We’ve looked at how aperture impacts the depth of field of your photographs, and how shutter speed addresses motion blur. Now it’s time to add the last corner of the Exposure Triangle: ISO. Very simply, your ISO setting changes your camera’s sensitivity to light and affects how much grain or noise there will be in your photo.
If you ever shot with a film camera, the concept of ISO will be familiar. Back then, you picked the sensitivity of your film that you needed for your shooting conditions. Low light? ISO 800 or 1600 film. Bright light? ISO 100 film. Wanted to add graininess for interest? Pick that high ISO film.
Now that we are (happily) in the digital age, ISO is a setting on your camera that impacts the amount of light reaching your sensor. The higher the ISO, the more light it lets in. Each “jump” in ISO is usually double that of the previous setting. ISO works together with aperture and shutter speed to properly expose your photo.
I look at ISO as the way to almost have your cake and eat it, too. When you can’t compromise on your depth of field (or have no larger aperture to choose on your camera) and you need a certain shutter speed to avoid motion blur, your only other option for getting additional light into your camera is to raise your ISO. The only trade-off is how grainy or noisy your picture is going to be.
Different cameras have a different ISO range. My point-and-shoot goes from ISO 80 to ISO 6400. My dSLR ranges from ISO 100 to ISO 25600. Unless your camera is spectacular at dealing with high sensitivities, try to avoid going over ISO 3200. The folks over at DPReview have a great feature for comparing ISO results: Studio ISO Comparison. In identical lighting conditions, they tested different cameras to see how they dealt with ISO. Look your camera up and zoom in on the images for each of the ISO settings they provide—you’ll notice how grainy your camera will get at high ISO settings.
Here's a quick explanation of the ISO scale (again, each camera has a different ISO range):
How much does noise matter in a photograph? I firmly believe that only the photographer can answer that question for each photo. Sure, noise can be distracting and take the sharpness out of a photograph, but sometimes that is okay. Other times, we want to add graininess to get a rougher feel to the picture. Often when I sacrifice colour and convert a photo to black and white if I think it’s too grainy, I end up loving the picture more.
Play with your ISO settings and see what happens; hands-on learning is the best way to understand photography.
Original hero image via Wikimedia Commons.