Canon 7D, Selecting the Proper ISO

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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 5.4 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.
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 5.6 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.
FIGURE 5.7 The grain in this ISO 3200 image is very noticeable, detracting from the quality of the photograph.