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#1
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Basic question about contrast and gamma
Hi ng,
I'm just begining to dive into the photographic process, so my practical experience is still scarse. - I try to figure out the basics in a theoretical way first. In case of the negative/positive process (both b&w and color) there is the relationship of contrast and gamma and I simply can't get that into my head, no matter how much I read and think (maybe I read the wrong papers?!). One thing is that negative gamma and print gamma should combine to 1 if the contrast reproduction should match the scene (or 1,1/1,2 if you take into account that most people prefer a slightly exaggerated contrast of the midrange values). If thatīs true and the negative is developed to a kind of normal gamma of 0,6, print gamma must be about 1,7. In the case of b&w contrast must be balanced between the negative and the print if contrast is to be reproduced correct. So if the neg has, say, 1,3 log units it must be printed on a paper that is also capable of 1,3 log units. Not more, not less. So if you chose the paper grade according to the neg contrast, doesnīt that mean that you might have to chose a grade that leads to a gamma which doesnīt combine to 1 with the neg? Another thing that give me headaches in the same context are color prints. In the case of the color reproduction chain, the negative is normally developed to a gamma of about 0,6 and most papers have a gamma of more than 2,5, so the combined gamma is 1,5. - So how does that work? And why are color papers not available in different grades to balance with the neg contrast? You see Iīm kind of lost with those concepts and I really hope that someone could shed some light on them to show me whatīs going on. Monica Schulz |
#2
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Basic question about contrast and gamma
On Thu, 21 Feb 2008 15:33:56 -0800 (PST), Monica Schulz
wrote: And why are color papers not available in different grades to balance with the neg contrast? Color materials have a longer scale than black-&-white materials. Also there is in fact a significant difference in Kodak's RA4 Endura, Supra and Ultra papers. The Ultra is about 1 grade higher in contrast when compared with Supra which is about 1/2 grade higher than Endura Metallic. JD |
#3
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Basic question about contrast and gamma
On 2/21/2008 3:33 PM Monica Schulz spake thus:
I'm just begining to dive into the photographic process, so my practical experience is still scarse. - I try to figure out the basics in a theoretical way first. [snip theoretical worrying] If I might be so bold, it seems to me that if you're just starting out (at least in b&w photography, let's say), you're waaaay overthinking this one. Unless you absolutely need to feel comfortable with the theoretical basis of making negatives and prints, it would be far better to just go out and start shooting, assuming you have all the stuff you need. Shoot a bunch of rolls of film. Make contact sheets. Make some prints. Put them up and look at them. In other words, make mistakes. The more mistakes you make, the better (because you'll learn something from each one in a way that no theoretical discussion can match). What's that old saying? "My first 10,000 prints were garbage"? |
#4
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Basic question about contrast and gamma
"Monica Schulz" wrote in message ... Hi ng, I'm just begining to dive into the photographic process, so my practical experience is still scarse. - I try to figure out the basics in a theoretical way first. In case of the negative/positive process (both b&w and color) there is the relationship of contrast and gamma and I simply can't get that into my head, no matter how much I read and think (maybe I read the wrong papers?!). One thing is that negative gamma and print gamma should combine to 1 if the contrast reproduction should match the scene (or 1,1/1,2 if you take into account that most people prefer a slightly exaggerated contrast of the midrange values). If thatīs true and the negative is developed to a kind of normal gamma of 0,6, print gamma must be about 1,7. In the case of b&w contrast must be balanced between the negative and the print if contrast is to be reproduced correct. So if the neg has, say, 1,3 log units it must be printed on a paper that is also capable of 1,3 log units. Not more, not less. So if you chose the paper grade according to the neg contrast, doesnīt that mean that you might have to chose a grade that leads to a gamma which doesnīt combine to 1 with the neg? Another thing that give me headaches in the same context are color prints. In the case of the color reproduction chain, the negative is normally developed to a gamma of about 0,6 and most papers have a gamma of more than 2,5, so the combined gamma is 1,5. - So how does that work? And why are color papers not available in different grades to balance with the neg contrast? You see Iīm kind of lost with those concepts and I really hope that someone could shed some light on them to show me whatīs going on. Monica Schulz First of all "gamma" is a measure of contrast but is not often used now. Gamma is the slope of the straight line portion of the film or paper characteristic but, since most characteristics have only small straight line sections, or sometimes none another measure is used. This is average contrast or G-bar, the slope of a stright line drawn between a shadow and a highlight point on the curve. Kodak goes one step further and uses Contrast Index or CI which is the average contrast between two specified points on the curve. Since these are constant CI of different materials is easy to compare. You are right that the overall contrast of a print should be on the order of 1 but reflection prints are limited in the range of brightness they can reproduce so the unity contrast is usually true only of a range of mid tones, the shadows and highlights generally being compressed or simply going out of the range the paper can reproduce. Since transparencies don't rely on ambient light their range can be greater but the target contrast is still 1. However, the eye tends to judge "correct" reproduction of tones by the gray tones and, for the most part ignors deep shadows and bright highlights, so, if the mid-tone contrast is right the eye will see the print as being right even though the overall range of brightness that can be reproduced is substantially less than in the original scene, or, for that matter, less than what the negative will record. For many scenes of relatively high contrast a low contrast print that attempts to reproduce the full range of tones recorded on the negative will simply look flat. To some degree this can be compensated for by localized printing of details, either holding dark areas back to brighten them or printing in (burning in) highlights, but this must be done with skill or the manipulation will be obvious to the viewer. For black and white work, where there is little standardization, it is common to develop negatives to a CI of anywhere from about 0.4 to 0.7 depending somewhat on the kind of illumination used in printing. The printing light is important because of something called Callier effect. This is the effect on effective density caused by the relative diffusion or specularity of the light source. Because most B&W films have an image made up of descrete silver particals, which can scatter light, there is a fairly large Callier effect. Between a completely diffuse light source, like a color head or a cold light head, and the usual partly condensing head, the contrast difference is about one paper grade, the condenser head having the greater contrast. Note that Callier effect has no effect on the one reproduction scale other than overall contrast which can be completely compensated for by either a change in development time of the negative or choice of paper contrast. The method of compensation is a matter of convenience. Because color films, and here I include chromogenic B&W film, the image is made up of semi-transparent dye, there is very little Callier effect so the printing light makes little or no difference in effective contrast. While there is some choice in contrast of color printing materials it is much less than for B&W. This is because the eye is much more sensitive to variations in the contrast of color. If color is too low or too hight in contrast the effect is quite obvious. So, the contrast of color recording and printing materials is much more highly standardized than for B&W. In addition to this the three recording layers (some films have many more) for the three basic colors must have characteristics which match very closely if the color is to track for all brightnesses. Anothere way of putting this is that the neutrality of a gray scale must be constant. For this reason also there are strict limits on the contrast range of color materials. Color negative materials, which are intended to printed to make postives, have relatively low contrast and can be controlled to some extent in processing, as long at the gray scale tracking mentioned above remains acceptable. Color positive materials are generally meant for direct viewing and are much higher in contrast, generally slightly more than 1 to compensate for flare. There ARE some transparency materials intended for duplication or other printing which are lower in contrast bu they are rarely encountered by amateur photographers. Some color materials intended for direct reversal printing from positive transparencies offer a lower contrast grade for use when the usual grade is too contrasty. This was sometimes the case with Cibachrome/Ilfochrome. Another method of reducing contrast is masking, the use of a very low contrast negative image overlying the postive image to reduce the contrast. I think I am getting deeper into this than you want to go. You can find some elementary material on sensitometry, the name for the study of tone reproduction from film, on the Kodak site. Search for H-740. If you can't find it write me privately and I will send it to you. -- --- Richard Knoppow Los Angeles, CA, USA |
#5
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Basic question about contrast and gamma
On 22 Feb., 10:57, "Richard Knoppow" wrote:
snip You are right that the overall contrast of aprint should be on the order of 1 but reflection prints are limited in the range of brightness they can reproduce so the unity contrastis usually true only of a range of mid tones, the shadows and highlights generally being compressed or simply going out of the range the paper can reproduce. snip While there is some choice in contrast of colorprinting materials it is much less than for B&W. This is because the eye is much more sensitive to variations in the contrast of color. If color is too low or too hight in contrast the effect is quite obvious. So, the contrast of color recording and printing materials is much more highly standardized than for B&W. On the risk of beeing pushing I must still insist on the relationship between neg contrast and paper contrast. As we agree that both must balance to about 1 and color negative film is normally developd to about 0,6, the paper should show gamma of 1,7. But almost all color printing papers have gamma values far in excess of 2. So how does the steeply raised contrast of the print fit into your above statement? Does the contrast have to be raised that much so offset the unavoidable flare? If thatīs true paper contrast cannot be calculated by simply using the reciprocal of the neg. Maybe thatīs all what blocked me from recognizing the true connection?! Best regards! Monica Schulz |
#6
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Basic question about contrast and gamma
In article
, Monica Schulz wrote: On the risk of beeing pushing I must still insist on the relationship between neg contrast and paper contrast. Neg contrast= 1.45 max Paper contrast .85 Gamma is not a good indicator of true paper capability try .85 or if you must use film gamma subtract .20 from the bottom and top of the Characteristic curve your using, use a shoulder around 1.45 or less more like 1.25 As we agree that both must balance to about 1 and color negative film is normally developd to about 0,6, the paper should show gamma of 1,7. But almost all color printing papers have gamma values far in excess of 2. So how does the steeply raised contrast of the print fit into your above statement? Does the contrast have to be raised that much so offset the unavoidable flare? If thatīs true paper contrast cannot be calculated by simply using the reciprocal of the neg. Maybe thatīs all what blocked me from recognizing the true connection?! Best regards! Monica Schulz Flare should not be an issue when using a modern lens if using something older producing flare you should replace it with a modern Apo lens. -- Reality is a picture perfected and never looking back. |
#7
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Basic question about contrast and gamma
"Monica Schulz" wrote in message ... On 22 Feb., 10:57, "Richard Knoppow" wrote: snip You are right that the overall contrast of aprint should be on the order of 1 but reflection prints are limited in the range of brightness they can reproduce so the unity contrastis usually true only of a range of mid tones, the shadows and highlights generally being compressed or simply going out of the range the paper can reproduce. snip While there is some choice in contrast of colorprinting materials it is much less than for B&W. This is because the eye is much more sensitive to variations in the contrast of color. If color is too low or too hight in contrast the effect is quite obvious. So, the contrast of color recording and printing materials is much more highly standardized than for B&W. On the risk of beeing pushing I must still insist on the relationship between neg contrast and paper contrast. As we agree that both must balance to about 1 and color negative film is normally developd to about 0,6, the paper should show gamma of 1,7. But almost all color printing papers have gamma values far in excess of 2. So how does the steeply raised contrast of the print fit into your above statement? Does the contrast have to be raised that much so offset the unavoidable flare? If thatīs true paper contrast cannot be calculated by simply using the reciprocal of the neg. Maybe thatīs all what blocked me from recognizing the true connection?! Best regards! Monica Schulz I may be going over territory I covered in private e-mail but want to answer it here if possible. First of all I am not sure about your numbers for negative and paper contrast. 0.6 is probably about right for B&W negatives but I think color negs are somewhat lower, perhaps around 0.4. In any case gamma can be misleading because it applies only to the straight line portion of the film or paper characteristic. More modern methods of stating "effective" contrast are to use an average. This is done by drawing a straight line between the shadow density and highlight density and stating its slope. Average contrast, or G bar, does not have a fixed interval between highlights and shadows, the Kodak method called Conrtast Index does. G-bar and CI can be the same. In general gamma will not always agree with either. The idea is that multiplying negative contrast and positive contrast will give the contrast of the final print. Target contrast is usually somewhere around 1.0 but can be varied for a special effect or to compensate for something such as flare. Because the range of brightness that can be reproduced by a reflection print under normal illumination is, in general, less than the original scene some compression must take place. Compression means some part of the image is reproduced with lower contrast than the original. Since the eye tends to judge the "correctness" of an image based on its mid gray values or equivalent in color, the compression is typically in the shadows and highlights. Since the eye tends to compress these values in direct viewing the results are acceptable. If the mid tone contrast is too little, in an attempt to compress the entire tone scale, the results will look "flat" even though details are visible in shadows and highlights that are not in a more "normal" looking print. If the print is made to have higher contrast than the original scene it will still not look natural but may be acceptable if the original scene was very low in contrast. Since they eye is constantly compensating for brightness variations our impressions of original scene contast may not be too accurate. For instance, on a brightly lighted day with deep shadows the eye will adjust somewhat when looking into the shadows and see detail there that is not visible when looking at bright objects, but, because one's attention is not concentrated in the shadows then we are not generally aware of it. Film, OTOH, is fixed in its sensitivity. It can record a wide range of brightness but that range is centered according to the exposure its given. Then, when a reflection print is made, there is a further reduction of range since the reproducing medium is not capable of reproducing the full range of original scene brighnesses. Note that I am talking about the presentation to the eye. If one prints a long scale negative will onto a low contrast paper its possible to reproduce the full range of tones on the negative, but, the print will look very flat to the eye because the original brightness scale will be compressed. There are times when such prints are desirable, for instance for scientific or evidentiary purposes but, in general, they are not acceptable for pictorial purposes. Where there is not a clear reference for the eye distortion of contrast is acceptable. This is very true of B&W but somewhat less for color. If there is no psychological expectation the eye simply accepts what is presented. There is a great deal more to this and I have oversimplified to some extent but I think the main answer is that I think the gamma numbers you have are somewhat off. -- --- Richard Knoppow Los Angeles, CA, USA |
#8
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Basic question about contrast and gamma
"____" wrote in message ... In article , Monica Schulz wrote: On the risk of beeing pushing I must still insist on the relationship between neg contrast and paper contrast. Neg contrast= 1.45 max Paper contrast .85 Gamma is not a good indicator of true paper capability try .85 or if you must use film gamma subtract .20 from the bottom and top of the Characteristic curve your using, use a shoulder around 1.45 or less more like 1.25 Gamma is a measure of the slope of exposure vs: density of the straight line portion of a film or paper curve. Since nearly all film or paper characteristics are curved and have either small or no straight line portions gamma can be misleading about overall contrast. More modern measures are average contrast or G-bar and Contrast Index or CI. These two are very similar but CI has a specified interval of log exposure. In any case, whether one is using gamma or G-bar or CI the numbers combine by multiplying, not addition or subtraction. That is, a gamma of 0.5 will combine with a gamma of 2.0 to result in a gamma of 1.0. A gamma of 1.0 means that the contrast is the same as the original subject. As we agree that both must balance to about 1 and color negative film is normally developd to about 0,6, the paper should show gamma of 1,7. But almost all color printing papers have gamma values far in excess of 2. So how does the steeply raised contrast of the print fit into your above statement? Does the contrast have to be raised that much so offset the unavoidable flare? If thatīs true paper contrast cannot be calculated by simply using the reciprocal of the neg. Maybe thatīs all what blocked me from recognizing the true connection?! Best regards! Monica Schulz Flare should not be an issue when using a modern lens if using something older producing flare you should replace it with a modern Apo lens. Flare comes from other sources than just lenses, for instance light scattered in the optical path of a projector. Also, ambient light in a room where slides are being shown has an effect similar to flare. Projection transparencies and motion picture prints are often slighly higher than a gamma of 1.0 to compensate a bit. Another note: The prefix APO on most modern lenses in meaningless. It is permitted in German made lenses because of an error in the German DIN standard which mis-represents the term "apochromatic" and allows its use for lenses which are not so corrected. I think this has been changed but the terminology continues. A true apochromat is corrected to bring three colors to a common focus. The curve of longitudinal chrmomatic aberration will be S shaped crossing the zero line three times. Most lenses are achromats, that is they are corrected for two colors. The curve for an achromat will cross the zero line twice. More important than the number of zero crossings is the deviation from focus between the corrected points: the less the deviation the better the lens is corrected. Its possible to have an achromat that has better overall correction than an apochromat. In any case, nearly all of the current crop of lenses with APO as a prefix are NOT apochromats but are simply well corrected achromats. I think the term APO used for such lenses is misleading and should be eliminated. In any case, an APO lens, regardless of its true color correction, has no less flare than any other. Flare is mainly a function of the number of glass-air surfaces but other things can contribute such as reflections from the inside of the lens mount. Surface reflections from the glass can be controlled by anti-reflection coating. Such coatings have been known for over a century but were made economically practical only after WW-2. Modern coatings are complex and are effective over the entire range of colors of interest in photography. Modern complex lenses, especially zoom lenses, would be impossible without advanced coating techniques. Even single coated lenses have substantial reduction in flare and modern multiple coated lenses are nearly flare free. Probably the use of a modern lens would pretty much eliminate having to increase print gamma to compensate for flare but one might still want to compensate for other factors such as the viewing conditions. -- --- Richard Knoppow Los Angeles, CA, USA |
#9
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Basic question about contrast and gamma
On 24 Feb., 23:29, "Richard Knoppow" wrote:
Gamma is a measure of the slope of exposure vs: density of the straight line portion of a film or paper curve. Since nearly all film or paper characteristics are curved and have either small or no straight line portions gamma can be misleading about overall contrast. More modern measures are average contrast or G-bar and Contrast Index or CI. These two are very similar but CI has a specified interval of log exposure. * * *In any case, whether one is using gamma or G-bar or CI the numbers combine by multiplying, not addition or subtraction. That is, a gamma of 0.5 will combine with a gamma of 2.0 to result in a gamma of 1.0. A gamma of 1.0 means that the contrast is the same as the original subject. Uuh, now I see what you mean. You talked about the difference between gamma and contrast index before but I didnīt realize itīs importance - sorry for the delay! So using the contrast index method leads to other figures for the negative and the paper and they combine to about 1. So I finally got rid of this problem I had with the curves of these materials. - Thank you! Now there is only one point left from my starting question: Why is there not such a wide variaty of paper grades in color reproduction as there is in b&w? As far as I understand, in b&w we need this wide variaty to make the dark tones of a low contrast negative as black as the dark ones of a high contrast neg. Whatīs thze difference in color? Is it because a high density there really means a more saturated color and not so much a darker color? Best! Monica Schulz |
#10
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Basic question about contrast and gamma
Maybe I did not make myself clear enough in my last post. What I mean
is that there is no need for such a wide variaty of paper grades in color printing as there is in b&w because contrast canīt be manipulated that much in color as in b&w. I came thinking about that after reading another old thread so I will use the important words of the relevant post. To adjust contrast means to make the image brighter or darker. The only way to do that is to make the dyes thinner. This lets more light reflect off the paper. But as a dye gets, say, thinner it removes less of its anti-color until it finally disappears and no anti-color is removed. And the other way around if the dye gets thicker. So the scale doesnīt go from, say, a dark magenta to a light magenta but from dark magenta to white. Or from a saturated magenta to an unsaturated magenta. The digital process can compensate for this unwanted increase or decrease in saturation if contrast is adjusted in the luminosity channel of lab-mode. In this case neither hue nor saturation is beeing changed. There is a nice little example of that on http://www.cambridgeincolour.com/tut...hop-curves.htm. But we canīt do the same in analog color printing. It could well be that this is complete garbage as far as the subtractive mixing of the analog process is concerned and if so Iīd be happy if someone tells me so that I can get this idea out of my head and can think in another direction. Best regards! Monica Schulz |
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