This is part three in a series I started awhile ago. If you haven’t read the Av (Aperture) and Tv (Shutter Speed) posts yet, feel free to do so now.

The Short Version

The Av setting controls the aperture, or the size of the hole that lets light hit the sensor on your camera. The Tv setting controls the amount of time the sensor is exposed to light. ISO is the speed at which the sensor collects light in order to form an image.{C}{C}

History

Remember back in the day, when everyone shot film? You’d go to the store and pick up 100-speed film to shoot outside on bright sunny days, or 400-speed film if you were going to be shooting indoors. My main issue with this is that I might start a roll outside in the sun, but would want to finish the roll somewhere with less light. Sure, I could just waste the rest of the frames on the roll and switch film, or try to rewind the roll just enough to be able to take it out of the camera, (remembering which frame I was on when I put it back later.) Most of the time, though, I’d just shoot what I had in the camera and take my chances.

This is one of the reasons I love digital photography. No more wasting film or taking chances. You can tell your camera what ISO to use any time you want.

100 ISO is still great for brightly lit conditions, and 400 is still good for most indoor photos. Today’s DSLR cameras go far higher than 400, though.

Why not shoot everything at 400 ISO or higher? Speed comes at a price. The higher the ISO, the grainier the image.

Trinity

The trick is to balance all three; ISO, Aperture, and Shutter Speed all work together to create an exposure. There’s no one right way to do it. It comes down to what you’re shooting, and what your priorities are.

If you’re shooting portraits, and need to have a pristine, noise-free photo, go with the lowest ISO possible. Open up your aperture if you have to, lower your shutter speed (within reason–camera shake is worse than grain.)

If you’re shooting sports, and you can live with a little grain as long as you get the shot (and need to use high shutter speeds to stop the action,) crank the ISO. Sports often require a smaller aperture (for a larger depth of field,) so experiment to find the best ISO/Aperture combination that still gives you the shutter speed you need.

Post Processing

The good news is that many DSLRs, and photo editing applications have a noise removal feature. Lightroom’s noise reduction is one of the best I’ve used. So even if you do have to shoot at high ISOs, you can still eliminate a good amount of grain from the final image.

Homework

Try it for yourself: Take a picture at 100 ISO, and again at your camera’s max ISO. Load these pictures on your computer and zoom in. You’ll see the grain in the high ISO image, especially in the dark areas.