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Contrast Index Question: Newbie
Can someone explain to me contrast indexes. Are they published for each B&W
film, or do I have to calculate it by myself? TIA. |
#2
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Contrast Index Question: Newbie
"In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no...
Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
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Contrast Index Question: Newbie
Thanks, a few more questions though, how is the CI derived?. What do I use
to measure it? What does the number actually mean? Is there a chart somewhere? I've got a condenser head on my MX45 and I've just been developing as per their instructions on the box of film, so probably, I've been doing everything at .57. Could I correct this by reprinting using my vari contrast filters or will that make the problem worse? Thx. "Michael Scarpitti" wrote in message om... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
#4
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Contrast Index Question: Newbie
One more thing. If I'm reducing dev times should I also be rating my film
down too. i.e. 400-320 and 125 down to 100? Or what happens in this instance. "Michael Scarpitti" wrote in message om... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
#5
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Contrast Index Question: Newbie
"In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. Contrast Index, or CI, is a method of specifying contrast devised by Kodak to be applicable to a large variety of materials. Contrast is the slope of the characteristic curve. This curve shows the relationship between exposure and density. It varies with development: the more the film is developed the steeper the curve will be and the higher the contrast. At one time contrast was defined as the slop of this curve at a point on the straight line portion. Actual film curves vary in slope from low to high densities. The difference in slope is determined by the way the emulsion is made and represents the distribution of sensitivity of the silver halide particals which make up the emulsion. The shape of the curve is decided on by the manufacturer to yield a certain tonal rendition. The curve has three parts: the toe, the straight line, and the shoulder. The toe is the low density part. In general this has a lower contrast than the straight line portion. The shadows are recoreded in this region. The straight line portion represents most of the film's recording range. It records differences in brightness as a linear change in density. This is the region used for most of the brightness information in the photo. The shoulder is the high exposure- high density part of the curve. Again, the contrast is lower than in the straight line portion. Most modern films have such a long range of densities that they never reach the shoulder in normal use but many older films did resulting in "blocked" highlights. Until about the 1940's contrast was usually measured as "gamma" Gamma is the contrast of the straight line portion of the curve. However, most films record part of the image in the toe region and many films have relatively long toe sections. Some, like Tri-X sheet film and the discontinued Plus-X sheet film, have no straight line portion, the contrast increasing with density at all values. For films like this measuring contrast as gamma can be very misleading about the printing quality of the negative. For instance, a long toe film, like Tri-X Pan sheet film, when compared to a short toe film, like T-Max 400, will have much lower highlight density when developed to the same gamma. One solution to this was the development of Average Contrast, or bar-G (i.e., the letter with a dash over it to indicate its an averaged value). Average contrast is, essentially, a straight line drawn from the minimum density to some maximum density. For the ISO method of measuring film speed bar-G is defined indirectly by the specified exposure range and expected resulting density range. For this method the range of exposure is log 1.30. The film is developed so that the density resulting from the maximum exposure is log 0.9 above base density plus fog. The speed is measured between this point and a minimum of logd 0.1 above base density and fog for a density range of log 0.8. Contrast Index is similar to bar-G but it specifies a minimum density of log 0.2 above base and fog and is the slope measured from this point to a high density point corresponding to a range of LogE 2.0 from the exposure needed to produce the low density reference. This is a more realistic range of exposure in normal photography and the reference point of logD 0.2 above base and fog is far enough up the toe to eliminate problems with very low contrast toe regions (i.e., no shadow detail). Contrast Index is applicable to all sorts of film, including special purpose films, like high contrast copy or graphic arts films. While CI can be determined from a film curve it is easier to measure using a graphic overlay designed by Kodak Labs. These used to be available from Kodak but have probably been discontinued or, perhaps, are available from Silver Pixel Press. CI was originally described in a paper published by the Kodak Research Labs. "Contrast Index" C.N.Nelson and J.A.C.Yule, _Photographic Science and Engineering_ V. 10, No. 1, January-Febuary 1966 It is also described in detail in several books on photographic science. I should clarify that two negatives on different films developed to the same contrast index will not look the same. The curve of the film has a strong influence on tonal rendition, however, at the same CI both negatives will print on the same grade of paper and yield reasonably good prints. Where they are developed to the same _gamma_ they will not print on the same paper grade and the long toe film may have inadequate shadow detail on any paper. Kodak uses CI for its contrast specifications. Agfa seems to still use gamma, and Ilford and others use bar-G. -- --- Richard Knoppow Los Angeles, CA, USA |
#6
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Contrast Index Question: Newbie
Of course you can calculate it. But it is not simple. You calculate it by
contact printing a step wedge on your film and developing to whatever time the manufacturer recommends or to the time you want. Now you measure the density of the wedge images and plot the film curve and then use the definition of contrast index to calculate what you got. It is not easy but it is doable. Kodak gives CI values for its recommended developing times but they are for specific types of enlargers (like the OP said, diffusion enlargers) and you may end up with a negative that is too contrasty if you use a condenwer enlarger. Kodak is the only manufacturer that publishes CI values for its films, as far as I know. Perhaps there are a couple of others who do the same but I am not familiar with them. If you are interested there is a book "Beyond the Zone System" by Phil Davis that goes into the subject at great depth. It is basically applied sensitometry which is a good defintion of the Zone system. Mr. Scarpitti's postings are considered to be garbage by the majority of the posters in this newsgroup. Once in a while he says something worthwhile but it is lost in the noise of his hatred for the Zone System. I suggest you take them with a grain of salt until you have read a few more of his posts and then you can make your own opinion. Best regards Bert In The Trenches wrote: Thanks, a few more questions though, how is the CI derived?. What do I use to measure it? What does the number actually mean? Is there a chart somewhere? I've got a condenser head on my MX45 and I've just been developing as per their instructions on the box of film, so probably, I've been doing everything at .57. Could I correct this by reprinting using my vari contrast filters or will that make the problem worse? Thx. "Michael Scarpitti" wrote in message om... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
#7
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Contrast Index Question: Newbie
In The Trenches wrote:
One more thing. If I'm reducing dev times should I also be rating my film down too. i.e. 400-320 and 125 down to 100? Or what happens in this instance. No, you reduce the times to reduce the contrast. If you should lower the film speed you have just undone what you did by reducing the developing times. CI is a measurement of contrast, similar to gamma but the measurements are taken differently and the two values are not exactly interchangeable. I will check to see if I can find the definition of CI. Basically it is the slope of a line drawn in the H&D curve between two specified points. OK, here it is, taken from "Beyond the Zone System" p29 and p30. The definition of CI is the slope of a line drawn from a point in the curve at 0.1 units above base+fog to a point that is 2.0 units away from the first point. You may have to see the graphic to understand this. Bert "Michael Scarpitti" wrote in message om... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
#8
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Contrast Index Question: Newbie
I've some some more info at http://www.infodotinc.com/photographyadv/19.htm
They've even posted a Contrast Index chart. I'm still grappling with what all this means in practical terms to my darkroom work but I think it's worth knowing. I have a darkroom and contrast is a big part of developing so might as well know it all or as much as possible. Do people who develop their own film regularly consult contrast indices? Or are they consider something to know about and forget.? "BertS" wrote in message . net... In The Trenches wrote: One more thing. If I'm reducing dev times should I also be rating my film down too. i.e. 400-320 and 125 down to 100? Or what happens in this instance. No, you reduce the times to reduce the contrast. If you should lower the film speed you have just undone what you did by reducing the developing times. CI is a measurement of contrast, similar to gamma but the measurements are taken differently and the two values are not exactly interchangeable. I will check to see if I can find the definition of CI. Basically it is the slope of a line drawn in the H&D curve between two specified points. OK, here it is, taken from "Beyond the Zone System" p29 and p30. The definition of CI is the slope of a line drawn from a point in the curve at 0.1 units above base+fog to a point that is 2.0 units away from the first point. You may have to see the graphic to understand this. Bert "Michael Scarpitti" wrote in message om... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. 'Contrast Index' (CI) is a measurement of contrast of B&W film, and varies with the degree of development. Kodak's times given in their instructions usually produce a CI of about 0.57, which is about optimum for diffusion enlargers. CI 0.43 or so is ideal for condenser enlargers, and is obtained by reducing the developing times given by about 25%. You don't calculate this number or use it at all. It is simply for measurement purposes. |
#9
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Contrast Index Question: Newbie
Excellent post. Clearly I need to do some more reading. But I think you've
provided a starting point and historical perspective as well. In general terms should I be manipulating the contrast on the negative or is it better to do it post development using the enlarger? "Richard Knoppow" wrote in message ink.net... "In The Trenches" wrote in message news:BqMsc.588797$Ig.273737@pd7tw2no... Can someone explain to me contrast indexes. Are they published for each B&W film, or do I have to calculate it by myself? TIA. Contrast Index, or CI, is a method of specifying contrast devised by Kodak to be applicable to a large variety of materials. Contrast is the slope of the characteristic curve. This curve shows the relationship between exposure and density. It varies with development: the more the film is developed the steeper the curve will be and the higher the contrast. At one time contrast was defined as the slop of this curve at a point on the straight line portion. Actual film curves vary in slope from low to high densities. The difference in slope is determined by the way the emulsion is made and represents the distribution of sensitivity of the silver halide particals which make up the emulsion. The shape of the curve is decided on by the manufacturer to yield a certain tonal rendition. The curve has three parts: the toe, the straight line, and the shoulder. The toe is the low density part. In general this has a lower contrast than the straight line portion. The shadows are recoreded in this region. The straight line portion represents most of the film's recording range. It records differences in brightness as a linear change in density. This is the region used for most of the brightness information in the photo. The shoulder is the high exposure- high density part of the curve. Again, the contrast is lower than in the straight line portion. Most modern films have such a long range of densities that they never reach the shoulder in normal use but many older films did resulting in "blocked" highlights. Until about the 1940's contrast was usually measured as "gamma" Gamma is the contrast of the straight line portion of the curve. However, most films record part of the image in the toe region and many films have relatively long toe sections. Some, like Tri-X sheet film and the discontinued Plus-X sheet film, have no straight line portion, the contrast increasing with density at all values. For films like this measuring contrast as gamma can be very misleading about the printing quality of the negative. For instance, a long toe film, like Tri-X Pan sheet film, when compared to a short toe film, like T-Max 400, will have much lower highlight density when developed to the same gamma. One solution to this was the development of Average Contrast, or bar-G (i.e., the letter with a dash over it to indicate its an averaged value). Average contrast is, essentially, a straight line drawn from the minimum density to some maximum density. For the ISO method of measuring film speed bar-G is defined indirectly by the specified exposure range and expected resulting density range. For this method the range of exposure is log 1.30. The film is developed so that the density resulting from the maximum exposure is log 0.9 above base density plus fog. The speed is measured between this point and a minimum of logd 0.1 above base density and fog for a density range of log 0.8. Contrast Index is similar to bar-G but it specifies a minimum density of log 0.2 above base and fog and is the slope measured from this point to a high density point corresponding to a range of LogE 2.0 from the exposure needed to produce the low density reference. This is a more realistic range of exposure in normal photography and the reference point of logD 0.2 above base and fog is far enough up the toe to eliminate problems with very low contrast toe regions (i.e., no shadow detail). Contrast Index is applicable to all sorts of film, including special purpose films, like high contrast copy or graphic arts films. While CI can be determined from a film curve it is easier to measure using a graphic overlay designed by Kodak Labs. These used to be available from Kodak but have probably been discontinued or, perhaps, are available from Silver Pixel Press. CI was originally described in a paper published by the Kodak Research Labs. "Contrast Index" C.N.Nelson and J.A.C.Yule, _Photographic Science and Engineering_ V. 10, No. 1, January-Febuary 1966 It is also described in detail in several books on photographic science. I should clarify that two negatives on different films developed to the same contrast index will not look the same. The curve of the film has a strong influence on tonal rendition, however, at the same CI both negatives will print on the same grade of paper and yield reasonably good prints. Where they are developed to the same _gamma_ they will not print on the same paper grade and the long toe film may have inadequate shadow detail on any paper. Kodak uses CI for its contrast specifications. Agfa seems to still use gamma, and Ilford and others use bar-G. -- --- Richard Knoppow Los Angeles, CA, USA |
#10
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Contrast Index Question: Newbie
In The Trenches wrote:
In general terms should I be manipulating the contrast on the negative or is it better to do it post development using the enlarger? It depends on exactly what you're doing, and what type of equipment that you're using. If you're using large format (4x5" or bigger), then most people very the development for each sheet of film. You want to give the exposure and development that will best convey the qualities that you want in your print. Usually this means exposing and developing your negatives with grade 2 paper in mind. That way you give yourself the greatest range for manipulation of the print using different printing contrasts. With roll film (120mm or 35mm), it's difficult to give each frame different development. People who vary development with these films either have multiple cameras/film backs, or they try and shoot whole rolls of film under similar conditions. Another consideration is that developing a film longer gives bigger grain. In large format this is often not a problem, but it can be with the smaller formats. Often times roll film users aim to print on grade 3 paper. Doing so will lead to shorter film development, which minimizes grain size. -Peter De Smidt |
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