ISO Speed (Sensitivity)

When you cannot get the right exposure with the aperture and shutter speed, it is time to think about the ISO sensitivity (also called ISO speed). It defines how sensitive the camera sensor is to light. With the same amount of available light, the higher the ISO sensitivity is, the more light will be captured by the sensor.
The ISO sensitivity is measured in… numbers! 🙂 I do not know what exactly these numbers mean, never needed it (now, when writing this, I became curious ). The “normal” ISO speed depends on your camera. In my experience it is either ISO 100 or ISO 200. This is what you start with. If you cannot get the right exposure by altering aperture and shutter speed, try to adjust the ISO speed. I cannot remember a situation when I needed to reduce the normal ISO speed. Aperture and shutter speed can reduce the amount of light as much as needed in most cases, with rare exceptions. There are also other ways to reduce the amount of light, such as lens filters. The ISO sensitivity control is usually useful with insufficient light. When ISO 100 is not enough, try ISO 200, ISO 400, etc.
This is not so simple, however. Aperture and shutter speed controls can be considered safe, because they alter a natural measure: the amount of light. The ISO sensitivity is more artificial, its increase comes at a price. The more sensitive the sensor becomes, the more image noise it creates. This is when the equipment quality becomes important in photography: expensive professional cameras usually allow much higher ISO speeds before the image noise becomes prohibitive. With my entry-level DSLR Nikon D80 I never shot above ISO 800. With Nikon D700 I often get to ISO 1600 (twice more sensitive) and sometimes even to ISO 3200.