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More neutral than almost all other gray cards

Cem_Usakligil

Well-known member
Hi, Cem,

Well, I wasn't qualified to denounce them, so I thought I would (via satire, as is my habit, and I understand how that worries you) illuminate them, hoping that the more qualified here could actually nail them (as you have already).

Best regards,

Doug
Hi Doug,

Satire is good, satire is excellent. It has the unfortunate property of being totally misunderstood by some people. Like, for a second there, I thought that you've actually called me a qualified person!

Live long and prosper Doug Kerr :).

Cheers,
 

Doug Kerr

Well-known member
Hey, Jamie

Hi, Jamie,

All carping about your observations about CMY(K) and RGB aside, your piece on Drew's forum on the use of the non-neutral patches on the ColorRight MAX for making attractive shifts in skin tone is very useful to me.

Your observations about the problems caused by two little red (ooh, sorry, too much cyan) in the images seem to make sense, looking at your examples. And of course, all the non-neutral patches on the ColorRight MAX shift chromaticity in a direction that is away from magenta and even more so, away from cyan.

I am still trying to figure out Drew's aphorism, "red is the enemy of good skin tone."

Maybe you can help me understand it.

Best regards,

Doug
 

Drew Strickland

New member
This thread is a Riot!

Can we take this show on the road?

Doug, Drew, Andrew, Bart, Jamie and everyone else. It's an all swim event.

We can all enhance our characters even further. Doug is the wiseass. Drew is the all too serious simpleton. Andrew is the wise wizard. Bart is the smart, but curt European and Jamie will be the polite Canadian.

I'm sure someone wandering into this thread at this point has no idea which way is up, but it sure is entertaining.
 

Doug Kerr

Well-known member
My conclusions

Well, I was "off the air" for about 24 hours as a result of a miscue by an AT&T technician working nearby. (By the way, my conjecture as to what was done wrong proved to be right on the money!)

So I have a lot of catching up to do on the matter of CMY(K) and RGB outlooks on recipes.

But I am going to jump all that and report my conclusions on the matter of the ColorRight MAX (which, as those of you with very good memories might recall, was where this thread started).

1. It certainly appears that, by making an arbitrary departure from "theoretical" white balance color correction, according to one of the six "offset vectors" provided by the ColorRight MAX, we can very often conveniently attain a "more attractive" skin rendering than with the "theoretical" white balance color correction.

2. To accept this, we need not have recourse to "recipes" expressed in any particular color space ("M before Y, except after C").

3. Thus, questionable notions of which color spaces are the best vehicles for expressing empirical recipes of chromaticity shift in the interest of "attractive skin tone", and questionable slogans that strive to capsulize the "recipe" outlook ("red is the enemy of good skin tone"), are counterproductive in promoting the benefits of this pragmatic technique. In other words, "Quit while you're ahead".

4. Don't piss off scientists and engineers.

To put it more succinctly: Drew's product is much more useful than his dogma.

Drew: feel free to quote me on that last (but only in its entirety).

Best regards,

Doug
 

James Roberts

New member
Hi Doug,

These statements are so fundamentally wrong that referring to them even jokingly constitutes a risk. Can you imagine that some casual and uninformed reader might think that they might be correct since you are fascinated by them? Oh, the horror of it! ;-)

Cheers,


I guess you guys haven't been actually reading this thread. Sorry. You should catch up on the conversation Andrew and I have been having on this topic.

As for my method, it works for me still, and while I'm discovering ways to render "RGB percentages" to apply the proper ratios in applications other than lightroom (which I don't use), I will stand by the usefulnesss of my original statements... however unenlightened about RGB correction I was at the time (though I always said it was possible).

I'll thank Andrew once again for clarifying the RGB way to do this, and I agree it has its advantges.

So I'm always learning, in other words. But is the technique fundamentally wrong? Well, as Joyce says, "nes, yo" :)

Sorry Cem, I remain "un-nailed" :)
 

James Roberts

New member
{stuff snipped}

4. Don't piss off scientists and engineers.

To put it more succinctly: Drew's product is much more useful than his dogma.

Drew: feel free to quote me on that last (but only in its entirety).

Best regards,

Doug

There is no dogma here, only practical advice that you can ignore, from my perspective at your own risk.

So let's go over the issue once more:

1) There is a tradition (oh no--an ART!) of printing good skin tones that is bounded by culture and practice. It is not unmethodical, or filled with "hand-waving" but has to do with the way people and cultures understand what constitutes good skin tones.

Customers--being part of culture in general--can often tell the difference between good and bad skin tones even if they can't articulate it. More and more photographers these days have no clue whatsoever what constitutes good skin tones or how to print them. Why? Because huge film companies like Kodak and Fuji--and good labs--used to take care of this problem for them.

Traditionally--and before computers and before colour management--these corrections for photography colour prints were made with Cyan, Magenta and Yellow filters in an optical system. Perhaps you remember them from your own colour film printing?

As such, and having had experience in that area, I still think in Cyan, Magenta and Yellow terms when it comes to skin.

2) Enter computers. And colour correction. Neutrals can now be measured very easily (in RGB terms or LAB terms). And so can skin tone ratios. That people don't get this, or think it can't work, is fine with me. Within the boundaries I've described in this thread, the techniques do work, and the skin tone ratios still obtain for fine printing.

Now, Andrew says the better, more reliable way, to measure these ratios would be in RGB, and Lightroom presents an easy way to measure ratios (not numbers) whereas my applications dont... I have to calculate the ratio from the measurement. In my preferred applications, CMY is still easier to measure, if, I admit, perhaps less reliable, even though in my preferred workflow, with my applications, where the gammas of two RGBs are different (ProPhoto and sRGB), we get some pretty wild variation.

So: advantage Lightroom for an RGB measurement workflow for skin.

But nothing Andrew wrote discounts the idea of this fundamental skin tone correction.

3) Which begs the question: a correction from what? From colorimetrically neutral white balance, of course, or as close as you can get with any number of devices. There are many times when neutral (NOT THEORETICAL) white balance is perceptually wrong.

Skin tones are almost always off, even with neutral WB.

Now, in THEORY, if digital cameras had the same perceptual mechanics as our eyes (or even as other devices in the chain, such as printers), then we could 1) take a neutral WB and 2) get the 'right' colour all the time.

Unfortunately, this is not the case.

4) Where Michael Tapes and I parted company--and I respect Michael immensely--is that a profile system in the RAW converter should take care of the shifts to get good skin tones from a neutral WB.

I have found this to be practically impossible, given luminance variation on the job and given the range of skin details. Some profiles are very, very good in one or two lighting conditions, but they don't work for all (and here I'm talking about ICC profiles--I simply don't know enough about Adobe's new profiling method).

5) Enter ColorRight Max. Whether you believe or not that CMY or RGB is the best way to measure skin tones, this device works. I was SO ready to say it wouldn't, but it does, and it does under conditions where, by my way of thinking, it shouldn't.

NOW--it's not a perfect devce, and this is important. Without some kind of measureable ratio or guide, you will mess up even with the device. You need to understand the art of skin tone rendering to understand how to do it. No whibal or ColorRight or GM chart alone will do it. But the ColorRight gets closest, in my opinion, and is therefore a workflow boon and well worth the money.

So there we are. There is no dogma here.

However, in my experience, and in the experience of many, many other photographers who are used to film, digital camera systems with neutral WB regularly produce skin that looks dead (too cyan or blue) sunburned (wayyy too red or magenta) or, especially in African American skin, too Yellow (too much green and not enough blue).

So Drew quoting Margulis, probably, on red being a problem for digicams, is quite fundamentally correct, in my view as well. They *are* too red sensitive, most of the time; IR sensitivity only exacerbates the problem.

All in my opinion, Doug. I think you should post more shots, personally... you too, Cem :)

YMMV.
 

Doug Kerr

Well-known member
Hi, Jamie,

. . .while I'm discovering ways to render "RGB percentages". . .

One problem we often have in this area is the matter of "percentages. It's like the old "percent" vs "percentage points" in talking about changes in statistical measures. Just to give an example, suppose that in one year 36% of all building permits were not thoroughly reviewed. But now we hear that, as a result of a increased vigilance by the Building Permit office, that number has fallen by 33%. What, to 3%? Or to 24%?

Many applications report the coordinates C, M, Y, and K on a scale of 0-255. Some report them on a scale of 0-1.0. Some report them on a scale of 0-100%. Usually, what is meant is fairly clear from the context.

Then, we have people who talk about "C-M-Y percentages", in which case they may be talking about the coordinates themselves, but are more likely talking about the ratios between them.

For example, one place we might hear it said that "C should perhaps be no more than 25% of M". And, said fully like that, it is not ambiguous. For example, if (in a setting where C, M, and Y are reported on a "percent" scale) and the value of M is 48%, then the guideline would suggest a value for C of no more than 24%. So far so good.

But when when we abstractly hear about "guidelines for percentages", which is meant? Guidelines on the values (which might be expressed in percent terms), or on the ratios.

My only point is to urge care in stating whatever it is that you wish to present. A really good name for ratio is "ratio".

Best regards,

Doug
 

James Roberts

New member
{snipped}My only point is to urge care in stating whatever it is that you wish to present. A really good name for ratio is "ratio".

Doug--couldn't agree more, actually.

What I'm trying to get at is some kind of RGB independence from the actual numbers 0-255 in deriving the ratio across 3 channels.

So I like exactly what Andrew showed in LR... the percentage of R, G and B elements present at a sample point. This is akin to the way PS (or C1) shows CMY values--as percentages.

It's quite easy to derive a ratio from percentages across the channels. Not as easy, for me at least and in my head, to derive it from a scale of 0-255
 

James Roberts

New member
Hi, Jamie,


What does "neutral white balance" mean?

Best regards,

Doug


To my way of thinking, it means an unshifted WB--or whatever people mean when they say they've created a colorimetrically "correct" WB from a near-neutral reference, like a WhiBal. ColorRight or GM card.
 

Doug Kerr

Well-known member
Hi, Jamie,

To my way of thinking, it means an unshifted WB--or whatever people mean when they say they've created a colorimetrically "correct" WB from a near-neutral reference, like a WhiBal. ColorRight or GM card.

Well, that's what I thought. Probably not a bad "colloquial" term.

Thanks.

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jamie,

When you say:

"CMY ratios are a perfect way to measure skin tone because they're RGB independent.

. . .

Again, CMY(K) values are nice to use to measure because regardless of your RGB colourspace, the CMY values (or LAB values) will be the same.
"

Do you mean that if we properly convert to some specific CMY color space from an RGB color space, the interpretation of colors in the CMY color space does not depend on which RGB color space was the earlier host of the image?

Or do you mean that if we have made the skin colors in an image, interpreted in a CMY color space that relates to the printing process we contemplate using, follow empirically-determined guidelines of CMY ratios for "pleasing" skin color, then the image as printed accordingly will exhibit "pleasing" skin colors, regardless of what CMY color space that is?

Or maybe both?

Both would seems sensible to me.

Best regards,

Doug
 

Doug Kerr

Well-known member
Well, the 2000th visitor to this thread . . .

. . . wins a free donut.

Asher, if you'll be so kind as to certify who that is, I'll be glad to mail it to him.

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jamie,
It's quite easy to derive a ratio from percentages across the channels. Not as easy, for me at least and in my head, to derive it from a scale of 0-255

Indeed.

Of course, all the math is done with scales of 0-1, and to express that with a unit of "1/100" (1%, if you will) makes it handier but just as clear.

Best regards,

Doug
 

Doug Kerr

Well-known member
That pesky red

I think I've got it, thanks to Jamie:

Red - having too much or to little is bad for pleasing skin tone.

Who's have thunk it!

Best regards,

Doug
 
I think I've got it, thanks to Jamie:

Red - having too much or to little is bad for pleasing skin tone.

Who's have thunk it!

And when you think you've got it, allow me to raise a related question.

Who decided what pleasing skin tone actually is?

Was there a majority vote, which I apparently missed. Was there a panel that had different choices put in front of them, and they consistently rated some deviation from accurate as more pleasing? Was it decided because some images were reordered more due to some deviant color reproduction? Were there ethnic preferences that differ between ethnicities? Does solid research exist about the issue, where is it published? Other sources?

Just raising a serious question (sorry for that),
Bart
 

James Roberts

New member
And when you think you've got it, allow me to raise a related question.

Who decided what pleasing skin tone actually is?

Was there a majority vote, which I apparently missed. Was there a panel that had different choices put in front of them, and they consistently rated some deviation from accurate as more pleasing? Was it decided because some images were reordered more due to some deviant color reproduction? Were there ethnic preferences that differ between ethnicities? Does solid research exist about the issue, where is it published? Other sources?

Just raising a serious question (sorry for that),
Bart

Bart--those are excellent serious questions. And I'll try to answer them the best way that *I* know how... there are others, no doubt, who could answer them more fully.

It's not a "majority vote" exactly, but years of printing practice in the photographic, fine art and publishing fields.

Perception doesn't vary by ethnicity, per se, to my knowledge though I daresay it could for all I know, but I know it does by culture. Lee Varis, an accomplished photographer and the author of a book called "Skin"

http://www.amazon.com/Skin-Complete-Digitally-Photographing-Retouching/dp/047004733X

has a great chapter on two mistakes he made as an American with creating (not recording) appropriate or pleasing skin tones in prints from digital cameras.

The first was in shipping a bunch of files to be corrected in China, and when they came back they were extremely pale and cyanotic looking to his eyes--and to his clients. Evidently, extreme paleness is highly prized (or was when the story happened, perhaps) and so that's how they corrected.

Another story he tells is how he had an Indian client, and in a similar fashion, he found his clients wanted to look a bit lighter as a mark of "higher class."

Here in NA, for a long time, a nice tan on caucasian subjects has signified health and wealth (leisure). I read recently that 100 years ago, a tan was a sign of working outside and associated with lower classes :)

African Americans often react to skin that appears too yellow for their taste, which is a common occurence again, with the way the "digital system" records skin tones. FWIW, it's common for me to book clients based on how I treat skin in prints, and just a couple of weeks ago I had an African Canadian hire me based on my appreciation of skin tones in printing. This is not uncommon.

So this appears to be just an art, and craft, but like all arts, it has its technological underpinnings. For many of us, for many years, the science came from people at Kodak and Fuji and Afga and Ciba. They created films that--in the right light and with the right processing--could guarantee good skin tones. And photographers got used to that...

In fact, in some ways with film the photographer's job was to control or create the right light for the film. Extreme examples of control for colour's sake would fall to the movie industry and things like the Technicolor process, which required experts on set with certain colour paints, fabrics, etc... all to ensure a certain look from the final result.

Even on film, though, certain films had reputations for different ways to reproduce colour. Personally, for wedding skin tones, where I want something elegant and realistic (in the lighting too) I would never have used something that looked like Velvia :)

Now--enter digital sensors. In some ways, the collective memory of film results vanish nearly overnight! Some people shooting today have never seen a fine colour print, and few new photographers work with an established lab. Some labs even, can't make the transition from film to digital.

You yourself said they're more prone to IR (which is true) and overall, they react to light differently than film.

But there's no going back. As photographers, we can now work with or provide every different kind of light--the choices are liberating! But we now are our own perceptual colour experts as well.

There are days when I long to just throw my VPS III or Portra at the lab and get fabulous prints back :) Sigh.

So the tradition of good skin tones, including the ratios of colour and correction we've been mentining in this thread, go back a long way.

Do they change over time? Yes. Especially when I read European fashion magazines, they tend to balance far more cyan (sorry--very little red and a lot more blue) in their skin.

It looks "dead" to me, but obviously some people like it, and like the "otherworldly" effect (in truth it looks almost like a subtle cross=process to me, which was used a lot in fashion advertising for effect).

And over time, I've noticed a very pronounced and solid "brown" coming into NA fashion magazines (Vanity Fair). Annie L's work., in particular, is very consistent as printed. This surprised me--even her pix of Queen Elizabeth II have a more definite brown tone (less magenta in the mix) than I expected. I don't expect her to look deathly pale or anything, but also not like she lives in Carmel :)

So the ratios for skin tones are a way of describing a historical tradition in representing skin, but like many things about "taste" are less subjective than you might think.

People really do notice if their skin displeases them, or if you make their uncle look drunk because he has rosacea and that's what the frigging infra-red sensitive digital camera actually "made worse" with a "neutral" white balance!

So you know the old expression ars longa, vita brevis? It applies here, I think.

If anyone has any better information, I'd love to hear it.

I know many of the best photographers have workflows to painstakingly reproduce good skin tones, and a lot are loathe to tell their "secrets"; the whole point of something like the ColorRight is to make that process faster and more consistent, and it does a very good job of that.
 

James Roberts

New member
{snipped}

Do you mean that if we properly convert to some specific CMY color space from an RGB color space, the interpretation of colors in the CMY color space does not depend on which RGB color space was the earlier host of the image?

Or do you mean that if we have made the skin colors in an image, interpreted in a CMY color space that relates to the printing process we contemplate using, follow empirically-determined guidelines of CMY ratios for "pleasing" skin color, then the image as printed accordingly will exhibit "pleasing" skin colors, regardless of what CMY color space that is?

{snipped}

Hey Doug--

Sorry for the confusion, I mean both, but with caveats...

First, I meant that for a given CMYK default colorspace, the ratios wont change as wildly in the way they appear to if you use different RGB spaces.

Andrew Rodney rightly pointed out that the variance I was seeing was not due to RGB per se, but rather the gamma difference between my two most commonly used RGB spaces.

For my applications / software, as I've said earlier, I can't easily see RGB percentages, either, though you can in Lightroom.

Secondly, as long as you're not doing something too wild with the CMYK color space, then yes--if you adjust the skin to follow the ratios, then you will end up with much superior skin tones (assuming, of course, that your print process is controlled / colour managed).

Andrew also pointed out that their *are* many wild CMYK colourspaces (though I use the boring US Web defaults from PS CS3 / 4), and so again, you could use the RGB as a guide to the right skin tone ratios (and that Lightroom makes that easy... and so, it appears, does the ColorRight Max).
 
Bart--those are excellent serious questions. And I'll try to answer them the best way that *I* know how... there are others, no doubt, who could answer them more fully.

Hi Jamie, thanks.

This is a useful startingpoint, and I'd love it if some others with references/sources could add their view/data on the matter.

Pleasing is still a bit too much of a moving target to me, so for the time being I still perfer to start with a known baseline (the calibrated camera/raw converter's known response), and adjust from there.

Bart
(apparently a curt European to some)
 

Doug Kerr

Well-known member
Hi, Jaimie,

So you know the old expression ars longa, vita brevis? It applies here, I think.

Well, said.

I find it instructive to consider a popular translation of the fuller aphorism (he labeled it exactly that) from Hippocrates:

"Life is short, the art long [the inference being "long to master"], opportunity fleeting, experience misleading, judgment difficult. The physician must not only be prepared to do what is right himself, but also to make the patient, the attendants, and externals cooperate."

Unfortunately, many draw the wrong inference from the short citation, often something like "art lasts but artists (maybe even clients) don't".

So, do you think "first, do no wrong" (perhaps the most familiar quotation from that philosopher) means to apply colorimetrically-correct white balance color correction first?

Best regards,

Doug
 

James Roberts

New member
{snipped}
So, do you think "first, do no wrong" (perhaps the most familiar quotation from that philosopher) means to apply colorimetrically-correct white balance color correction first?

LOL!!

Actually Doug--my "first do no harm" would be not to make the patient (customer) look worse due to colorimetrically correct white balance than they should!

As Jack Flesher implied in this thread (before it became the other thread?) with portraits it's not so much about a caring about a shift in clothing or something as getting the expression and skin right.
 

Doug Kerr

Well-known member
Hi, Jamie,

As Jack Flesher implied in this thread (before it became the other thread?) with portraits it's not so much about a caring about a shift in clothing or something as getting the expression and skin right.

We'll be looking forward to a new plugin for image editors to provide improvement in expression!

All kidding aside, I have often needed to "turn that frown upside down" and find that I am not very good at it, not having any skills as an illustrator (even my stick figures don't look like stick figures of humans).

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jamie,

. . .or if you make their uncle look drunk because he has rosacea and that's what the frigging infra-red sensitive digital camera actually "made worse" with a "neutral" white balance!

Is the phenomenon there that the skin has a fairly high reflectance to the near infra-red and the camera is sensitive to the reflected IR, or is it that the skin fluoresces under the near IR and emits long-wavelength visible light as the result?

Best regards,

Doug
 

Jack_Flesher

New member
Is the phenomenon there that the skin has a fairly high reflectance to the near infra-red and the camera is sensitive to the reflected IR, or is it that the skin fluoresces under the near IR and emits long-wavelength visible light as the result?

Neither -- it's the fact the heat signature of warm blood coursing close to the skin is passed by (and thus over-excites) the red channel via the bayer matrix, converting normally invisible red erroneously into a spectrally visible red in the final image...

And in the spirit of turning the frowns upside down by using a popular brand of satire here on OPF, I guess something like that is what you meant to ask.

Best regards,
 
Neither -- it's the fact the heat signature of the blood coursing close to the skin is passed by (and thus over-excites) the red channel in the bayer matrix, converting normally invisible red erroneously into spectrally visible red in the final image... But I guess that's what you meant to ask.

Sorry to disagree with that assumption, it's a common misunderstanding. For heat to register on film or a sensor at near-IR frequencies, we probably need object emission temperatures like 600 degrees Celcius temperature (1112 Fahrenheit, or 873 Kelvin) or more. These are rough estimates, I haven't done these calculations since 30 years ago and silicon was not used as a sensor as much then. For those interested, one needs to calculate the energy of a blackbody emission at the relevant wavelengths and factor in the sensitivity of the sensor at those wavelengths. I'll leave the exercise for others.

Silicon sensors have IR sensitivity to about 1000nm wavelengths. They become transparent at longer wavelengths. Therefore they are capable of directly recording lower temperatures than mentioned, but most sensor arrays also have an IR filter to block that.

We have to cope with different efficiencies of those IR filters. That is what constitutes a variable between camera's, their blocking efficiency of near IR reflection (usually reflection, not emission).

Doug raises a technically good point (although not that important for our subject, IMHO); is the near IR skin reflection caused by direct reflection, or fluorescence from another wavelength? It would be relevant if there are fluorescence differences between skin pigmentation. Research seems lacking sofar, maybe I need to dig further myself.

For our topic of skin color, it's the cut-off frequency and the steepness of the transition that's important. Another important factor is that the Bayer CFA filters are transparent to IR, and thus all R/G/B filtered sensels register residual IR as if it were R/G/B light.

Also, as I've shown in a diagram, human skin reflects longer wavelengths more than shorter wavelengths (in the bandpass region we're interested anyway). This makes the near-IR (blocking) efficiency a factor with variable color temperatures.

Cheers,
Bart
 

Doug Kerr

Well-known member
Hi, Bart,

For our topic of skin color, it's the cut-off frequency and the steepness of the transition that's important. Another important factor is that the Bayer CFA filters are transparent to IR, and thus all R/G/B filtered sensels register residual IR as if it were R/G/B light.

So does that mean it might be advantageous to provide, on the lens, an IR cut filter with a lower-wavelength cutoff (farther into the visible spectrum) and steeper rolloff than that in the camera? If so, is something suitable readily available?

Best regards,

Doug
 

Doug Kerr

Well-known member
Chrmaticity offsets of the ColorRight MAX

This information was originally in a separate thread, but recent discussions here set a nice context for it, so I have put this update here.

This figure shows, on the familiar du'dv' chromaticity difference chart, the offsets from "colorimetrically ideal" white balance color correction that would be given by the use of the six "skin tone control" patches on the ColorRight MAX.

colorright_max_04.gif


These have not been taken from measurement of an actual specimen unit, nor from any basic data provided by the manufacturer. Rather, they have been inferred by analysis of the chromaticity of the various sectors of the target as recorded in one of the raw files Drew has provided for us to play with. I make no claim that they represent the authentic chromaticities of actual current product devices.

(Earlier-published charts were based on the "product photos" on the ColorRight Web site, but we worried that perhaps these might have been taken of reproduction units with different characteristics.)

The offsets are shown as relative to the chromaticities the image would have if given white balance color correction predicated on the "neutral" sectors of the tool.

Remember that these points show the chromaticity shifts that would occur by using the corresponding sectors for a "pseudo white balance" operation during raw development. They are not the relative chromaticities of the tool sectors themselves (but are in fact precisely their opposites).

All these shifts include movement in the "more yellow" direction (in different amounts), accompanied by movement in the "more red" ("less cyan") direction (in different amounts relative to the yellow shift).

The lower-numbered (odd numbered) sector in each "group" is the one nearest the outside of the tool. Except for the "darker skin" group, these are the sectors providing the lesser degree of chromaticity shift.

Jamie Roberts' recent demonstrations suggest that by the use of this repertoire of six predetermined shifts one can, in a practical way, provide improvements in the "desirability" of the corrected skin tone in a range of situations. He relates this "empirical" process to the use of recognized numerical guidelines for "skin tone improvement".

I have not myself had a chance to experiment with this technique. We do have available lovely Cherokee-English skin, in a decidedly non-planar configuration, for such work when we get the opportunity.

Best regards,

Doug
 
So does that mean it might be advantageous to provide, on the lens, an IR cut filter with a lower-wavelength cutoff (farther into the visible spectrum) and steeper rolloff than that in the camera? If so, is something suitable readily available?

Hi Doug,

No, I don't think so (unless one has a Leica M8). The commercially available filters are not without drawbacks, and I don't think that they are all that much different from what the internal filtration already achieves. They are different, but it's on top of what's already provided, so the additional effect is minor. It is mainly a variable of unknown magnitude between camera brands/models, so it could be used to reduce the inter-model differences as long as it is stricter than the built-in solutions.

I think it is much better to have a good Raw converter that knows the specifics of the camera model, and optionally a profile with a bit of a skin-tone tweak (at the expense of accuracy) for a given type of lightsource (color temperature).

Skin tone will still change with the lightsource's color temperature, at a different rate than the White Balance does, due to its unbalanced reflection along the wavelengths.

Remember, (my) skin reflects much more Red than Blue:
SkinRemissionSpectrum.png


And here are the spectral compositions of several Standard illuminants (source www.brucelindbloom.com):
D40.png

D50.png

D65.png


As can be seen, the weight of Blue wavelengths is more significantly changed than the already high Reds. Part of that is covered by White Balancing, but the unbalanced skin reflectance de-leverages the differences.

For example:
at 400nm, skin reflectance 13%
at 700nm, skin reflectance 37%

That has the following effect with different iluminants, at the 2 extreme ends of the spectrum:
D40 at 400nm, 26% * 13% skin reflectance = 3.4%
D50 at 400nm, 50% * 13% skin reflectance = 6.5% (= +92% vs D40)
D65 at 400nm, 82% * 13% skin reflectance =10.7% (= +215% vs D40)

D40 at 700nm, 120% * 37% skin reflectance = 44.4%
D50 at 700nm, 92% * 37% skin reflectance = 34.0% (= -23.3% vs D40)
D65 at 700nm, 74% * 37% skin reflectance = 27.4% (= -38.3% vs D40)

(Near-)IR registers to all wavelengths, thus desaturating the colors to different degrees between camera brands/models. That makes it difficult to design a one-fix-fits-all solution. The need to tweak remains, unless one stays with one camera model.
Skin color functions as a de-leverage to the change in wavelength distribution. Differences in skin color are leveraged by different color temperatures (and White Balance).

Therefore, a generic WB change has an effect on skin color and on all other colors, a skincolor tweak allows a more accurate overall result. But then, we already knew that.

Bart
 
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