined end points on the curve (S and T). For black and white negative film, it is the point where the minimum gradient it .3x the average gradient of the film curve. By adhering to the contrast parameters of the black and white film standard, this point will always fall 0.29 log-H units to the left of the speed point (attachment - B W film speed standard and Fractional Gradient point.jpg). Knowing the position of these points is important toward producing a film speed that has any relevance to the film and to quality results. By adjusting the exposure placement through a change in the constant and not a change in the speed point maintains this relevance.

    Now that the critical points with each of the film types are known, the next step is to determine where to place the point of exposure averaged from the scene being photographed where all the values from the scene will fall within the tested limits.

    The luminance range of the statistically average scene is 2.20 logs (7 1/3 stops). The average luminance of the scene will fall where there are just a little over three stops above the average luminance (0.92) and just a little under 4 1/3 stops below (1.28).

    I hope that I was able to begin to convey how the film speed is important to exposure placement. Now, we need to determine the placement of the exposure. This involves understanding something about meter calibration and camera exposure.

    Here are the next set of questions:
    What is the calibration luminance for an reflection meter?
    What is the calibration illuminance for an incident meter?
    What is the camera exposure equation and how does the calibration luminance plug into it?
    And from my first post:
    Also keeping in mind that exposure meters are designed to produce one data value for the scene, what should that exposure be for a given film speed?
    Attached Thumbnails Attached Thumbnails B W and Color Reversal film speed standards.jpg   B W film speed standard and Fractional Gradient point.jpg  

  2. #22

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    Quote Originally Posted by Stephen Benskin View Post
    Hm and HR is the exposure in meter candle seconds at the measured point for the respective film speeds. The reason why it needs to be a sensitometric exposure is to produce known values to work with. Few people have this capability, but itís possible to determine the necessary values for Hm and HR for any film speed by dividing the aim film speed into the constant.
    Meter-candle-seconds is also known as lux-seconds.

    I know several of the Minolta Flashmeter series can make incident measurements that can be converted into lux. You then multiply the exposure by lenght of the exposure, and then you have an exposure that has a known lux-seconds value.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  3. #23

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    Quote Originally Posted by Stephen Benskin View Post
    Why change the constant and not the speed point? Why not just shift the speed point to the right by 0.10 logs? One of the purposes in film speed testing is to define the limiting points on the film curve that are critical to the perception of quality in the finished image. For color reversal film, it is the two defined end points on the curve (S and T). For black and white negative film, it is the point where the minimum gradient it .3x the average gradient of the film curve.
    The speed point is a function of the characteristics of the film - as you say, "the limiting points".

    For reversal films, going from the 60s to the 70s, reversal films were able to capture a much large range of exposure. That is, the distance between S and T would have increased over films of a few years earlier.

    That means the film speed number could be determined with a differenent equation to better account for this increased range.

    For B&W neg films, the the standard printing paper LER did not change, so the speed point calcs would have remained the same.
    Last edited by Kirk Keyes; 01-01-2012 at 10:01 AM. Click to view previous post history.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  4. #24

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    Fujichrome Family.pdf

    Just for fun, here's a set of curves I measured for Provia RDP III, Velvia 100, Astia RAP 100F, and RTP II 64T (Tungsten film). I'm afraid I didn't plot speed points (you can see the S and T points of these films on the graphs), but I plotted just a composite curve for all 3 colors layers of each film is plotted instead. Note how there are slight differences in speed, despite 3 out of the 4 being 100 speed films.

    Note that Velvia has the highest contrasta of all 4, RTP the lowest. And that Velvia 100 and RDP III have nearly the same highlight response, but then Velvia kicks up the contrast in the shadows. Also note that while Astia and Provia are very similar in the shadows/darker areas, Astia is much lower in contrast in the highlights with a much longer "toe".
    Last edited by Kirk Keyes; 01-01-2012 at 10:03 AM. Click to view previous post history.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  5. #25
    Stephen Benskin's Avatar
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    We've discussed how the determination of film speed is not just about the sensitivity of the film, but also defines the critical limits of exposure placement. We've also discussed how the speed constant can be used to adjust the exposure placement so that the average exposure range falls in a desire portion of the curve.

    The next step is to determine where the exposure meter wants to place the exposure regardless of the film speed, and then with the film speed calculated in. First, we need to determine the average Luminance for the reflection meter and the average Illuminance for the incident meter. These can be considered the calibration values and as such can be determine using the exposure meter calibration equations.

    Even though the refection meter reads Luminances, it possible to determine what is considered the average scene Reflectance in one of two ways. After converting the average Luminance to Footlamberts (* pi), divide the Luminance value by the Illuminance value. The second way is to divide the reflection meter's constant (converted) by the incident meter's constant.

    Click image for larger version. 

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    Now, that we have the average Luminance value, all that is necessary is to plug it into the camera exposure equation to determine the exposure value.

  6. #26
    Stephen Benskin's Avatar
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    According to the exposure meter's calibration equation, the mean scene Luminance is 297 cd/ft^2 or 3196 cd/m^2. Whatever the meter points at, the f/stop and shutter speed half of the equation wants to adjust itself to balance the equation to equal the same output as with the 297 cd/ft^2 at the Sunny 16 rule - f/16 and the shutter speed as the reciprocal of the film speed - 1/film speed.

    If you remove the shutter speed out of the picture, the exposure meter wants to produce a single value that, according to Connelly, "can be considered either the average image illumination required for light sensitive material having unity film speed when exposed for a time of one second, or a constant reference exposure (P)." This value is what makes exposure meters work for all materials. It wants to produce a single exposure value.

    It is the film speed of the photographic material determines where the average point of exposure or the metered exposure point will fall. As we've seen, it's not just a value representing the sensitivity of the photographic material, but also includes information about the limits of useful exposure of the film type, and most importantly an adjustment by the speed constant to place the metered exposure point at the point on the film's characteristic curve so that the entire scene's exposure range will fall on the curve where it will be most advantageous.

    Now that we know the average Luminance or Lg, we can determine the constant P and then calculate the exposure value for all the different film speeds.

    Click image for larger version. 

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    There's a short cut to determine the metered exposure value for different film speeds. Simply divided the film speed into the constant P.

    P/100 = 0.080
    P/125 = 0.064
    P/200 = 0.040
    P/400 = 0.020

    Now that we know the positions of both the speed point and the meter exposure point, it's now possible to determine the relationship between the speed point, film speed, and the metered exposure point.

  7. #27
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    For black and white negative film, it is the point where the minimum gradient is .3x the average gradient of the film curve. By adhering to the contrast parameters of the black and white film standard, this point will always fall 0.29 log-H units to the left of the speed point (attachment - B W film speed standard and Fractional Gradient point.jpg).
    I can't get my head around this. All black and white negative film has a similar shaped toe under the 0.1 speed point when developed to ASA gradient? What of long-toed and short-toed films? And if you use the 0.3x average gradient for a speed point... do you select a value for mcs that falls 0.29 to the right of the 0.3x point?

  8. #28
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    I can't get my head around this. All black and white negative film has a similar shaped toe under the 0.1 speed point when developed to ASA gradient? What of long-toed and short-toed films?
    Bill,
    Excellent question. It all has to do with the ISO contrast parameters. The log-H range of 1.30 is designed to measure the lower portion of the film curve. Long toed curves with their slow upsweep require a higher CI to fit the the ISO conditions than a short toed film. When people refer to the ISO contrast parameters producing an gamma, contrast index, or even an average gradient of 0.61 because of the .80 / 1.30 = 0.615, they are in error as those measurement methods include a greater portion of the film curve. While this usually won't make much of a difference with the primarily straight lined short toed curves, it will with the longer toed curves.

    Click image for larger version. 

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    And if you use the 0.3x average gradient for a speed point... do you select a value for mcs that falls 0.29 to the right of the 0.3x point?
    If you are using the fractional gradient method, then yes. But it's not as simple as that. The two primary changes in the 1960 standard were the elimination of most of a safety factor and to simplify the determination of the speed point. For simplicity sake, let's just say the fractional gradient speed point falls 1 stop below where today's ISO speed point falls, and let's use the Hm value for a 125 speed film - 0.0064. This would make the .3G speed point exposure value 0.0032. If you calculate the speed using the ISO equation, the resulting film speed would be .8/.0032 = 250, but that's not the .3G speed equation. It was (1/Es) / 4 or for our example (1/.0032) / 4 = 78. So even though the .3G speed point falls ~1 stop to the left of the ISO speed point, the speeds created were ~ 1 stop slower.

    For the 1960 standard, they simply could have changed the speed constant to 0.4 and divided the .3G exposure value into it: .4 / 0.0032 = 125. The reason why they didn't was because the .3G speed point was difficult to find. That's one of the reasons why they went with a density of 0.10 over Fb+f. It's exactly 1/3 stop over the film base. It's easy to determine. But the only reason why they could accept this simplified speed point was that under certain contrast conditions, the 0.10 speed point falls at a uniform distance from the .3G speed point. So, if the film is processed to those conditions, and the 0.10 speed point is used to determine the film speed, it will produce the same film speed as from the fractional gradient method. In other words, if you are adhering to the ISO contrast parameters, you are essentially using the fractional gradient method.

    The implication of this is that any use of the 0.10 speed point with the film processed outside the ISO contrast parameters will no longer relate to the .3G speed point and no longer have the same correlation to the psychophysical pictorial speed tests and therefore be less accurate.

    Delta-X Criterion is a method that uses a mathematical adjustment which allows the use of 0.10 as the speed point for any processing condition while still retaining the accuracy of the fractional gradient speeds.

    I hope this explanation also helps clarify how the speed constant works and how the speed point isn't necessarily associated with the placement of the camera exposure. It's for testing purposes to determine the placement of the exposure.
    Last edited by Stephen Benskin; 01-04-2012 at 12:08 AM. Click to view previous post history.

  9. #29
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    So ... the .3G speed point falls ~1 stop to the left of the ISO speed point...

    ... under certain contrast conditions, the 0.10 speed point fell at a uniform distance from the .3G speed point...
    Great, that makes sense.

    The .3G and .1 speed points correlate to each other... by being a certain distance apart.

  10. #30
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    The .3G and .1 speed points correlate to each other... by being a certain distance apart.
    Yes, but I would like to emphasize the 0.29 only applies when the ISO contrast parameters are met, otherwise the relationship is different. Although it's not random. You can determine the Delta X value by the Delta D value while maintaining the 1.30 log-H range.

    Click image for larger version. 

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    Last edited by Stephen Benskin; 01-04-2012 at 07:27 AM. Click to view previous post history.

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