What is the name of the camera function that allows you to increase or decrease the brightness of the picture?

The camera function that allows you to increase or decrease the brightness of the picture is referred to as exposure. Exposure controls the amount of light that reaches the camera sensor and is influenced by three key settings: aperture, shutter speed, and ISO (which is related to gain). Adjusting the exposure setting will result in a brighter or darker image based on how much light you allow into the camera.

While "gain" can also be used to refer to increasing brightness, it specifically pertains to amplifying the signal in digital cameras or when using certain types of video equipment, rather than directly controlling the amount of light coming into the camera. In contrast, exposure is a broader term that encompasses the collective effects of various settings including gain, but is not limited to just that. Therefore, exposure is the term most commonly used to describe the overall brightness control function in photography.