A lower ISO means less noise, greater dynamic range, and better image quality. As such, a lower ISO is almost always preferable — almost. ISO is only one of the three exposure parameters, and if you do not consider it in tandem with them, it can seriously affect your image quality. This excellent video tutorial will show you why you should not be afraid to raise your ISO when the situation calls for it.
Coming to you from Jan Wegener, this great video tutorial will show you why being willing to embrace high ISOs is so crucial for a range of scenarios. A common mistake beginners make is keeping their ISO too low in an attempt to raise image quality and, as a consequence, using too slow a shutter speed for the action at hand or the focal length in use. This then results in blurry images. The important thing to remember is that while you can do a lot to reduce the noise in a high-ISO image (especially with modern software), there is not much you can do to save a shot that is blurry because of camera shake or subject motion. Check out the video above for the full rundown from Wegener.