Just like Green Camera mode, cameras come from the manufacturer set to Auto ISO. Even if you go directly to Manual mode to start using your camera Auto ISO will still be active. Find the ISO settings in your camera’s menus and turn off Auto ISO. In some cameras there is a setting for ISO as well as another setting for Auto ISO. If your camera has both settings it will not matter what you set the ISO to if you have not turned off Auto ISO.
By setting the ISO you are taking control of how sensitive the sensor in your camera is to light. It may not sound like much but this is very important step toward taking full control of your camera. Setting the ISO is as easy as choosing a number in the ISO settings menu. Which number you should choose has to do with the amount of light available to make images. In general it is always best to use the lowest ISO possible to get the image you want under the lighting conditions you have. As a rule of thumb 100 – 200 ISO is good for outdoors while 400 – 800 ISO is generally good for indoors. Newer digital cameras, those made within the past three to four years (currently 2013 as I write this), have great High ISO capabilities.
To make things a little easier for this discussion lets say the ISO range for your camera is from 100 to 3200 ISO. Check your camera’s owner’s manuel for the ISO range of your camera (the ISO on some cameras can be adjusted in 1/3 of a stop increments). The options you see in the ISO settings menu (other than Auto) will be: 100, 200, 400, 800, 1600, 3200. Notice that as the numbers increase they double. This is because with each higher number the camera’s sensor is twice as sensitive to light as was with the number before it. For example, when the ISO setting is changed from 200 to 400 the sensor needs half as much light as it needed at 200 ISO. The general ideas is the more light you have the lower the ISO can be.
Can you use ISO 1600 on a bright sunny day? Sure you can! Will you get the best detail possible in your image? Probably not, but if you are looking for an interesting effect experiment a little. Make several exposures at different ISO of the same subject and compare the different images. Experimenting with different ISO settings is a great way to see what is possible when want different effects that different ISO settings can provide.