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#31
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New Freestyle Premium film ID?
"John" wrote in message m... Jean-David Beyer wrote: John wrote: Jean-David Beyer wrote: I thought they did make a better B&W T-Grain film. It TMax films are both sharper, finer-grained, and have a straighter D/H curve than any I have ever used in sheet sizes, anyway. That straighter curve has me puzzled. Doesn't it compress luminance/density values? Maybe habit has prejudiced me. I'll have to try some in 4x5 so that perhaps I'll understand why some like it. No, the toe of longer toe films compresses the shadows, and the shoulder (if present) in other films compresses the highlights. The only film I ever used that had a shoulder in the ighlights (in the useful range) was Panatomic-X. With a very short toe, you do have to be careful not to underexpose because if you do, you get nothing. Please don't beat me up just yet. Do we actually use either toe? If one were to take the actual ranges of exposure that are generally useful to printing would they include either toe? IOW, make a rectangle of the bounded (useful) area and slide it up and down the curve. Toe? No toe? OK, now you can beat me up. The toe and shoulder are areas that have lower contrast than the rest of the curve. The area in-between is called the straight line portion even though its not really straight in many films. The contrast of both shoulder and toe change gradually, the toe becomming greater as it approaches the straight line portion and the shoulder slowly loosing contrast as it is approached. For many modern films the range between the end of the toe and beginning of the shoulder is more than ten stops but it also depends on the developer, especially the shoulder end. Some developers do not produce as high a maximum density as others. There is a minimum exposure to get the shadows up far enough on the toe to get good shadow detail. Many decades ago a researcher from Kodak Research Labs named Loyd A. Jones (that's the correct spelling) conducted extensive experiments over a period of decades to determine the optimum exposure of the negative for good tonal rendition on the print. What he discovered was that the deepest shadow that was to have any detail had to be at a point on the toe where the gamma (contrast of a point) was about 1/3rd of the contrast of the straight line section of the curve. Greater exposure made little difference up to an increase of perhaps ten stops but even a small reduction resulted in poor shadow detail and a noticable reduction in print quality. The judgement of print quality was done by double-blind testing of prints made from actual scenes. I no longer remember the number of observers but it was considerable and there were hundreds of scenes of various sorts. From this data Jones developed a speed measureing system. This was used internally at Kodak for many years. Jones wanted to find the _minimum_ exposure for good tone rendition because relatively thin negatives have an advantage in looking sharper and having somewhat less grain than denser ones. This was more true of the thick emulsion films of the time (1920s through 1940s) than it is now, but its still considered good practice to make negatives on the thin side provided the tone rendition is good. In 1943 the Jones/Kodak system was adopted by the American Standards Association (ASA) as its speed system. Unfortunately, the ASA decided to add a one stop fudge factor to the speeds. This resulted in excessively dense negatives. Probably the reason for the fudge factor was an attempt to make sure that casual photographers got a printable image even if it was not of optimum quality. In about 1958 the ASA adopted a different system. The main reason was the difficulty of measuring speeds by the Jones method. It required measuring two curves simultaneously. The system which replaced it was a modification of the second version of the German DIN system (there was an older DIN system which was different). The DIN system based its speeds on a certain minimum density above gross fog and the base density. The ASA found, after surveying nearly all the pictorial films on the market at the time, that if a multiplier of 1.4 was added to the speed needed to get a density of log 0.1 above fog and base density the point would be, in virtually all cases, identical with the one found by the Jones minimum gradient system and would be a lot easier to measure consistently. This new system was adopted and at the same time the fudge factor was dropped. As a result the speeds of all films jumped by a full stop! The current speed standard promulgated by the ISO and, in the USA, by NIST, is a ammended version of the 1958 standard. The main difference is that no standard developer is specified. The measurement can be made with any developer the film manufacturer wants as long as that developer is stated with the results. In fact, the new system still incorporates Jones' idea of making exposures so that the darkest shadow to have detail is still on a part of the toe having sufficient contrast to print well. For many films many photographers find that a small increase in exposure gives them better shadow detail. This is probably due to variations in exposure measurement, flare, and processing than in the film or speed method. Note that the speed measurement is made at what ammounts to a fixed, standard, contrast. This contrast is about right for contact printing and diffusion enlarging. If some other contrast value is desired and a different developing time is used to obtain it the _effective_ speed of the film will change. The standard specifies a range of exposure at the film surface which will translate to a specified range of densities on the developed film. Development during testing is adjusted to obtain this range of densities. Of course, if development is varied the range of densities will also vary, this is, by definition, a change in contrast. Generally a lowering of contrast will reqire more exposure and raising of the contrast will require less. The ISO standard contains a chart for calculating the variation in effective speed (sometimes called exposure index) which results from this variation. A note on the shape of film curves. Kodak's published curves appear to be genuine and not approximations. They can be compared and some idea of what the variations will do to the reproduced gray scale can be seen. Some film and developer combinations result in a slight hump in the mid gray range and some films, notably ISO-350 Tri-X, have a delibrately very long toe, meaning the gamma increases gradually all throughout the curve until reaching extremely high values of density (out of the usable range). If the same scene is photographed on films of different curves and the shadow and highlight points are chosen to be the same, the effect will be that long toe films will tend to reproduce the mid-grays darker and the humped films reproduce them lighter, than in the original scene. A stright line film will reproduce them linearly. All three types have their places. The long toe film will tend toward dramatic highlights where the humped film will tend toward luminous gray values. Of course, the same sort of variations occur in printing paper but there the effect is the reverse of those from the film. A paper with a long toe will tend to reproduce mid grays brighter than one with a staight line. -- --- Richard Knoppow Los Angeles, CA, USA |
#32
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New Freestyle Premium film ID?
Thank you, Richard, for the cogent article. You have added to the body
of knowledge by placing 'the curve', in this case, into something I can understand. |
#33
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New Freestyle Premium film ID?
"John" wrote in message m... Thank you, Richard, for the cogent article. You have added to the body of knowledge by placing 'the curve', in this case, into something I can understand. See if you can find an old Kodak film handbook somewhere. These had a very good section on basic sensitometry in them explaining how the curves are generated and their relavance to the results. A similar article on print sensitometry appeared in the _Professional Printing_ book. These sometimes show up in used book stores or maybe eBay. There are good summaries of Jones' work in the classic _Theory of the Photographic Process_ C.E.K Mees, the "revised edition is best for this. Jones' work was scattered in several professional journals. I don't have a complete bibliography but there are many citations in Mees's book. More may be found in later editions. When Mees founded the Kodak Research Laboratories in 1912 it was decided to publish technical and scientific papers in established, peer reviewed, journals, rather than in a house organ. This lent immediate credibility to the papers but can make them hard to find. Much of Jones stuff appears in the _Journal of the Franklin Institute_ and also in the _Journal of the Optical Society of America_. For many years Kodak published summaries of scientific and technical literature in photography and related fields. There were several and I can cite them if you like. I don't know of any on-line source for these and the local libraries seem to have burried them in dead storage. They are helpful if one is doing serious research. -- --- Richard Knoppow Los Angeles, CA, USA |
#34
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New Freestyle Premium film ID?
"Richard Knoppow" wrote in message m... "John" wrote in message m... Thank you, Richard, for the cogent article. You have added to the body of knowledge by placing 'the curve', in this case, into something I can understand. See if you can find an old Kodak film handbook somewhere. These had a very good section on basic sensitometry in them explaining how the curves are generated and their relavance to the results. A similar article on print sensitometry appeared in the _Professional Printing_ book. These sometimes show up in used book stores or maybe eBay. There are good summaries of Jones' work in the classic _Theory of the Photographic Process_ C.E.K Mees, [...] Thank you. I have ordered the book from our state library. He wrote at least two more books which we have in our university library. I have some reading for the weekend! |
#35
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New Freestyle Premium film ID?
"Geoffrey S. Mendelson" wrote in message ... jch wrote: I surely agree with that last comment. I really liked Panatomic-X. It made very good B&W slides as well. Would buy a batch of 100 rolls to start with. Wonder what would be involved for Kodak to swing a Tri-X line around to make a run of Panatomic-X? Is it just the emulsion, or the emulsion /and/ the base? I expect that the base made today would be very different than the base used in Pan-X, but IMHO it would not matter. As long as it was a reasonably clear base (remember the Efke dark grey base?), it's to me the emulsion that would count. If it's really Lucky and not Kodak that made it for $2.00 a roll, lots of people would buy it. I fit were Kodak themselves and they had to charge $10 a roll, a lot less of them would be sold. Geoff. The same sort of base is used now and would make no difference to the emulsion. For reversal purposes it would be good to use a clear base with the under the emulsions type of anti-halation coating used for color film. The same equipment is used to coat different films at various times although I am sure the color line is not used for B&W. Film and paper is made in batches which are stored until needed. I have no idea how much of anything Kodak would find economical to make. I suspect these days it would be quite a bit. I wonder if they even remember how to make Panatomic-X. Note, 35mm negative B&W film is often coated on a support which has a pigment incorporated into it. The pigment is not removable in processing. The purpose of the pigment is to prevent "light-piping" or the tansverse transmission of light in the support. The reason for this in 35mm film is because one end of the film is often exposed to light when loading. Light can be conducted some way into the film causing fogging. This pigment is in addition to the anti-halation dye in the back coating. Anti-halation dye is to prevent reflection of light from the back surface of the support. Since the anti-halation coating is on the back of the film it can not act to prevent light-piping so both are needed. Many color films have an anti-halation coating on top of the support and under the emulsion. This is very effective in stopping both light-piping and halation since light conducted through the support can not reach the emulsion and no light can get through the coating to be reflected. I don't know why this system is not used for B&W film. It may be that the dye in the coating is removed or decolorized by one of the bleach steps where the dye in B&W is converted to a colorless form by the sulfite in both developer and fixing bath. -- --- Richard Knoppow Los Angeles, CA, USA |
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