ISO refers to the light sensitivity of your camera. There are different numbers indicating how sensitive your camera is to light – 100, 200, 400, 800, 1600, etc. The lower the number the less sensitive your camera is to light. The higher the number the more sensitive your camera is to light.
A high ISO doesn’t only mean your camera is very sensitive to light, but it also means that the grain in your images will increase. So keeping your ISO down results in less grainy pictures – something you want to remember. Grainy photos aren’t exactly pretty ( see the last photo – you can see the graininess of the house ).
However, sometimes you will have to shoot with a high ISO, especially in low light situations – e.g. inside a room with little ambient light, or when it’s getting dark outside. Little light means you want your camera to capture as much of the light that’s available as possible, in order to get a well exposed picture. So you will have to use a high ISO in order to increase the light sensitivity of your camera. You can remember: little light available = high ISO. Much light available = low ISO.
Even just a cloudy sky can force you to increase your ISO. London usually treats us with clouds rather than sun, so on a normal day my standard ISO is around 400.