ISO is the sensitivity of the matrix to light rays. The higher this setting, the more sensitive the camera is to how light enters the frame. For example, in order to take a high-quality picture at ISO 15, you need sufficient room lighting. But if you set ISO 1600, the camera will take in more light, and the dark picture will become brighter.
High ISO can spoil the frame, as well as add digital noise. The latter is the main problem of all digital matrices. Image graininess at high ISO is one of the main indicators of matrix power.
Now the noise level at maximum ISO for SLR cameras is low, but not good enough.
Selecting a shooting mode
Modern cameras provide 3 shooting options:
Manual mode (M) – you can control three parameters: aperture, shutter speed and ISO.
Aperture priority (Av or A) – aperture and ISO are set, the camera controls the shutter speed.
Shutter Advantage (Tv or S) – Shutter speed and ISO control, aperture controlled automatically.