For me, one of the greatest advantages of shooting digital is the ability to change your “film speed” from picture to picture. Being able to adjust your ISO from picture to picture gives you incredible control in changing light situations. I used to shoot slides, and usually bought Kodachrome 64. It was beautiful-when there was enough light. When it got a little dark or you went indoors you had to find a flash or hope things would stay still long enough to shoot. Back then you had to choose between wasting the rest of the roll and switching to the right film or just really pushing it and hoping for the best. ISO is just a measure of how sensitive the film, and now the CCD sensor in our cameras is to light. Lower speeds such as 100 or 200 mean that your camera needs more light for a well exposed image. Higher speeds such as 800, 1200 or more mean the camera can shoot in much lower available light.
So why wouldn’t you want to keep your camera cranked up as high as it will go? The answer is image quality. At the higher speeds, the pictures start showing a lot of grain or noise. Here is an example of one of the first shots I made trying out the low light capability of my camera. It was shot inside, at EPCOT center’s Mexico. I cranked the ISO up to 1600 and rested the camera on a wall. It looks ok, but when you look at the detail, it has lots of big chunks in the sky and in the shadows. That is usually where the noise will show up.
There are times when you want to increase the speed and it doesn’t hurt the picture. If you want to shoot at a fast shutter speed, like at a sporting event, you might have to increase the ISO until your camera has enough light. Here is one from a Washington Nationals game, shot from way up, at night, with a long lens. I set the ISO to 1000 and was happy with the results. Not the game as unfortunately the NATS are truly terrible, but the image.
All cameras have a designed best ISO and it can be found in the manual. For my Nikon D300, it is ISO 200 and that is where I try and keep it set. Cameras now will give you the option to set auto-ISO. You can get some pretty funky settings on when Otto goes to work—ISO 346 for example. I recommend that you turn that off and control it yourself. Set up your camera and take multiple images of something, changing the ISO for every shoot. Process them and see when you can start to tell the difference. Sometimes you might want the effects, but mostly we want good sharp pictures.