Color management guide by Arnaud Frich

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Updated on April 10, 2017
 

The gamma

Our eye, as our monitors, "has" a gamma! It simply means that our eye reacts to light in a nonlinear way. What?! In fact, our eye is not sensitive to the same increase of light when it is in a dark room and when it is exposed to direct sunlight. Light a second candle in a dark room and you will see more light BUT not twice more light. That's right! Finally, light the same candle in the same room but in broad daylight: it does not change much and yet you bring as much light as in the dark room. Surprising, right, even if you never pay attention to it?

 

 

Key points if you are a beginner ...

Here are the key points to remember about gamma. The rest of this page will be dedicated to those who want to go into more details.

What is Gamma ?
The eye reacts to light in a weird manner. This reaction to light is not regular - it is said to be non linear. What does that means ?

  • The human eye is more sensitive to small variation of light in a dark environnement than in a bright one.
  • If you light two candles instead of one in a dark room, you will not percieve twice more light, but only more light. The eye response is not linear.

That's what gamma expresses.

But usually we talk about gamma for a monitor and not for the eye?
It is precisely to try to reproduce the behavior of the eye that monitors have a gamma.

Hopefully, everything is done automatically in our computers and there is nothing to do, except in specific cases that are of no interest for beginners and they will therefore not face!

Next key point: color spaces suivre


It is especially when talking about monitors that the notion of gamma is used. Although unfortunately its content remains quite uncertain, it is now important to pay closer attention to this vocable to understand the eye and human vision and thus choose your monitor's gamma with all parameters in sight: 1.8 or 2.2, the two most famous gammas to calibrate your screen and to question the so peculiar rendering of an HDR photo?

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Vocabulary and general notions

The vocabulary of color management is full of new words compared to the vocabulary that was used in a lab for film-based photography: now we're talking of gamma, response curve...

Gamma

The notion of gamma - of an eye, of a monitor or of a scanner - is a concept that comes up regulary in the vocabulary of color management, particularly when calibrating the monitor. It is simply a mathematical curve (a function) that enables to know the connection - the correlation - between an input signal and the response of a sensor, for instance, our eye. This function is written as follows:

 

output signal = input signal gamma

(The output signal equals the input signal to the power of gamma)

 

Indeed, our eye has this singular characteristic not to have the same sensitivity (in quantity) in low light and high light. The same difference in brightness (eg 10 lumens) will be seen in a dark environment, but not at all or almost not in a bright environment. The eye is much more sensitive to small differences of brightness in low lights than in high lights. If you light a second candle in a dark room, you will see more light but it will not even be noticable in a room in broad daylight. But in both cases, you will have added the same amount of light.

 

Therefore the response is not linear.

 

Finally it should also be noted that this non-linearity is different depending on the light environment, so this "gamma" differs depending on the lighting ambiance, as shown in the figures below.

 

Contrast according to light environment

Contrast and light environment: the two grey squares have the same RGB values of 160 and 100 on a white background on the left and on a black background on the right. It clearly shows that the contrast between these two squares is more important on a white background!

 

Summary! The gamma of the eye is continuously changing depending on the light around and the contrast of the scene viewed. It is thus different on a foggy day and in plain sunshine or in front of a screen and when it looks at a photo print. It varies between about 2 and 3, with the famous 2.2 when watching a screen.

Here's one surprising specificity about the human eye but it is rather a specificity of the human body and not only of its eye. This characteristic enables the human body to feel important differences without saturating its sensorial sensors too fast.

Dynamic of the eye

Iris of the eyeWhy? The reason is very simple. To adapt to different light environments while continuing to see something without saturating the nerve cells too fast, the eye has a necessary appendix: the iris. It can open up (slowly) and close (almost instantly) when the light variation is too important - you can feel it when you leave a room where you stayed long enough in summer, for example. Under these conditions, the iris measures approximately one millimeter in diameter and lets few light pass through, while at night, after a good quarter of hour of adaptation, the iris fully opened measures six-seven millimeters in diameter. But for a given iris diameter (from 1 to 7 mm), the eye is able to see about 14-16 equivalents of photo stops out of the range of 24 stops. The absolute range of the eye is thus 24 stops, but thanks to the iris, you can only access 14-16 stops at the same time. Remember that the best digital cameras today are capable of shooting a maximum of 11 to 13 stops, like for instance the famous Nikon D810 but also the Pentax 645Z or also the Hasselblad 50C!

So why do HDR images have such a characteristic rendering?

Well, quite simply, because the gamma of the eye watching a monitor isn't the same as the gamma of this same eye when it was in front of the scene shot. Since we're trying to reproduce using this technical process called HDR technique and because it is "technically" possible to merge multiple images together (it isn't uncommon to merge 6 photos with a diaphragm gap between two shots) and it is possible to "recover" a "crazy" dynamic of over 16 diaphragms that we're trying to watch with a 2.2 gamma hence not adapted to view over 16 diaphragms. The images thus doesn't seem very contrasted, weird-looking. HDR, in fact!

 

 

So in other words, a photo with an important dynamic - over 14-15 diaphragms - will always have this typical rendering displayed on our screens with fixed gamma.

HDR rendering with only one photo in RAW format! The dynamic of certain recent camera bodies with Sony sensors - Nikon D810, Pentax 645Z, Sony A7R2, etc. - is so important - about 13.5 diaphragms but just under the limit of 14/15 diaphragms - that when you'll move the cursors "High lights recovery" and "High shadows recovery" in Camera Raw or Lightroom to the maximum, you bypass this limit and end up with an HDR image in one photo. What it must be with the brand new PhaseOne diaphragm, announced at 15 diaphragms!!!

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Response curves and gamma

Curves of gammaSo, if I send to my eye a light signal located exactly between 0 and 255 (minimum and maximum levels that can be seen), hence at 128, it will have the sensation of seeing a dark grey of around 0.2 on a scale from 0 to 1 and not 0.5 as one might think. This correction is thus applied to the signals that are sent to graphic cards to fit this specificity of the eye.
Only raw files - RAW - of digital cameras have a gamma of 1 because their sensors have a linear response to the light they receive (blue curve). This is important to know if you are shooting in RAW.

Control of the gamma of your monitor

When getting close enough to the monitor, you can see that some samples seem to blend into the checked image used as a background. If you get far enough from your monitor - about a meter - you'll see that one or two samples seem to disappear into the background of the image and the area seems to become uniform. At this viewing distance, you just have to locate the area - so the gamma - which seems the most homogeneous and where you no longer distinguish the patches from the background.

 

Test of gamma

Look for the patch that fades into the background of the image. This is a fun test when you compare it to the gamma chosen during calibration. There is often a slight difference, not really annoying though.

 


To be remembered!

The gamma of the eye varies a lot depending on its light environnement.

It is close to 2.2 when watching a monitor. That is why most of the monitors are at 2.2.

It is close to 1.8 when reading a book. Graphic art monitors, for printing professional for instance, are thus often at 1.8.



Let's keep on going with the specific vocabulary of color management: color spaces
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