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#81
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Improved T-Max 400
On Nov 1, 8:48 pm, Peter Irwin wrote:
Note that the inertia based Weston speed also tracked pretty well with the Jones Kodak Speed and Old ASA. The normal relationship would be that Weston 40 equaled Kodak 200 and Old ASA 50. Many films seem to have fit that relationship perfectly and I'm unaware of any that were more than 1/3 stop off. I was wrong about this. I looked up official Weston speeds from 1940 and compared them to Kodak speeds from 1943 - and they just aren't always that close. The Weston speed system was ok, but it wasn't that good. Weston speeds quoted by Kodak in the 1943 book are just converted Kodak speeds - so of course those match. Old DIN (1936) I was also wrong about the date of the original DIN speed standard. DIN 4512 was 1934 not 1936. Peter. -- |
#82
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Improved T-Max 400
On Nov 1, 8:48 pm, Peter Irwin wrote:
Richard Knoppow wrote: This is getting long but I can't find anything to snip. I hope that means that it was mostly ok. I don't know when Kodak began using Jones' method internally but it did begin to publish Kodak speeds around 1939. Jones actually worked out his system much earlier. I'm sure that's right, but published Kodak speeds from before 1939 are a different system. The Kodak speeds listed in the 1938 edition of "Eastman Professional Films" are not the Jones system. The pre-1939 Kodak system was based on inertia (Like Weston and H&D) and is equal to ten times the Weston speed. The ASA adopted the system in 1943 with a safety factor of 2. That's what you've said several times. But - In "Kodak Films" fifth edition 1951 from the Kodak Reference Handbook on page 16, it says "For the black-and-white continuous tone negative materials covered by the standard, a safety factor of 2.5 is used." In "Kodak Films" seventh edition 1956, it says the same thing on the top of page 21. ASA speeds were 1/4 of Kodak speeds. The resulting number could be used with either Weston or General Electric meters of the time with insignificant error. However, the safety factor increased the exposure by a stop over the Jones speed point. A film rated Kodak 400 would be an ASA 100 film by this standard. In its data sheets of this time Kodak stated that the exposure could be reduced a stop if work was carried out carefully. That's true, but I expect you have the same booklets I have where it says the safety factor is 2.5. The second ASA standard changed the method of measurement from the Jones minimum usable gradient to a fixed minimum density method as adopted by the DIN in the early 1950's (don't have the exact date at hand). The 1958 Ilford Manual by Horder has the date for revised DIN standard as 1957 (p. 284, 287). The older 1936 DIN system used the same minimum density requirement, but specified development for maximum speed, rather than a standard development representing something like normal use - as was used from 1957. I think the ASA Standard is ANSI PH2.5-1960, at least it is cited that way in Photographic Sensitometry by Todd and Zakia (1974 p.164). These would just be publication dates, the actual work would always be a bit earlier. However, they wanted to accomplish two things: first was to make the speeds compatible with earlier ASA speeds, That made a lot of sense given the number of meters already in use. and secondly, to maintain something like the Jones idea of the minimum gradient. It doesn't use the 0.3 times average gradient criterion though, it uses the 0.1 density above base + fog criterion. The ASA conducted extensive surveys of films of the time and found that there was a nearly constant ratio between the fixed minimum density, that is log 0.1 above fog plus support density, and the Jones point as found using the Jones method. It seems to hold good within a third of a stop based on the ratings of films just before and just after the change. Note that the inertia based Weston speed also tracked pretty well with the Jones Kodak Speed and Old ASA. The normal relationship would be that Weston 40 equaled Kodak 200 and Old ASA 50. Many films seem to have fit that relationship perfectly and I'm unaware of any that were more than 1/3 stop off. The key seems to be normal development. Weston, Old ASA, and ISO (New ASA/New DIN) use different criteria, but track quite well. They all use normal development. Old DIN (1936) tracked the other systems very badly. It used development for maximum speed. The same criterion, but with normal development, works very well. This obviated the difficult Jones measurement. The ratio turned out to be about 1.25 times the exposure required to reach the DIN density point. The math in my previous post showed that an ISO 400/27 film has a DIN density point at -2.7 log lux seconds and that an Old ASA 200 film had a Jones Point at -2.9. Unless someone shows that my formulas or math are wrong, I'm sticking to those figures. The Jones Point for a typical film is thus 0.2 log units to the left and represents 2/3 of a stop less exposure than the DIN point. So, a factor of 0.8, the reciprocal of 1.25 is introduced into the calculation of the speed in the new ASA method. That was to get the numbers they wanted, but the actual difference between the DIN point and the Jones point for films where New ASA is double Old ASA is 0.2 log units, 2/3 of a stop or a factor of around 1.6. In effect the speeds were now double those measured by the old ASA method and half of the Kodak speed. True for most films. I reiterate that the factor in the current speed method is NOT a safety factor but rather to bring measurements made by the method into agreement with the speed that would be measured by the Jones/Kodak method and reverts to Jones' original idea of finding the minimum exposure that results in good tone rendition. Yes but if the "Kodak Films" booklets are correct that Old ASA had a safety factor of 2.5, then the doubling of the speed ratings should have reduced this to 1.25. I agree that this has nothing to do with the 0.8 in the formula. The 0.8 in the formula just puts the scale where they wanted it. The original Kodak method does not seem to have had a fixed contrast, however, contrast does affect the speeds measured by either method. The Kodak method does specify a fixed exposure interval, much the same as the current method. Yes, but changes in development time tend to affect the contrast in the toe and the overall contrast at the same time. Since the Jones method depends on the ratio between the slope of the curve at the Jones point and the overall slope, changes in development will have less effect on the Jones point than on the DIN point. Since the old ASA method and the new ISO method are compatible as to speed point even though measured by different techniques, its possible to translate old ASA (pre 1958) speeds to equivalent modern speeds by simply multiplying by 2. Keep in mind that the speeds in both systems are rounded off as were old Weston speeds so there may not be an exact agreement. Also, even though Kodak used the same trade names for decades the emulsions were changed many times. Current Plus-X is not the same as the product of, say, 1948 although its broad speed category is about the same and its intended use is about the same so speed comparisons must be made with some care. Plus-X 35mm did vary in rated speed over the years. Kodak 200 in 1943 (Old ASA 50) ASA 50 in 1951 ASA 80 in 1956 But the speed of both Plus-X 35mm and Verichrome Pan just before the change was ASA 80 and both were ASA 125 just after the change and forever after. I'm aware that both systems round to the nearest 1/3 stop so that a 1/3 stop difference in reality may be hardly anything at all. My impression is that Plus-X 35mm, while now greatly improved from the film introduced in 1938, never underwent any rapid obvious change from year to year. Every now and then the new stuff would be just a little better. A bunch of minor changes over 69 years can add up a lot. Then again, it was always around 100 speed by today's standards. It was always a double coated film, always type B panchromatic, always had a high acutance, and was always fine grained for its time. What is surprizing is how fast films of the mid 1940's were. The difference was, of course, grain. A 1940's film which would measure 400 on the ISO system would be extremely grainy compared to a modern film of that speed. Medium speed films were actually pretty ok. I have some of my father's Verichrome negatives that he took with his Brownie in the 1940s. Even with department store processing, the negatives are still a bit finer grained than today's Tri-X in D-76 1:1. Peter. -- Everything I have ever read says the safety factor before 1959 was 2.5. It was reduced to 1.25 in the new system. The right factor is about 1.75. |
#83
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Improved T-Max 400
"UC" wrote in message ups.com... On Nov 1, 8:48 pm, Peter Irwin wrote: Richard Knoppow wrote: This is getting long but I can't find anything to snip. I hope that means that it was mostly ok. I don't know when Kodak began using Jones' method internally but it did begin to publish Kodak speeds around 1939. Jones actually worked out his system much earlier. I'm sure that's right, but published Kodak speeds from before 1939 are a different system. The Kodak speeds listed in the 1938 edition of "Eastman Professional Films" are not the Jones system. The pre-1939 Kodak system was based on inertia (Like Weston and H&D) and is equal to ten times the Weston speed. The ASA adopted the system in 1943 with a safety factor of 2. That's what you've said several times. But - In "Kodak Films" fifth edition 1951 from the Kodak Reference Handbook on page 16, it says "For the black-and-white continuous tone negative materials covered by the standard, a safety factor of 2.5 is used." In "Kodak Films" seventh edition 1956, it says the same thing on the top of page 21. ASA speeds were 1/4 of Kodak speeds. The resulting number could be used with either Weston or General Electric meters of the time with insignificant error. However, the safety factor increased the exposure by a stop over the Jones speed point. A film rated Kodak 400 would be an ASA 100 film by this standard. In its data sheets of this time Kodak stated that the exposure could be reduced a stop if work was carried out carefully. That's true, but I expect you have the same booklets I have where it says the safety factor is 2.5. The second ASA standard changed the method of measurement from the Jones minimum usable gradient to a fixed minimum density method as adopted by the DIN in the early 1950's (don't have the exact date at hand). The 1958 Ilford Manual by Horder has the date for revised DIN standard as 1957 (p. 284, 287). The older 1936 DIN system used the same minimum density requirement, but specified development for maximum speed, rather than a standard development representing something like normal use - as was used from 1957. I think the ASA Standard is ANSI PH2.5-1960, at least it is cited that way in Photographic Sensitometry by Todd and Zakia (1974 p.164). These would just be publication dates, the actual work would always be a bit earlier. However, they wanted to accomplish two things: first was to make the speeds compatible with earlier ASA speeds, That made a lot of sense given the number of meters already in use. and secondly, to maintain something like the Jones idea of the minimum gradient. It doesn't use the 0.3 times average gradient criterion though, it uses the 0.1 density above base + fog criterion. The ASA conducted extensive surveys of films of the time and found that there was a nearly constant ratio between the fixed minimum density, that is log 0.1 above fog plus support density, and the Jones point as found using the Jones method. It seems to hold good within a third of a stop based on the ratings of films just before and just after the change. Note that the inertia based Weston speed also tracked pretty well with the Jones Kodak Speed and Old ASA. The normal relationship would be that Weston 40 equaled Kodak 200 and Old ASA 50. Many films seem to have fit that relationship perfectly and I'm unaware of any that were more than 1/3 stop off. The key seems to be normal development. Weston, Old ASA, and ISO (New ASA/New DIN) use different criteria, but track quite well. They all use normal development. Old DIN (1936) tracked the other systems very badly. It used development for maximum speed. The same criterion, but with normal development, works very well. This obviated the difficult Jones measurement. The ratio turned out to be about 1.25 times the exposure required to reach the DIN density point. The math in my previous post showed that an ISO 400/27 film has a DIN density point at -2.7 log lux seconds and that an Old ASA 200 film had a Jones Point at -2.9. Unless someone shows that my formulas or math are wrong, I'm sticking to those figures. The Jones Point for a typical film is thus 0.2 log units to the left and represents 2/3 of a stop less exposure than the DIN point. So, a factor of 0.8, the reciprocal of 1.25 is introduced into the calculation of the speed in the new ASA method. That was to get the numbers they wanted, but the actual difference between the DIN point and the Jones point for films where New ASA is double Old ASA is 0.2 log units, 2/3 of a stop or a factor of around 1.6. In effect the speeds were now double those measured by the old ASA method and half of the Kodak speed. True for most films. I reiterate that the factor in the current speed method is NOT a safety factor but rather to bring measurements made by the method into agreement with the speed that would be measured by the Jones/Kodak method and reverts to Jones' original idea of finding the minimum exposure that results in good tone rendition. Yes but if the "Kodak Films" booklets are correct that Old ASA had a safety factor of 2.5, then the doubling of the speed ratings should have reduced this to 1.25. I agree that this has nothing to do with the 0.8 in the formula. The 0.8 in the formula just puts the scale where they wanted it. The original Kodak method does not seem to have had a fixed contrast, however, contrast does affect the speeds measured by either method. The Kodak method does specify a fixed exposure interval, much the same as the current method. Yes, but changes in development time tend to affect the contrast in the toe and the overall contrast at the same time. Since the Jones method depends on the ratio between the slope of the curve at the Jones point and the overall slope, changes in development will have less effect on the Jones point than on the DIN point. Since the old ASA method and the new ISO method are compatible as to speed point even though measured by different techniques, its possible to translate old ASA (pre 1958) speeds to equivalent modern speeds by simply multiplying by 2. Keep in mind that the speeds in both systems are rounded off as were old Weston speeds so there may not be an exact agreement. Also, even though Kodak used the same trade names for decades the emulsions were changed many times. Current Plus-X is not the same as the product of, say, 1948 although its broad speed category is about the same and its intended use is about the same so speed comparisons must be made with some care. Plus-X 35mm did vary in rated speed over the years. Kodak 200 in 1943 (Old ASA 50) ASA 50 in 1951 ASA 80 in 1956 But the speed of both Plus-X 35mm and Verichrome Pan just before the change was ASA 80 and both were ASA 125 just after the change and forever after. I'm aware that both systems round to the nearest 1/3 stop so that a 1/3 stop difference in reality may be hardly anything at all. My impression is that Plus-X 35mm, while now greatly improved from the film introduced in 1938, never underwent any rapid obvious change from year to year. Every now and then the new stuff would be just a little better. A bunch of minor changes over 69 years can add up a lot. Then again, it was always around 100 speed by today's standards. It was always a double coated film, always type B panchromatic, always had a high acutance, and was always fine grained for its time. What is surprizing is how fast films of the mid 1940's were. The difference was, of course, grain. A 1940's film which would measure 400 on the ISO system would be extremely grainy compared to a modern film of that speed. Medium speed films were actually pretty ok. I have some of my father's Verichrome negatives that he took with his Brownie in the 1940s. Even with department store processing, the negatives are still a bit finer grained than today's Tri-X in D-76 1:1. Peter. -- Everything I have ever read says the safety factor before 1959 was 2.5. It was reduced to 1.25 in the new system. The right factor is about 1.75. I don't know what you mean by the "right factor" the numbers 1.75 do not exist in the standard. Also, the 0.8 or 1.25 factor in the new standard is _NOT_ a safety factor as I explained in my long previous post. It is meant to adjust the value gotten from the straight DIN method to the speed which would result from the Jones/Kodak minimum usable gradient method. The ASA adoption of the Jones/Kodak method did include a safety factor of 2.5. This was done to insure that there would be a developable image when applied by amateurs. Unfortunately it resulted in quite dense negatives. Jones worked on the basis of determing the minimum practical exposure for good tone rendition in order to obtain the best sharpness and grain characteristic from a film. He also found that increased exposure made little difference to the tone rendition but that underexposure by even a small amount resulted in poor rendition, so, the ASA and Kodak decided to lower the film speed by about a stop to make sure people would get acceptable results. Tone rendition was considered more important than optimum grain and sharpness. Again speeds _reported_ by the current method are about double those obtained by the original ASA method and about half of those obtained by the method used by Kodak internally. the division of the Kodak speed by a factor of two fits the resulting speed to the calibration of the exposure meter calculators current in the United States in the 1940's, i.e. Weston and General Electric. Weston had their own system of determining speed and all measurements were made, at least at first, by Weston, so there was no temptation for manufacturers to cheat, as they could with the earlier DIN, Schneiner, or H&D systems, all being used at the time. I have never found a description of the method used by General Electric. GE speeds were two numbers higher than Weston speeds and the original ASA sytem was designed to result in a number in-between these two so it could be used on either meter with insignificant error. Weston speeds were rounded off so that all films within a bracketed range had the same speed value. Probably GE did the same. Actually, the current ISO standard also has rounded off ranges so published speeds in any of these four systems are, and have always been, approximations. Again, the speed depends on the degree of development. The ISO and new DIN standard effectively specify a contrast index by specifying the density range to be obtained from an exposure range. Any change in the contrast will affect the effective speed. That's why the term EI, or Exposure Index should be used when a speed has not been determined by the ISO method. Also, common developers can affect speed. The range is around 1.5 stops overall for developers ranging from Microdol-X or Perceptol (low end of speed when used full strength) to Xtol, T-Max, Microphen (all at the high end of speed) with developers like D-76 being in the middle. The standard requires that the developer used be specified with the speed ratings. Kodak does this on its development charts but I have never seen a developer specified on a film box. In any case, the calculator of an exposure meter is designed to fit an average scene into the usable range of the film. Changing the film speed simply moves the exposure left or right along the curve. The mid gray value, which is often argued, is actually of little relevance, provided its in the linear part of the curve somewhere. More important is where shadows that are to have some detail fall. They must be recorded on the film characteristic at a point where the contrast is high enough to record the detail. If too far down on the toe the shadows become blank. Moving them up will improve their rendition but exposure must not be moved up the curve enough to make the highlights _which are to have detail_ fall onto the shoulder, where, again, the contrast is low. For modern film there is no practical shoulder unless the film is overexposed by a great many stops. Printing density is another consideration: overall density becomes higher as exposure is increased. Negatives which are so dense that they take very long printing exposures are undesirable even if they deliver good tone rendition. Worrying about small errors in exposure is useless for normal B&W, the important thing is to give the negative enough exposure for good shadow detail. Once this mimimum is met there is a long range of increased exposure which will still result in good tone rendition in the print. -- --- Richard Knoppow Los Angeles, CA, USA |
#84
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Improved T-Max 400
On Nov 2, 5:24 pm, "Richard Knoppow" wrote:
"UC" wrote in message ups.com... On Nov 1, 8:48 pm, Peter Irwin wrote: Richard Knoppow wrote: This is getting long but I can't find anything to snip. I hope that means that it was mostly ok. I don't know when Kodak began using Jones' method internally but it did begin to publish Kodak speeds around 1939. Jones actually worked out his system much earlier. I'm sure that's right, but published Kodak speeds from before 1939 are a different system. The Kodak speeds listed in the 1938 edition of "Eastman Professional Films" are not the Jones system. The pre-1939 Kodak system was based on inertia (Like Weston and H&D) and is equal to ten times the Weston speed. The ASA adopted the system in 1943 with a safety factor of 2. That's what you've said several times. But - In "Kodak Films" fifth edition 1951 from the Kodak Reference Handbook on page 16, it says "For the black-and-white continuous tone negative materials covered by the standard, a safety factor of 2.5 is used." In "Kodak Films" seventh edition 1956, it says the same thing on the top of page 21. ASA speeds were 1/4 of Kodak speeds. The resulting number could be used with either Weston or General Electric meters of the time with insignificant error. However, the safety factor increased the exposure by a stop over the Jones speed point. A film rated Kodak 400 would be an ASA 100 film by this standard. In its data sheets of this time Kodak stated that the exposure could be reduced a stop if work was carried out carefully. That's true, but I expect you have the same booklets I have where it says the safety factor is 2.5. The second ASA standard changed the method of measurement from the Jones minimum usable gradient to a fixed minimum density method as adopted by the DIN in the early 1950's (don't have the exact date at hand). The 1958 Ilford Manual by Horder has the date for revised DIN standard as 1957 (p. 284, 287). The older 1936 DIN system used the same minimum density requirement, but specified development for maximum speed, rather than a standard development representing something like normal use - as was used from 1957. I think the ASA Standard is ANSI PH2.5-1960, at least it is cited that way in Photographic Sensitometry by Todd and Zakia (1974 p.164). These would just be publication dates, the actual work would always be a bit earlier. However, they wanted to accomplish two things: first was to make the speeds compatible with earlier ASA speeds, That made a lot of sense given the number of meters already in use. and secondly, to maintain something like the Jones idea of the minimum gradient. It doesn't use the 0.3 times average gradient criterion though, it uses the 0.1 density above base + fog criterion. The ASA conducted extensive surveys of films of the time and found that there was a nearly constant ratio between the fixed minimum density, that is log 0.1 above fog plus support density, and the Jones point as found using the Jones method. It seems to hold good within a third of a stop based on the ratings of films just before and just after the change. Note that the inertia based Weston speed also tracked pretty well with the Jones Kodak Speed and Old ASA. The normal relationship would be that Weston 40 equaled Kodak 200 and Old ASA 50. Many films seem to have fit that relationship perfectly and I'm unaware of any that were more than 1/3 stop off. The key seems to be normal development. Weston, Old ASA, and ISO (New ASA/New DIN) use different criteria, but track quite well. They all use normal development. Old DIN (1936) tracked the other systems very badly. It used development for maximum speed. The same criterion, but with normal development, works very well. This obviated the difficult Jones measurement. The ratio turned out to be about 1.25 times the exposure required to reach the DIN density point. The math in my previous post showed that an ISO 400/27 film has a DIN density point at -2.7 log lux seconds and that an Old ASA 200 film had a Jones Point at -2.9. Unless someone shows that my formulas or math are wrong, I'm sticking to those figures. The Jones Point for a typical film is thus 0.2 log units to the left and represents 2/3 of a stop less exposure than the DIN point. So, a factor of 0.8, the reciprocal of 1.25 is introduced into the calculation of the speed in the new ASA method. That was to get the numbers they wanted, but the actual difference between the DIN point and the Jones point for films where New ASA is double Old ASA is 0.2 log units, 2/3 of a stop or a factor of around 1.6. In effect the speeds were now double those measured by the old ASA method and half of the Kodak speed. True for most films. I reiterate that the factor in the current speed method is NOT a safety factor but rather to bring measurements made by the method into agreement with the speed that would be measured by the Jones/Kodak method and reverts to Jones' original idea of finding the minimum exposure that results in good tone rendition. Yes but if the "Kodak Films" booklets are correct that Old ASA had a safety factor of 2.5, then the doubling of the speed ratings should have reduced this to 1.25. I agree that this has nothing to do with the 0.8 in the formula. The 0.8 in the formula just puts the scale where they wanted it. The original Kodak method does not seem to have had a fixed contrast, however, contrast does affect the speeds measured by either method. The Kodak method does specify a fixed exposure interval, much the same as the current method. Yes, but changes in development time tend to affect the contrast in the toe and the overall contrast at the same time. Since the Jones method depends on the ratio between the slope of the curve at the Jones point and the overall slope, changes in development will have less effect on the Jones point than on the DIN point. Since the old ASA method and the new ISO method are compatible as to speed point even though measured by different techniques, its possible to translate old ASA (pre 1958) speeds to equivalent modern speeds by simply multiplying by 2. Keep in mind that the speeds in both systems are rounded off as were old Weston speeds so there may not be an exact agreement. Also, even though Kodak used the same trade names for decades the emulsions were changed many times. Current Plus-X is not the same as the product of, say, 1948 although its broad speed category is about the same and its intended use is about the same so speed comparisons must be made with some care. Plus-X 35mm did vary in rated speed over the years. Kodak 200 in 1943 (Old ASA 50) ASA 50 in 1951 ASA 80 in 1956 But the speed of both Plus-X 35mm and Verichrome Pan just before the change was ASA 80 and both were ASA 125 just after the change and forever after. I'm aware that both systems round to the nearest 1/3 stop so that a 1/3 stop difference in reality may be hardly anything at all. My impression is that Plus-X 35mm, while now greatly improved from the film introduced in 1938, never underwent any rapid obvious change from year to year. Every now and then the new stuff would be just a little better. A bunch of minor changes over 69 years can add up a lot. Then again, it was always around 100 speed by today's standards. It was always a double coated film, always type B panchromatic, always had a high acutance, and was always fine grained for its time. What is surprizing is how fast films of the mid 1940's were. The difference was, of course, grain. A 1940's film which would measure 400 on the ISO system would be extremely grainy compared to a modern film of that speed. Medium speed films were actually pretty ok. I have some of my father's Verichrome negatives that he took with his Brownie in the 1940s. Even with department store processing, the negatives are still a bit finer grained than today's Tri-X in D-76 1:1. Peter. -- Everything I have ever read says the safety factor before 1959 was 2.5. It was reduced to 1.25 in the new system. The right factor is about 1.75. I don't know what you mean by the "right factor" the numbers 1.75 do not exist in the standard. Also, the 0.8 or 1.25 factor in the new standard is _NOT_ a safety factor as I explained in my long previous post. It is meant to adjust the value gotten from the straight DIN method to the speed which would result from the Jones/Kodak minimum usable gradient method. The ASA adoption of the Jones/Kodak method did include a safety factor of 2.5. This was done to insure that there would be a developable image when applied by amateurs. Unfortunately it resulted in quite dense negatives. Jones worked on the basis of determing the minimum practical exposure for good tone rendition in order to obtain the best sharpness and grain characteristic from a film. He also found that increased exposure made little difference to the tone rendition but that underexposure by even a small amount resulted in poor rendition, so, the ... read more » The 'new' standard reduced the safety factor, but did not eliminate it. It was reduced from 2.5 to 1.25. I have read this somewhere. A factor of 1.75 (basically, lowering the speeds by 1/2 stop) would put the shadows up a little higher on the curve and give better separation of shadow detail. |
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Improved T-Max 400
Richard Knoppow wrote:
The ASA adoption of the Jones/Kodak method did include a safety factor of 2.5. This was done to insure that there would be a developable image when applied by amateurs. It also tended to match what the Weston and GE systems were already doing. The Weston and GE systems were actually working quite well in practice. The change to a unified system with a really sound theoretical basis was obviously a good thing, but it was probably desirable at the time for the new system to give people answers which were close to what they were already getting from their meters. Even in the 1943 Kodak datasheets, there was a sentence "When it is desired to reduce the exposure to a minimum, these meter settings can be doubled with little danger of of serious underexposure." Unfortunately it resulted in quite dense negatives. My understanding is that a one stop increase in exposure typically results in an increased negative density of 0.2 or so. This seems to me to be a fairly small shift. Doubling the exposure time for the negative will less than double the exposure time for the print. Jones worked on the basis of determining the minimum practical exposure for good tone rendition in order to obtain the best sharpness and grain characteristic from a film. He also found that increased exposure made little difference to the tone rendition but that underexposure by even a small amount resulted in poor rendition, so, the ASA and Kodak decided to lower the film speed by about a stop to make sure people would get acceptable results. Tone rendition was considered more important than optimum grain and sharpness. Even then, the increased grain and loss of sharpness from 1 stop or so extra exposure must have been quite small. According to T.L.J. Bentley's "Manual of the Miniature Camera" (4th ed. 1953 page 102) - "The resolution obtainable with photographic film suffers to some extent with excessive exposure, but with a good film the effect is not so noticeable in practice as some statements suggest: it can be shown by test that, at X 16 enlargement under critical conditions, the deterioration with an increase in exposure to eight times the least exposure giving an enlargement of optimum quality is barely detectable under the closest scrutiny." Even if you think you might be somewhat fussier, it doesn't seem that the adverse effects of a single stop increase could have been very noticeable even in 1953. Again speeds reported by the current method are about double those obtained by the original ASA method and about half of those obtained by the method used by Kodak internally. the division of the Kodak speed by a factor of two fits the resulting speed to the calibration of the exposure meter calculators current in the United States in the 1940's, i.e. Weston and General Electric. Weston had their own system of determining speed and all measurements were made, at least at first, by Weston, so there was no temptation for manufacturers to cheat, as they could with the earlier DIN, Schneiner, or H&D systems, all being used at the time. In the early 20th century, Alfred Watkins produced lists of measured speeds for plates sold in the UK. In theory his numbers should have been 1.47 times higher than honest H&D speeds, but in practice they are generally lower than manufacturers' rated speeds. When I last had a look at this it appeared to me that Ilford H&D used a fudge factor of two. This appears to have remained the case from the early teens through the 1940s. Ilford H&D can be converted to Weston by dividing by 50, and roughly to modern ISO speeds by dividing by 20. Ilford seems to have been quite open about this: in the 1934 Ilford Manual on page 40, it reads "It might be mentioned, further, that for various reasons the original details laid down by Hurter & Driffield have been departed from to some extent, and in consequence the H&D speed number of an emulsion, though correctly only about two-thirds that of a Watkins number, is generally given as about one-third higher." European Scheiner inflation seems to have been a major problem. The DIN standard was supposed to correct this. DIN numbers were originally around 10 less than the Scheiner speeds, though the difference seems to have grown to 11-13 by the 1950s. I have never found a description of the method used by General Electric. I've never seen one either. There is a good discussion of the Weston method in the 1958 Ilford Manual. GE speeds were two numbers higher than Weston speeds and the original ASA sytem was designed to result in a number in-between these two so it could be used on either meter with insignificant error. Weston speeds were rounded off so that all films within a bracketed range had the same speed value. Probably GE did the same. Actually, the current ISO standard also has rounded off ranges so published speeds in any of these four systems are, and have always been, approximations. Again, the speed depends on the degree of development. The ISO and new DIN standard effectively specify a contrast index by specifying the density range to be obtained from an exposure range. Any change in the contrast will affect the effective speed. That's why the term EI, or Exposure Index should be used when a speed has not been determined by the ISO method. Also, common developers can affect speed. The range is around 1.5 stops overall for developers ranging from Microdol-X or Perceptol (low end of speed when used full strength) to Xtol, T-Max, Microphen (all at the high end of speed) with developers like D-76 being in the middle. According to a post by David Carper of Ilford on photo.net: http://photo.net/bboard/q-and-a-fetch-msg?msg_id=003WI4 Ilford HP5+ has an ISO speed of 500/28 in Microphen, compared to a speed of 400/27 in ID11. The speed at a Gbar of 0.75 is less than a third of a stop higher than it is at a Gbar of 0.62. The speed at a Gbar of 0.55 is less than 1/4 of a stop slower than at a Gbar of 0.62. So while changing development does change the speed point, it doesn't appear to change it by all that much in most cases. Printing density is another consideration: overall density becomes higher as exposure is increased. Negatives which are so dense that they take very long printing exposures are undesirable even if they deliver good tone rendition. A single extra stop isn't going to cause any trouble in the darkroom. An exposure of six stops over does cause a big problem: not only are the printing times too long - it can be hard to focus the enlarger. A six stop overexposed negative will actually print ok, but it is an absolutely ridiculous thing to do except as a test to prove a point. Peter. -- |
#86
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Improved T-Max 400
On Nov 5, 8:32 pm, Peter Irwin wrote:
Peter has made some excellent points, and my experience corresponds precisely with what he says. One stop increase in exposure above the bare minimum results in hardly any change except increased shadow detail. I use about 2/3 stop over the ISO exposure, i.e., I rate most ISO 400 films at 250. |
#87
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Improved T-Max 400
"Richard Knoppow" wrote in message ... [... snip generous article ...] the division of the Kodak speed by a factor of two fits the resulting speed to the calibration of the exposure meter calculators current in the United States in the 1940's, i.e. Weston and General Electric. At one point Weston changed the calibration or setting of the dial. Was it when ASA was introduced? Darned. I will get the big box of old Westons and compare and let you know what I find. |
#88
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Improved T-Max 400
In article , "pico" pico.pico.net
wrote: "UC" wrote in message ups.com... Here is the curve for TMY: http://www.kodak.com/global/en/profe...4016/f002_0507 ac.gif Here is the curve for Tri-X Pan: http://www.kodak.com/global/en/profe...4017/f009_0490 ac.gif Do you see the difference? What part of that line/curve is actually useful for making photographs? You never use the so-called shoulder - it is outside the useful exposure range. Film: You can use all of a curve that fits between .001 and 1.71 Ha ha less image area from .001 to .15 & the 1.00 to 1.71 areas! Paper: You must compress all film values to fit a range between .15 and 2.00 for a natural and real representation -- Reality is a picture perfected and never looking back. |
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