RustyBug Offline Upload & Sell: On
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Gotcha,
Yeah, B&W processing is an art unto itself. Bob has done quite a bit of exploration in this area. One thing I've tried to discern about mono processing is the impact that light color has on mono. Light color impact on color seems a bit more "obvious" to most folks. But, quantity (exposure), color (rgb / cmy / ab) and (diffuse / specular) of light all combine to formulate the cumulative amount of AI=AR for the light that strikes our subject, vs. the light reflected off our subject that gets captured within the view of our lens.
Exposure - really equates to the volume of flow of photons (area, time, density of photons, speed of light). This gives us a volume of photons reaching our capture media (sensor / film)
Color (of light) - equates to the amount of energy being carried by said photons
Color (of subject) - equates to the amount of energy being reflected (vs. absorbed) by the subject.
Quality - equates to soft / diffuse vs. hard / specular which is indicative of the degree to which our light is being broadly scattered about or laser focused.
The combination of the two color pieces of the puzzle will determine how much is being reflected toward our lens. NOTE: shooting a light source such as the sun or sky means that it is a direct light source, thus no reflection, thus no absorption (i.e. loss of energy).
Depending on the color of the subject reflecting the light, we can get various returns on the amount of light directed toward our lens.
Using a scale of 1-10 (for simplicity), it looks something like this:
If our light source contains 10 units of full spectrum white light.
Direct source - 100% of 10 = 10
Specular reflection - 99.9% of 10 = 9.99
White reflection - 95% of 10 = 9.5
Gray reflection - 50% of 10 = 5
Black reflection - 5% of 10 = 0.5
Absence of reflection (black hole) - 0% of 10 = 0
But, when our SUBJECT is not of a neutral color, it will NOT RETURN all colors of the full spectrum. Thus, we get a further reduced return of our light, as a product of the necessary absorption that occurs subtracting some portion of our full spectrum light, so that we only see a portion of the full spectrum light that is reflected.
RGB - RG (Blue subject absorbing red & green) = B
RGB - GB (Red subject absorbing green & blue) = R
RGB - BR (Green subject absorbing blue & red) = G
Nothing really earth shattering there, but as we move to consider colored light where our LIGHT is NOT NEUTRAL, it can go something like this, whereas, if full spectrum white light is a composite of RGB components, then (say, for example) B only light does not contain R&G components:
B - RG (Blue subject absorbing red & green) = B
B - GB (Red subject absorbing green & blue) = 0
B - BR (Green subject absorbing blue & red) = 0
B - 0 (Neutral subject absorbing no colors, i.e. reflecting all RGB) = B
As such, we only get maximum return of light from a color light source where the color of light befalling onto our subject is being reflected (vs. absorbed) by the subject.
Rarely are we working in a pure blue light for instance, but the point is that as our color of light deviates away from full spectrum, white light containing all colors of the spectrum, then it has a restrictive component to where / how much the light can be reflected / returned.
Thus, in our pic ... it seems that somewhat blue light befalling on somewhat dark yellow-red (i.e. brown) skin, has very little chance of returning much light being reflected (i.e. it is mostly being absorbed). Conversely, if the light color was full spectrum (or yellow-red), then the amount of light to be returned would consequently be greater.
As to the point about channel separation ... comparing the blue channel to the red channel can help reveal where your light has been absorbed extensively, and is being reflected maximally. This can serve as an aid to understanding how one can have a proper (or even over-) exposure, yet being "robbed" of the light returning to be captured.
So ... (long as usual ) ... the point becomes that light color can still influence the tonal values in monochromatic images. Although, this is nothing really new here either (monochromatic film shooters used colored filters to exploit this dramatically).
The short version is S&P to taste as always, but I find it helpful to understand which areas of the image are receiving which / what light as part of my decision making process to making those S&P determinations ... particularly in mixed lighting situations, such as where deep between the buildings may be much cooler light than in other areas of the scene. But, just general "blue hour" or "golden hour" still has an influence on your monochromatic values. Just that this one struck me as an pertinent example of where it happens (i.e. WHY so dark) and we can be left scratchin' our heads even when we know we "nailed" the exposure.
Many times, it is more subtle, but this one seemed distinct enough, that it could make a respectable example for discussion.
Short answer, red stuff will look darker in blue light than it normally would in white (or red) light.
HTH
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