Canon 7D, Selecting the Proper ISO

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).

FIGURE 5.4 I used a long exposure to photograph star trails, and there was enough ambient light to capture the image at a low ISO.

FIGURE 5.5 When zoomed in to 100%, there is some noticeable grain in this ISO 400 image, but it’s still a small amount when compared with a much higher ISO.

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).

FIGURE 5.6 I used a very high ISO in order to capture the stars at a fast exposure time and prevent creating star trails.

FIGURE 5.7 The grain in this ISO 3200 image is very noticeable, detracting from the quality of the photograph.