Gamma is an important but seldom understood characteristic of virtually all digital imaging systems. It defines the relationship between a pixel's numerical value and its actual luminance. Without gamma, shades captured by digital cameras wouldn't appear as they did to our eyes (on a standard monitor). It's also referred to as gamma correction, gamma encoding or gamma compression, but these all refer to a similar concept. Understanding how gamma works can improve one's exposure technique, in addition to helping one make the most of image editing.


1. Our eyes do not perceive light the way cameras do. With a digital camera, when twice the number of photons hit the sensor, it receives twice the signal (a "linear" relationship). Pretty logical, right? That's not how our eyes work. Instead, we perceive twice the light as being only a fraction brighter — and increasingly so for higher light intensities (a "nonlinear" relationship).

linear vs nonlinear gamma - cameras vs human eyes
Reference Tone Select:
Perceived as 50% as Bright
by Our Eyes
Detected as 50% as Bright
by the Camera

Refer to the tutorial on the photoshop curves tool if you're having trouble interpreting the graph.

Compared to a camera, we are much more sensitive to changes in dark tones than we are to similar changes in bright tones. There's a biological reason for this peculiarity: it enables our vision to operate over a broader range of luminance. Otherwise the typical range in brightness we encounter outdoors would be too overwhelming.

But how does all of this relate to gamma? In this case, gamma is what translates between our eye's light sensitivity and that of the camera. When a digital image is saved, it's therefore "gamma encoded" — so that twice the value in a file more closely corresponds to what we would perceive as being twice as bright.



smooth 8-bit gradient (256 levels) Original
Encoded using only 32 levels (5 bits)
linearly encoded gradient Linearly Encoded
gamma encoded gradient Gamma Encoded


Notice how the linear encoding uses insufficient levels to describe the dark tones — even though this leads to an excess of levels to describe the bright tones. On the other hand, the gamma encoded gradient distributes the tones roughly evenly across the entire range ("perceptually uniform"). This also ensures that subsequent image editing, color and histograms are all based on natural, perceptually uniform tones.



Despite all of these benefits, gamma encoding adds a layer of complexity to the whole process of recording and displaying images. The next step is where most people get confused, so take this part slowly. A gamma encoded image has to have "gamma correction" applied when it is viewed — which effectively converts it back into light from the original scene. In other words, the purpose of gamma encoding is for recording the image — not for displaying the image. Fortunately this second step (the "display gamma") is automatically performed by your monitor and video card. The following diagram illustrates how all of this fits together:

RAW Camera Image
is Saved as a JPEG File
image file gamma 1. Image File Gamma
JPEG is Viewed
on a Computer Monitor
display gamma 2. Display Gamma
Net Effect
system gamma 3. System Gamma

2. Depicts a display gamma equal to the standard of 2.2


2. Display Gamma. This refers to the net influence of your video card and display device, so it may in fact be comprised of several gammas. The main purpose of the display gamma is to compensate for a file's gamma — thereby ensuring that the image isn't unrealistically brightened when displayed on your screen. A higher display gamma results in a darker image with greater contrast.




linear RAW Linear RAW Image
(image gamma = 1.0)
gamma encoded sRGB image Gamma Encoded Image
(image gamma = 1/2.2)

If no color profile is embedded, then a standard gamma of 1/2.2 is usually assumed. Files without an embedded color profile typically include many PNG and GIF files, in addition to some JPEG images that were created using a "save for the web" setting.

Technical Note on Camera Gamma. Most digital cameras record light linearly, so their gamma is assumed to be 1.0, but near the extreme shadows and highlights this may not hold true. In that case, the file gamma may represent a combination of the encoding gamma and the camera's gamma. However, the camera's gamma is usually negligible by comparison. Camera manufacturers might also apply subtle tonal curves, which can also impact a file's gamma.



Recall that the display gamma compensates for the image file's gamma, and that the net result of this compensation is the system/overall gamma. For a standard gamma encoded image file (), changing the display gamma () will therefore have the following overall impact () on an image:

gamma curves chart with a display gamma of 1.0 Display Gamma 1.0 Gamma 1.0
gamma curves chart with a display gamma of 1.8 Display Gamma 1.8 Gamma 1.8
gamma curves chart with a display gamma of 2.2 Display Gamma 2.2 Gamma 2.2
gamma curves chart with a display gamma of 4.0 Display Gamma 4.0 Gamma 4.0


If you're having trouble following the above charts, don't despair! It's a good idea to first have an understanding of how tonal curves impact image brightness and contrast. Otherwise you can just look at the portrait images for a qualitative understanding.



CRT Monitor CRT Monitors
LCD Monitor LCD (Flat Panel) Monitors


LCD Monitors. LCD monitors weren't so fortunate; ensuring an overall display gamma of 2.2 often requires substantial corrections, and they are also much less consistent than CRT's. LCDs therefore require something called a look-up table (LUT) in order to ensure that input values are depicted using the intended display gamma (amongst other things). See the tutorial on monitor calibration: look-up tables for more on this topic.



Other important points and clarifications are listed below.

  • Gamma Correction. The term "gamma correction" is really just a catch-all phrase for when gamma is applied to offset some other earlier gamma. One should therefore probably avoid using this term if the specific gamma type can be referred to instead.
  • Is Gamma Required? No, linear gamma (RAW) images would still appear as our eyes saw them — but only if these images were shown on a linear gamma display. However, this would negate gamma's ability to efficiently record tonal levels.

For more on this topic, also visit the following tutorials:


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