Contents > Colour basics 4 / 10
 

Gamma in digital photography

 

The gamma
Published on April 15, 2015 / Updated on October 21, 2018

 

Our eye, like our monitors, "owns" a gamma! It simply means that our eye reacts to light in a non-linear way. What?! In fact, our eye is not equally sensitive to the same increase in light when it is in a dark room and when it is in full sunlight. Light a second candle in a dark room and you will see more light BUT not twice as much. Yes, I assure you ! Now light the same candle in the same room but in broad daylight: it won't change much and yet you bring as much light as in the dark room. Amazing isn't it ? even if you don't pay attention to it any more.


It is especially when we talk about monitors that we use the notion of gamma. If unfortunately its content remains unclear enough, it is important now to look at this term to better understand the eye and human vision and thus decide, in full knowledge of the facts, the gamma of the screen to choose: 1.8 or 2.2, the two most famous gammas to calibrate your screen and to question the particular appearance of an HDR photo ?


 

 

Vocabulary and general terms

The vocabulary of color management is full of new features compared to the vocabulary used in a silver laboratory : we now speak of gamma, response curve...

The gamma

The notion of gamma - of an eye, a screen or a scanner - is a notion that regularly comes up in the vocabulary of color management, especially when calibrating the screen. It is simply a mathematical curve (a function) that makes it possible to know the link - the correlation - that there is between an emitted signal (perceived light) and the response of a sensor, (for example our eye). This function is written as :

 

Output signal = input signal gamma

(The output signal is equal to the gamma power input signal)

  

Indeed, our eye has a singular particularity : it does not have the same sensitivity (here in quantity) to an increase or decrease in light in a weak or bright environment. The same difference in brightness (e. g. 10 lumens) will be perceived in a dark environment but not or only slightly in a brighter environment. The eye is much more sensitive to small differences in brightness in low light than in high light. If you light a second candle in a dark room, you will see more light while it will go completely unnoticed in a room in broad daylight. However, in both cases you will have brought the same amount of extra light.

 

The eye's response to light is therefore not linear.


It is therefore necessary to note a consequence of this non-linearity according to the lighting environments : "gamma sound" differs according to the lighting environments as shown in the figures below :

   
  Difference of perception of the eye according to the light environment
 
 

Contrast and light environment: the two grey squares have the same RGB values 160 and 100 on both a white background on the left and a black background on the right, but it is clearly shown that the contrast between these two squares is nevertheless higher on a white background!

The gamma of the eye constantly varies according to the lighting environment and the contrast of the scene being viewed. Thus, it is different on a foggy day and in full sunlight or in front of a screen and when he looks at a photo print. It varies from about 2 to 3 by way of the famous 2.2 in front of a screen.

This is an amazing feature of the human eye, but it is more of a feature of the human body than just its eye. This feature allows the human body to feel important differences without overloading its sensory captors too quickly.

The dynamics of the eye

To adapt to different lighting environments while continuing to see something without saturating its nerve cells too quickly, the eye also has an essential appendix : the iris. It can open (slowly) and close (almost instantly) when the variation in light is too great - when you leave a room where you have been in the middle of summer for a long time, for example. Under these conditions, the iris measures about one millimetre in diameter and therefore allows little light to pass through, while at night, after a good 15 minutes of adaptation, the fully open iris measures six-seven millimetres in diameter. However, for a given iris diameter (from 1 to 7 mm), the eye is able to see about 14-16 diaphragms (depending on the contrast of the scene and the lighting environment - light or dark - as we have just seen above) equivalent to a photo and this, therefore, over 24 diaphragms of total amplitude. The absolute amplitude of the eye is therefore about 24 diaphs but, thanks to the iris, at each moment only 14/16 diaphs. Remember that the best digital cameras today are capable of taking 11/13 slides, including the famous Nikon D810 but also the Pentax 645Z or the Hasselblad 50C, Fuji GFX 50!

So why do HDR images have such a characteristic look (on non-HDR screens) ?

Well, quite simply because the gamma of the eye looking at a screen is not the same as that of the same eye when it was in front of the scene photographed. As we try to reproduce by this technical process called the HDR technique on the one hand and because we can "technically" merge many images together (it is not uncommon to merge 6 photos with a gap of one diaphragm between two shots) and because we can "recover" a "crazy" dynamic of more than 16 diaphragms that we try to visualize with a gamma of 2.2 therefore unsuitable to visualize more than 16 diaphragms. The image thus seems to be of little contrast, strange. HDR what !

   
Photos with or without HDR rendering from the same photo
   
 

So in other words, a photo with a lot of dynamics - more than 14/15 f-stops - will always have that typical look displayed on our fixed gamma screens.

Response and gamma curves

Gamma curvesThus, if I send to my eye a light signal located exactly between 0 and 255, therefore at 128, (minimum and maximum levels that it can see), it will have the sensation of seeing a rather dark grey around 0.2 on a scale from 0 to 1 and not 0.5 as one might think. This correction is applied to the signals sent to the graphics cards to adapt to this particularity of the eye.
Only raw files - RAW - of APNs have a gamma of 1 because their captors 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 its screen


As you approach the monitor, you can see that some samples seem to blend into the checkered image that serves as the background. If you move far enough away from your screen - about one meter - you notice that one or two samples seem to disappear into the background of the image and the area seems to become uniform. At this observation distance, it is sufficient to identify the area - and therefore the gamma - which seems to be the most homogeneous and which no longer makes it possible to distinguish the patches from the bottom.


Gamma verification test of a screen


Look for the dot that disappears in the background of the image. It is a fun test compared to the gamma that was chosen during calibration. There is often a slight delay, not very embarrassing, actually.

 
     
   
  Let's continue with the specific vocabulary of color management with colorimetric spaces or color spaces - 5 / 10   Suivre
   
 
 

To be remembered !


 The gamma of the eye varies greatly depending on the light environment : light or dark, contrasted or not.

 It is close to 2.2 when he looks at a monitor. That's why most monitors are in 2.2.

 It' s more like 1.8 when looking at a book. The Graphic Art screens, for printing professionals for example, are therefore mainly in 1.8.

 
     
 
 
Through these 10 pages we will learn all the vocabulary related to colour management: colour spaces, ICC profiles, gamuts, etc...
 
- Introduction to colour management
- Eye and colour perception
- Colours and computer science
- Gamma - 4 / 10
  - Vocabulary and general terms
- To be remembered...

- Colour spaces
- ICC profiles
- Assign an ICC profile
- Convert an image
- Relative and Perceptual Conversion Modes
- What is calibration ?!

 

- 2018 monitors buying guide !
- My 21 full monitor review


 

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