The mapping between numbers and light
A digital image stores each pixel as numbers. A screen has to turn those numbers into light. The rule that does this is the tone response, and the most common way of describing it is gamma.
You might expect that half the signal would produce half the light. On most screens it does not. The usual relationship is a power curve: the light output is the signal level, scaled from 0 to 1, raised to a power called gamma. The shape of that curve is the gamma curve.
A worked example
Take a screen with a gamma of 2.2 and look at a signal halfway up the scale, which is 0.5. Raised to the power 2.2, 0.5 gives about 0.22. So a signal at 50 percent produces about 22 percent of the maximum light.
With a gamma of 2.4, the same signal gives 0.5 raised to 2.4, about 0.19, so 19 percent of maximum light. With a gamma of 1.0, which is a straight line, it would give 50 percent.
The difference between 22 percent and 19 percent looks small, but it shifts the whole midtone range. A higher gamma makes the midtones darker, which can make the image look deeper and more contrasty in a dim room. A lower gamma makes the midtones lighter, which can make shadows look open or washed out. The numbers are standard arithmetic, but how a given difference looks depends on the room and the viewer.
Why gamma exists
There are two reasons, one historical and one practical.
Historically, older tube screens naturally responded to the input in roughly this power-law way. Image standards were built around that behavior, and later flat screens were made to imitate it so that existing content would still look right.
The practical reason continues to matter. Human vision is more sensitive to small changes in dark areas than to the same changes in bright areas. A power curve spends more of the available signal values on darker tones. That makes better use of a limited number of steps, which is part of why a reasonable bit depth can still produce smooth-looking gradients. If steps were spaced evenly in light, the dark areas would show visible banding sooner.
What gamma does to what you see
Gamma does not change the black level or the white level. It changes how the steps in between are distributed. This has several visible effects.
- Shadow detail. With a high gamma, dark tones are pushed down, and details near black can disappear. With a low gamma, they are lifted and easier to see, but blacks can look gray.
- Midtones. The overall lightness of faces, skies and walls depends on the middle of the curve.
- Perceived contrast. Even with the same white and black, a different curve can make an image look flatter or punchier. This is why gamma is considered together with contrast, as described in the brightness and contrast article.
- Color. The curve is applied to each color channel, so errors in the curve can shift colors as well as brightness.
Common targets
You will see a few typical values. Many computer and web images are made for a curve that is close to a gamma of 2.2. Some video and film workflows in dim rooms use a value near 2.4. The standard that defines the common computer color space uses a curve that is slightly different from a pure power law, which makes it close to but not exactly 2.2. Some professional workflows use other curves for particular purposes.
There is no single correct gamma for every situation. The right target depends on what the content was made for and on the room. Dim surroundings tend to suit a higher gamma, and bright surroundings can make the shadows seem too dark, which is part of why people raise brightness or use a lower value during the day. These are general tendencies and not rules.
Gamma and high dynamic range
Standard gamma was designed for screens with a limited range between black and white. Content for high dynamic range uses different tone curves that can describe a much wider span of light, including very bright highlights. These curves are meant to be used with matching signaling, and a screen that shows such content incorrectly can look too dark or too bright. The article on high dynamic range goes into this in more detail.
Checking gamma
Gamma can be checked by eye or with an instrument.
By eye. A common test is a patch of alternating thin black and white lines next to a patch of solid gray. From a distance, the lines blend into an average gray. If the average matches the solid patch at about the expected level, the response is close to the target. Many test images use this idea. A second test is a stepped gray ramp, where you check that each step is distinct from its neighbors, especially at the dark and light ends. Test patterns are discussed in a separate article.
With an instrument. A measuring device reads the luminance of a series of gray levels, and software fits a curve to the results. This gives an actual curve rather than a single number, and it can show that a screen follows the target in the midtones but departs in the shadows.
Conditions matter. Room light adds a veil over dark tones, and a screen that is warmed up measures differently from one that has just been turned on. Make sure picture modes that alter contrast are off, and see the article on picture modes for why.
Adjusting it
Many screens offer a gamma setting in their on-screen menus, sometimes as a list of numbers or labels such as low, medium and high. Their numbers may not match true measured gamma. Calibration software can build a more exact correction. Some screens apply it inside the display, and other setups rely on the computer.
Keep in mind that adjusting gamma alone is not a full calibration. White point and color also play a part, as seen in the articles on color accuracy and on white point and color temperature. A gamma setting also cannot add bits that the signal chain does not carry.
A short summary of the idea
Gamma is a curve that decides how much light each signal level produces. Raising it darkens the midtones; lowering it lightens them. It rarely changes the extremes but strongly affects how an image feels, which is why it is checked in nearly every serious display test.