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#1
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Shooting Black & White with Digital
I'm interested in experimenting with Black and White prints. I have a
Canon 10D which (unlike the 20D) will take only in colour, so I need to convert afterwards. I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? ie, if you take a standard Ilford B&W film, is it capturing the luminosity of the image across all 3 RGB channels equally (analogous to desaturating a colour image in Photoshop), or is it insensitive to some parts of the spectrum (in which case what parts), or is it actually sensitive to any parts of the spectrum (eg towards the infra-red) which normal colour film (or a CCD sensor) is not? The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. Thanks for any comments on this. David |
#2
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David French wrote: I'm interested in experimenting with Black and White prints. I have a Canon 10D which (unlike the 20D) will take only in colour, so I need to convert afterwards. I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? ie, if you take a standard Ilford B&W film, is it capturing the luminosity of the image across all 3 RGB channels equally (analogous to desaturating a colour image in Photoshop), or is it insensitive to some parts of the spectrum (in which case what parts), or is it actually sensitive to any parts of the spectrum (eg towards the infra-red) which normal colour film (or a CCD sensor) is not? The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. Thanks for any comments on this. David Google for emulsion response curves. All the info you need. Colin |
#3
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David French wrote:
I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? Most film datasheets contain spectral sensitivity graphs, but the different manufacturers use different methods. Ilford displays wedge-spectrograms under 2850K light, while Kodak displays spectral sensitivity based on the amount of energy needed to produce a specific density. The results are difficult to compare, but it is safe to say that the spectral response of most Ilford films is not wildly unlike Tri-X. The wedge spectrograph method used by Ilford produces rather different looking results because 2850K tungsten light is rich in red and deficient in blue and the optical elements in the set-up absorb ultra-violet resulting in an apparent sharp drop-off in the UV which is not inherent in the films. See Kodak graphs: Tri-X Pan 400: http://www.kodak.com/global/en/profe...009_0506ac.gif Plus X Pan: http://www.kodak.com/global/en/profe...009_0431ac.gif T-Max 100 http://www.kodak.com/global/en/profe...002_0547ac.gif The curve for Tri-x is fairly typical of the classic panchromatic films. The UV response almost certainly remains fairly strong out to 230nm where the gelatine in the film absorbs UV, even though Kodak only shows it down to 300nm. The response of the film is stronger in the blue/violet region than it is in the yellow/green or orange/red. Human vision has its strongest response to the yellow green part of the spectrum, so typical pan films show blues and violets as much lighter than they appear to the human vision, and orange to medium-red as somewhat lighter than the eye sees them. The curve for Plus-x shows that it has a UV filter layer built in, which is a fairly modern feature in a black and white film, but it also has stronger response to the violet-end than Tri-X.. The Tmax films have a more even spectral response than the older style films. but still show extra sensitivity at the violet end of the spectrum. Almost all modern panchromatic films (except for those designated as extended-red) have a deep red response that drops like a stone at around 650nm. The reason for this is that the combination of extended-red film and tungsten light (which is rich in the longest wavelengths of red) can give some rather unnatural looking effects. Many photographers use a medium yellow (Wratten 8 or K2) filter in daylight to correct for the excess response to blue and violet. The yellow-green (Wratten 11 or X1) is also popular: it corrects both the blue-violet end and the red end of the spectrum. The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. I think that mucking about with mixing the channels, perhaps even adjusting the contrast on each channel is probably your best strategy. There is no real way of duplicating the spectral response with the three colour channels, but there is nothing particularly "correct" about the way panchromatic films render colours. Panchromatic film response is based on the intersection of what is technically possible with good overall sensitivity and with what appears pleasing. With b&w conversion from digital, you only have to worry about what is pleasing. Peter. --- |
#4
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David French wrote:
I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? Most film datasheets contain spectral sensitivity graphs, but the different manufacturers use different methods. Ilford displays wedge-spectrograms under 2850K light, while Kodak displays spectral sensitivity based on the amount of energy needed to produce a specific density. The results are difficult to compare, but it is safe to say that the spectral response of most Ilford films is not wildly unlike Tri-X. The wedge spectrograph method used by Ilford produces rather different looking results because 2850K tungsten light is rich in red and deficient in blue and the optical elements in the set-up absorb ultra-violet resulting in an apparent sharp drop-off in the UV which is not inherent in the films. See Kodak graphs: Tri-X Pan 400: http://www.kodak.com/global/en/profe...009_0506ac.gif Plus X Pan: http://www.kodak.com/global/en/profe...009_0431ac.gif T-Max 100 http://www.kodak.com/global/en/profe...002_0547ac.gif The curve for Tri-x is fairly typical of the classic panchromatic films. The UV response almost certainly remains fairly strong out to 230nm where the gelatine in the film absorbs UV, even though Kodak only shows it down to 300nm. The response of the film is stronger in the blue/violet region than it is in the yellow/green or orange/red. Human vision has its strongest response to the yellow green part of the spectrum, so typical pan films show blues and violets as much lighter than they appear to the human vision, and orange to medium-red as somewhat lighter than the eye sees them. The curve for Plus-x shows that it has a UV filter layer built in, which is a fairly modern feature in a black and white film, but it also has stronger response to the violet-end than Tri-X.. The Tmax films have a more even spectral response than the older style films. but still show extra sensitivity at the violet end of the spectrum. Almost all modern panchromatic films (except for those designated as extended-red) have a deep red response that drops like a stone at around 650nm. The reason for this is that the combination of extended-red film and tungsten light (which is rich in the longest wavelengths of red) can give some rather unnatural looking effects. Many photographers use a medium yellow (Wratten 8 or K2) filter in daylight to correct for the excess response to blue and violet. The yellow-green (Wratten 11 or X1) is also popular: it corrects both the blue-violet end and the red end of the spectrum. The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. I think that mucking about with mixing the channels, perhaps even adjusting the contrast on each channel is probably your best strategy. There is no real way of duplicating the spectral response with the three colour channels, but there is nothing particularly "correct" about the way panchromatic films render colours. Panchromatic film response is based on the intersection of what is technically possible with good overall sensitivity and with what appears pleasing. With b&w conversion from digital, you only have to worry about what is pleasing. Peter. --- |
#5
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Even though my camera does have a black and white mode, I prefer to
take all shots in color mode, and greyscale them myself in software. That way, I can make some hue corrections before greyscaling, giving differences in the resulting black and white image. You can, as you say, study the responses others have suggested a source for, but eventually you will want to adjust away anyway, since what you may envision as what the scene should look like will vary with light color temp and whatever responses either your camera or software default to. |
#6
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Even though my camera does have a black and white mode, I prefer to
take all shots in color mode, and greyscale them myself in software. That way, I can make some hue corrections before greyscaling, giving differences in the resulting black and white image. You can, as you say, study the responses others have suggested a source for, but eventually you will want to adjust away anyway, since what you may envision as what the scene should look like will vary with light color temp and whatever responses either your camera or software default to. |
#7
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I don't think I can add much to this very informative response, or to
your conclusions. However I will support them with the additional observation that, in addition to variations in films and lens filters, there is also variation in printing papers, enlarging filters, and printing techniques that affects how we perceive black and white photos. So pursuing the "authentic" black and white photo look can lead us in almost any direction we like. The original poster's desire to achieve objectivity "rather than just mess around with Channel Mixers until I get something I like" therefore seems doomed to subjectivity in more ways than one. Alan |
#8
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I don't think I can add much to this very informative response, or to
your conclusions. However I will support them with the additional observation that, in addition to variations in films and lens filters, there is also variation in printing papers, enlarging filters, and printing techniques that affects how we perceive black and white photos. So pursuing the "authentic" black and white photo look can lead us in almost any direction we like. The original poster's desire to achieve objectivity "rather than just mess around with Channel Mixers until I get something I like" therefore seems doomed to subjectivity in more ways than one. Alan |
#9
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David French wrote:
I'm interested in experimenting with Black and White prints. I have a Canon 10D which (unlike the 20D) will take only in colour, so I need to convert afterwards. I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? A photographic emulsion made up of only silver halides in a carrier has little or no response to the longer wavelengths of light. Adding certain dyes to the emulsion sensitizes the emulsion to the other wavelengths. There is more on htis at http://www.cheresources.com/photochem.shtml,, particularly in the section headed "Color Sensitizing". ie, if you take a standard Ilford B&W film, is it capturing the luminosity of the image across all 3 RGB channels equally (analogous to desaturating a colour image in Photoshop), or is it insensitive to some parts of the spectrum (in which case what parts), or is it actually sensitive to any parts of the spectrum (eg towards the infra-red) which normal colour film (or a CCD sensor) is not? It depends. Infrared emulsions are sensitive to wavelengths longer than the human eye can see. Panchromatic emulsions aim for a uniform response across the range of colors. The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. The behavior of each B&W film is tailored for a purpose. I think the same approach is appropriate for digital editing of images. Thanks for any comments on this. David |
#10
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David French wrote:
I'm interested in experimenting with Black and White prints. I have a Canon 10D which (unlike the 20D) will take only in colour, so I need to convert afterwards. I'm familiar with the use of Channel Mixers in Photoshop to convert the RGB channels to grey in varying proportions. What I'm interested in finding out is, how does a typical black and white film record the luminosity of the various colour channels it is receiving? A photographic emulsion made up of only silver halides in a carrier has little or no response to the longer wavelengths of light. Adding certain dyes to the emulsion sensitizes the emulsion to the other wavelengths. There is more on htis at http://www.cheresources.com/photochem.shtml,, particularly in the section headed "Color Sensitizing". ie, if you take a standard Ilford B&W film, is it capturing the luminosity of the image across all 3 RGB channels equally (analogous to desaturating a colour image in Photoshop), or is it insensitive to some parts of the spectrum (in which case what parts), or is it actually sensitive to any parts of the spectrum (eg towards the infra-red) which normal colour film (or a CCD sensor) is not? It depends. Infrared emulsions are sensitive to wavelengths longer than the human eye can see. Panchromatic emulsions aim for a uniform response across the range of colors. The reason I ask is, I'd like to understand how best to model the behaviour of a genuine B&W film, rather than just mess around with Channel Mixers until I get something I like. The behavior of each B&W film is tailored for a purpose. I think the same approach is appropriate for digital editing of images. Thanks for any comments on this. David |
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