Once the light has passed through the aperture and been filtered by the shutter speed, it reaches the sensor. This is where we decide how to set the ISO.

As you turn the ISO number up, you increase the brightness. But, at the same time, the image quality decreases. There will be more digital noise or “grain”.

So you have to decide upon your priorities in terms of exposure versus the grain.

For example, I would reduce the image quality if it meant that I could prevent motion blur in my photo. There’s no possible way to fix that in post-psrocessing.