ISO Sensitivity

ISO is the film's (or sensor's) sensitivity to light. A higher number is more sensitive. For example, ISO 800 is twice as sensitive as ISO 400. This means that if you switch from ISO 400 to 800, you can cut exposure time in half with the same results, or can reduce the aperture so that it lets in half the light with the same results. Importantly, increasing ISO results in more film grain in film cameras, and more noise in digital cameras. However, a high ISO image with noise almost always looks better than a low ISO image with motion blur. Interestingly, most modern digital cameras offer far higher ISO sensitivity than film cameras ever did. While film maxed out at ISO 400 or ISO 800 (for some specialty films), some modern digital cameras can give usable results at ISO 6,400 (a whole eight times more sensitive!!!). Interestingly, the original Kodachroms film had an ISO of 6. With such a high ISO possible, digital photographers don't have to carry lenses that are as "fast" as they used to. Increasing ISO is much cheaper than buying a f/2.8 telephoto that may set you back $5,000. Another bonus is that you can cange ISO on digital cmaeras at a whim, where with film cameras, you're stuck with that ISO for the whole roll of film.