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The gamma monitor
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.
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 :
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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
 Thus, 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.
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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.
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.
 Use note! The above visual test was performed for displays with resolutions ranging from 94 to 110 dpi (FullHD, QHD or WQHD). For it to work with 4K, 5K, UHD, iPad, tablets and other very high definition tiles, it is essential to use the native definition of your monitor (100% display). On iPad tablets, this does not work because you cannot change the display preferences of the screen.
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Let's continue with the specific vocabulary of color management with colorimetric spaces or color spaces - 5 / 10  |
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 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. |
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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 !
