One of the photo adjustment tools is called Gamma, but what does it do?

Gamma sets the overall brightness and contrast tonal curve for an image. In other words, adjusting the Gamma will adjust both the brightness and the contrast at the same time.

For example, imagine you have an image that has some light areas and some dark areas, but you want to adjust the dark areas so that you can "see" them better.

If you use only brightness to adjust a photo, the whole photo will get lighter. While this might work for the dark areas, it might make the light areas look "washed out" or too light.

Another option might be to try to adjust contrast. However, contrast adjustments will make the darks darker and the lights brighter. While this can help if your photo is mostly light or mostly dark, it won't help as much if it's got elements of both.

Instead of Brightness or Contrast, you may want to try Gamma.

Raising the gamma tonal curve is going to make the overall image lighter, lowering gamma is going to make the overall image darker, but it will also increase/decrease the contrast at the same time.

For example, for a picture that has a lot of pinks. If the gamma increases, it's going to change the total tonal curve and what the eye perceives. It will make dark red areas a bit less red but will also increase the white balance to equal out the tonal curve of the photo. For images that have a high saturation of color over most of the image, you might want to try Gamma. 

 

A special note about "out of gamut" colors: For best results, we recommend saving images in sRGB. Using other RGB formats can result in some distortion or loss of color in your images. For Adobe RGB, the difference will be small, but for some other formats like ProPhoto RGB, the difference will be very noticeable, both on the screen and in your printed book. However, there are certain shades of colors that can't be reproduced by our printing presses. Those include certain vivid, saturated colors, such as bright purple, hot pink, fluorescent colors, and neon colors, like the luminous blue of underwater photos.

These colors might look vivid on screen, but when printed, they may shift to less-vivid and more subdued colors. These colors are considered "out of gamut", which simply means our presses can't reproduce them. This isn't considered a print defect as it's ultimately a limitation of the print process.