We’ve already discussed setting your exposure and using a tripod; the next important factor to consider is the ISO. Since you’re already shooting with a slow shutter speed, you are free to shoot with a very low ISO, such as 100. By using a low ISO you’ll introduce very little digital noise into your images, thereby increasing the detail in your shots (Figures 5.4 and 5.5).
A high ISO will not only add grain to the image; you’ll also notice the colorful grain in the shadows. This is something you should generally try to avoid in order to get the highest-quality photographs you can (Figures 5.6 and 5.7).