Get up to speed with Auto ISO
How to set up Auto ISO on your DSLR for sharp shots in changing light
You’ll need fast shutter speeds for sharp wildlife shots, particularly when you’re working with telephoto lenses where every little movement will be magnified in the image.
In order to let enough light into the camera and hit those fast shutter speeds you’ll typically have to set the lens to its maximum (largest) aperture, such as f/2.8 or f/4. Be prepared to increase the ISO sensitivity too, as wild animals are often most active at dawn and dusk when light levels are low; don’t be surprised if you find yourself working at ISO3200 or higher.
The Auto option in the ISO menu automatically adjusts sensitivity to ensure a shutter speed fast enough to avoid camera shake when shooting in Aperture Priority. To prevent the ISO being pushed to image-degrading extremes you can limit the maximum sensitivity. Advanced EOS DSLRS also let you set a minimum (fastest) shutter speed of up to 1/250 sec. The camera then increases ISO to maintain this shutter speed until the light level is too low to achieve a correct exposure.