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This is a topic your editor first wrote about five years ago in Digital
Photographer magazine. The subject continues to generate much discussion
and we are frequently asked for advice and article reprints on the
features from Professional Imagemaker. It is therefore timely to revisit
the subject and add the mass of information gleaned in the intervening
period. There have been a number of changes. The use of RAW files has
increased and calibration has become more common for cameras, monitors and
printers. Our discussions for this feature have been wide-ranging and
include instrument makers, camera makers, dermatologists, artificial limb
makers, medical illustrators and a number of photographers.
Why Accurate?
As humans we are tuned to recognise certain colours with more discrimination – the so-called known colours. Skin is a known colour and its pleasing portrayal in an image is of paramount importance. There is a difference between an accurate colour (as in an image depicting a medical diagnosis) and a pleasing colour. Here we move in cultural preferences. Whist it is generally true that lighter skin is favoured by many cultures, very pale skin is only prized by certain ethnic groups. Let’s not beat about the bush, we are talking sexual attraction here – that is the point of wishing to look flattering in a portrait. One of the saying we came across in our research is “wealth lightens the skin” This is true for the darker skinned ethnic groups but most Europeans favour a more “tanned look”. The crossover between looking tanned and looking tacky is fine and movable, especially with the current campaigns to reduce exposure to damaging uv light. Today people are far more aware of the danger of skin cancer and a heavy, induced tan is looked upon as a sign of ignorance, a bit cheap. At the other end of the scale the dark-skinned ethnic groups stay out of the sun to stay as pale as possible. You will not see students walking about under parasols in a London university but you certainly do in Kuala Lumpur. Also recall that when the Khmer Rouge were running riot, the advice to academics was to get out into the fields and get a tan – pale skinned people were targeted as being part of the upper classes and more ruthlessly murdered.
Film manufacturers have endlessly pursued the accurate reproduction of skin colour. Starting with the move from Ortho Chromatic to Pan Chromatic film the topic always features in advertising literature. Here are some examples from a 1994 Fuji catalogue “reproducing natural skin tones with subtle shades”; “accurately reproduces a broad range of flesh tones”; “ideal for weddings, portrait and general purpose people photography”; “the most vivid colour reproduction and rich skin tones”; “sensuous flesh tones”
Note the key words here; natural, sensuous, rich, accurate, subtle. What does all this mean?
The arrival of digital has eased the mud off the bottom of the flesh
pond. While digital is demonstrably more accurate at rendering most
skin
tones it also carries the risk of drifting further from optimum in
unskilled hands. In addition there is the problem of un-natural reddening
of certain complexions when shot digitally as compared to negative-silver
halide reproduction – more on this later.
What is Skin?
The skin is the largest organ in the human body. It keeps all the bits inside that should be and all the bits outside that should be. It is waterproof when it needs to be, so that we don’t dissolve in the bath but not waterproof when we need to sweat to lose heat.
Whilst it is not appropriate to delve into medical science here, we do need a few pointers. The skin consists of a three layered structure, –subcutaneous tissue, dermis (cortium), and epidermis. The surface of skin itself is effectively matte because the uppermost level of skin is covered with dead cells causing no regular reflection. The glossiness of skin can be due to sweat, skin oil or an applied layer of cosmetic cream.
The matt skin colour appearance is influenced by the light filtering capabilities of three main colouring agents: melanin in epidermis, carotene in dermis and subcutaneous fat, and blood capillaries across the dermis. Melanin is a brown pigment and carotene gives an orange tint. Haemoglobin (an element of blood) can produce two different tints: if the haemoglobin is oxygenated (oxyhemoglobin), the tint is reddish or pinkish apart from when it is deoxygenated (reduced haemoglobin) then the tint is bluish. Like most natural objects, the skin has variability, which in this case is mainly due to amount, density, and distribution of melanin. Under the surface there are colourant particles which interact with light, producing scattering and colouration. The variation within a person can be quite marked, freckles being the classic example. There are also differences around the body. Production of melanin darkens the skin of the person to protect them against the harmful effects of the sun’s uv rays. For this reason the soles of the feet and palms of the hand contain less melanin and are pinker. The quantities of melanin vary with ethnicity and roughly correspond to the following variations:
Light Skinned Caucasians 1-3%
Well-Tanned Caucasians and Mediterranean 11-16%
Darkly Pigmented Africans 18-43%
L The lightness usually expressed as a percentage
a the redness to greenness of a colour
b the yellowness to blueness of a colour
There are huge advantages to separating the lightness values of a colour from its hue and saturation, which is what you can do by using Lab values. Ignoring the variations in different parts of the body, the values of a and b remain more constant around an image which may be lit is cross lighting. In this case the dark side of a face may have different L values but near identical a and b values to the lighter side.
There is another advantage to Lab. The concentration of melanin governs the “tan” of a skin colour by changes to the a value and the L value. The blood flow and concentration of blood vessels close to the surface effect the Lab b value. However just to confuse matters there are several types of melanin; a red and yellow sort. Red melanin is higher in Irish, Danish and Dutch populations which also colour red hair red! Within these variations, the presence of carotene (which is orange) influences the tone where it is stored in the subcutaneous fats and this underlying variability gives skin its variable texture and tone. Carotene concentration can be affected by diet – you may recall the little girl who turned orange after drinking way too much of a proprietary orange drink.
The problem which faces us as photographers is that we have to distil all this colour variability and conjure up a pleasing picture. To do these we need practical guidelines of how colour should be adjusted to achieve a particular “look”.
Skin Tone Analysis
According to some research, under normal, flat illumination, most Caucasian skin tones lie within the range shown below for mainly un-tanned parts of the body. These are lower than values we experience in “pleasing” portraiture and slightly lower than the average calculated by McCamy for the original Macbeth Chart (66%, 14, 17)
L 62-72%
a 2 - 10
b 11 - 22
The latest Macbeth Colour Checker SG has an additional 14 patches of “skin” tones which have a and b values in the range 11-28 (a) and 17-39 (b). These are plotted on the graph along with our original test spread as used in the colour audit process for Paper Chase. Note that the SG tones are more saturated and more yellow (tanned) than any other values. For completeness we have also added the skin tones for new born infants found in a medical research paper. As you might expect
On the Professional Imagemaker we have relatively little trouble in spotting off-colour images and, with prints, we can always resort to dropping them under the spectro for confirmation. Our monitors are calibrated to an average error of less than 1 ÄE overall and, on the main workstation, the errors for Macbeth Flesh tones are 1.39 for dark flesh and 0.62 for light flesh. First of all though here is our test workflow.
1. We always shoot RAW
2. We always use a Macbeth Color Checker (24 Swatch) (and also the new SG variant)
3. We start by neutralising the image with the eyedropper to bring the colour temperature to the correct value for the scene.
4. With the image opened into Photoshop as an Adobe RGB file, we set our Info Palette for Lab and HSB readouts.
5. We scan over the image with the eyedropper set to 5x5 Pixels looking to see if most of the exposed skin lies between 16° and 24° on Hue angle. At this stage we usually ignore the saturation value. We also avoid problem areas of an image eg.
Under coloured hats
Too close to green foliage
Too close to brightly coloured garments
Avoid ladies cheeks if a blush foundation has been used
Good places to test are just above the cleavage and the upper forearms of the dominant light side of the figure.
In reality this is just the starting point. The Lab a;b values are then measured and checked against the chart to ensure that we have not been fooled by surrounding colours in the image (they are important though, and have to be considered).
If a boost to the colours is desired the RAW file can be reprocessed.
The saturation slider in the RAW processor is proportional, that is the
higher the start saturation the bigger the amount added when it is
boosted. For example a 30% saturation becomes 40% with a +50 but a 60%
saturation is lifted to 100%. The table below shows the major points:
Thus for a modest boost, typical for “improving” a Caucasian flesh tone, a saturation increase of 10% would be appropriate and need a +25 to +30 point setting. Experience should tell you your values after some experimenting. Remember that the lighting ratio in the studio or overall scene contrast outdoors will affect this judgement.
Adjusting the Image
The most serious weakness of RAW files is the paucity of colour measurement numbers available as you tweak the handles. Whereas in Photoshop itself you can have RGB, CMYK, Lab or HSB, in RAW you are stuck with RGB, the least useful of all! This forces the photographer away from finalising adjustments in RAW to using Adjustment Layers in Photoshop. We recommend Adjustment Layers rather than straight image adjustments so that you can blip the correction on and off to prevent colour fatigue and chromatic adaptation conning you. As you will see later it also opens up the possibility of selective colour corrections of both colour range and in different parts of the image.
Pink Babies and Red Grandads!
Now for the tricky bit. In spite of our best endeavours we still find under certain situations that flesh tones do not come through accurately, even under controlled conditions. Given that you have shot RAW, click balanced for the colour temperature of your lights and then adjusted your exposure in the RAW processor against your Macbeth Chart you might be forgiven for thinking all would be well. Fat chance! Most experienced digital photographers will tell you that if they shoot a wedding party, some people will be standing out like beacons in the crowd. Your editor should know he is one of them!
After some deliberation and searching through literally hundreds of medical research journals we are starting to get some pointers. There are two potential sources for the increase in redness. The colour of the skin is determined by the melanin concentration, its colour and also the proximity of the blood vessels carrying oxyhaemoglobin, the red oxygenated type. These blood vessels are buried under the outer layers of the skin and the penetration of light through this layer and back out again influences the colour appearance. The colour balance of the skin has been described as Tan versus Rosiness. Given that only red heads (aka your editor) have the red melanin (pheomelanin) it seems a reasonable hypothesis that they will turn up more red under stronger illumination. There is a further hypothesis which, to date we have not tested. Detectors in digital cameras are more sensitive to infrared light and to overcome this and prevent excess reds in the final image, a “hot” filter is often employed to shield the near infrared light (NIR). If this effect is balanced for the majority of scenes it seems reasonable that under some conditions, extra NIR might get back to the camera.
Regardless of the underlying reasons for the effect, the evidence is easy to collect. In the 4-up image shown, your editor is excessively red in images [1] and [2]. In image [3] the overall red balance has been adjusted using selective colour on the reds with a hue adjustment of +9 on the reds. However this has dragged the Macbeth light skin swatch too yellow – it is not responding to the camera in the same way at all. This is the classic selective red effect. To overcome it the mask of the Colour Adjustment Layer has been painted, to control where the colour adjustments are active [4].
I
ABOVE: The graph shows the plotted data points for the girls above left. In the red ellipses the data from actual skin measurements are shown. The blue ellipses show where the calibrated image data lay. Note that Emily’s complexion is more yellow than that of Sally and that neither skin tone is rotated towards red from skin to image, even though there is a rotation towards red of the Macbeth swatch duen to the calibration profile. The estimated colour temperature was 5550°K. Overall the effects cause us to conclude that there is no over-reddening effect from either of the girl’s complexions.
In order to provide more background data we took the opportunity to photograph and measure the two young ladies featured in the picture above. We measured the skin specta of each girl and determined the Lab values on the forearms, fingers and the back of their hands. We are unsure of the accuracy of the technique but the two skin complexions were different in the way you would expect, with a rotation towards yellow for Emily (on the left). In the image file, which was corrected using a bespoke camera profile, Sally, on the right had hue angles of between 6° and 24° whilst Emily had angles between 24° and 27°. Both girls are relatively pale skinnned with saturation values of 15% for Sally and and 25% for Emily. This difference is just obvious in the original image and should survive through the magazine printing process.
And so to print
Having won your hard-earned, perfectly corrected image, the next hurdle
is to get it out onto the print in one piece. In our experience, most
profiled printers tend to reduce the saturation of the image while
maintaining the hue – that is the colour shift is down a constant hue
line. This is typified by the graph from the colour audit test target
shown on the right. When a printer/paper cannot hold a high Dmax, the
profile compensates by adjusting all saturations downwards. All of the
above assumes that you honour the profile as you go to print. If you
change up to a larger gamut volume the saturation will increase, if you
change down, it will decrease; this is the cause of a lot of grief at
external printing labs. Most unprofiled printers tend to reduce
saturation; you may care to compensate for this before you send your file
to print. If an unprofiled printer desaturates and is off colour balance
you are likely to be in trouble, especially if the error is towards green
or cyan.
Baby Flesh Tones
We collected data from a study of a large number of babies. In general the values lie on the red side of average for Caucasians with saturations that do not rise above 35%. In Lab
A similar colour-shift effect to that described above has been observed when photographing very young children. Whilst we expect babies to be pink, many digital photographs show an excessive colour change on the forearms (and feet). It is a specific effect as both arms are affected even though they are catching different levels of light. The overall colour shift was too much for a commercial picture, forcing photographer, Martin Sellars, to selectively mask and modify the colour of the arms. As shown in the dual 3-D image, the corrections are quite subtle but make a huge difference to the way the picture looks. In Martin’s experience, the problem is specific to digital; no similar issues have arisen in decades of film shooting! Your Editor is presently working with the medical specialists to resolve the puzzle. The consultant dermatologist we questioned was not surprised and provided a cogent explanation of why babies have a higher venous blood flow nearer the surface of their little arms. However whilst babies’ arms are undoubtedly bluer, the scale of the effect is at odds with measurements on actual baby skin, as shown in the graph. It seems possible therefore that the greater accuracy of digital is emphasizing an effect, which has previously been masked by the properties of film. Sadly this leaves us with a solution requiring selective (time-consuming) masking. Martin points out that the majority of photographers mistakenly judge the colour to be too magenta which would require an increase in green to correct it. The bias is, in fact, blue and requires an increase in yellow to correct it. Martin adds about 10 points of yellow to the mid-tones and 5 points of yellow to the highlights in the Color Balance dialogue box. This is applied through a selection, feathered about 10 pixels, applied to the arms and using natural folds and shadows to hide the joins. He estimates about 10 seconds to effect the correction.
In the absence of any better advice as to the source of this problem we are currently unable to formulate a rapid solution. Discussions with quite a large number of photographers have shown that the effects described for babies, and the reddening of some adult complexions, are common. One photographer related a situation in which a model was no longer engaged for commissions because her skin always turned orange. Others mentioned the possibility that the UV-screen formulations, commonly built into modern cosmetics, were responsible for some abnormal effects. TV cameramen related that they have selectively filtered particular cameras or lights to improve skin rendering. What is evident is that we have stumbled into an area in which there is a lack of solid evidence. One thing we are sure of – the effect is not that which occurs when the wrong profile is assigned or attached to a digital image. This typically occurs if an sRGB file is moved up into a larger colour space such as Adobe RGB. This produces an overall increase in saturation which is not the effect we are studying, a rotation of the hue angle. Anything which increases saturation will make any hue shift more apparent, so the effects described will seem worse. It is the selective shifts that we are concerned with. We intend to keep hunting!
Conclusions
To some extent the pages leading up to this conclusion are simply legitimising the advice we are about to give. We hear so much guff about the effect of Adobe RGB and sRGB, this RAW file processor or that, this white balance setting versus another, etc etc that it is timely to put the record straight! Here is our advice summary:
• It matters little which colour space that you use as long as you honour the profile as you move the file around.
• Moving to a bigger colour space increases saturation, moving to a smaller colour space decreases saturation. In some instances going the “wrong” way actually makes a more pleasing final print!
• If you use RAW files you have a better chance of controlling the eventual result. Some JPEG algorithms do funny thing to your files and colour space – beware!
• If you work in Adobe RGB and fit your skin tones within the bounds shown in the graph on the third page of this feature you are reasonably certain to be in the right ballpark. Most problem files we are presented with are outside that range eg shooting with the incorrect white balance will shift you out of the range if you start near to a boundary. If you work with both Lab and HSB co-ordinates in the Info Palette you will get a better idea of what is going on with your skin tones.
• One sound piece of advice is to always ensure that your yellow value (measured in CMKY in the Info palette) is always higher than your magenta value and that your cyan lies at less than 1/3rd of the magenta value.
• The average values for white Caucasians are shown in the table for HSB and Lab and also for sRGB and Adobe RGB. Note that Gretag seem to be varying their notion of “average”, the new SG chart does not have the same values as the older Color Checker 24. Be aware that to most Caucasians a slight tan is preferred but
the opposite is true for most Asian, Oriental and South American cultures, who prize lighter skin and generally avoid exposure to the sun.
• Although babies are pinker than their, parents make sure that their skin tones do not drift towards blue magenta. • At the present time we have no answer to the red-head problem other than selective masking as described – we are working on it!
Some other articles which you may be interested in reading:
Environmental Portraiture by Catherine Povaschuk
Photo Quote: Millions of men have lived to fight, build palaces and boundaries, shape destinies and societies; but the compelling force of all times has been the force of originality and creation profoundly affecting the roots of human spirit. - Ansel Adams
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