How to Use the ISO Feature in Cameras

"Really what you need to know about ISO is it's measuring the film sensitivity to light. There's a couple phrases that are used around this. You'll hear somebody say, "Oh, that's a slow film". Well a slow film has a low ISO rating, which may be as low as 50 or 100. Those are typically the slowest. Those are for portraits or for still lifes, where your image really isn't moving and it's extremely well lit, because that film is not as sensitive to light, so it needs to be exposed to the light longer. On the other end, there are fast films. Fast films have a very high ISO rating, maybe 800, 1600, some can even go as high as 3200. Those are used when you don't have a lot of light and you need the exposure to be quick so you can't leave the shutter open for long periods of time, so you use this higher speed film. Or, if you're shooting a sporting event and you want to stop someone in action a high speed film, again since the shutter doesn't need to be open as long, can open and close and freeze that person in the frame. With that, though, you have to make some choices. With a low ISO...On an actual piece of film there's these silver halide crystals. On a lower rated ISO, like 50 or 100 speed, there's virtually no sign of these crystals on the printed image. Where in a higher ISO there tends to be very visible what they call "grain". The way this translates to digital is pretty similar. You have to make a sacrifice. If you're shooting a portrait, you want to make sure your camera is set at a low ISO, and if you're shooting sports you want to change the ISO setting on your camera to a higher ISO. But the same kinds of things happen, you'll get a clearer, sharper image at the low ISO under the right lighting conditions, and at the higher ISO you may start to get what's called "noise". It's not exactly the same thing as "grain", because the grain was actually the little silver halide crystals. Where now it's more a disruption with the way that the sensor absorbs the light. So it can give you something similar to grain, but it's not exactly the same thing. But there's trade-offs, so you have to decide what's best for the type of image that you're shooting."