Knowing which ISO is best for low light depends on your particular circumstances and exactly how much light is available. In general, cameras with a more extensive ISO range will generally operate better in dimly lit spaces, but that doesn't necessarily mean you should be shooting at the highest ISO that your camera has. As you crank the ISO on your camera, the noise within your images will increase. Luckily, most modern cameras are good at reducing digital artifacts at ISOs as high as 10,000.