Now that we have aperture and shutter speed covered, the final part of your exposure triangle is your ISO setting. If you have any experience with film photography, your ISO is equivalent to the film speed in your camera. Essentially, your camera can increase your shutter speed artificially for those moments when you need an extra boost. The more you increase your ISO, the faster your shutter speed will be.
The downfall to increasing your ISO digitally is that you’ll also be increasing the noise in your image, also known as grain. It’s the camera’s way of reaching a faster shutter speed by having some pixels vary in brightness and color, which will usually give you an odd-looking photo when viewed at 100%. In film photography (especially black and white), the grain is usually pleasing and is used for artistic purposes. However, digital noise is of lesser quality and is usually considered a bad thing.
If you find yourself in a low-light situation (indoors usually) where you’ve opened your aperture as far as it will go and still can’t get a sharp enough image, it’s worth increasing your ISO in order to capture the moment. I consider it an emergency switch to pull when I’ve already pushed the limits of my camera – use only when necessary.
You can also increase your ISO when you have to keep your shutter speed at a certain setting, such as when you’re photographing a moving subject. If you’re looking to freeze the action, you may need the extra boost from your ISO if you’ve already opened up your aperture as wide as you can.
So with this new knowledge, you can see how your ISO, shutter speed, and aperture work together to create what is called the exposure triangle. If you change your ISO, you affect your shutter speed. If you change your shutter speed, you affect your aperture and vice versa. All three settings work in harmony to create the exposure you want.
It’s best to focus more on aperture and shutter speed and how those two work together by experimenting with your camera and taking as many manual photos as you can. Sooner or later, you’ll run across a situation where you can’t increase your shutter speed, which is when you should pull the emergency switch and bump up your ISO.
Also note that your ISO performance will rely heavily on the size of your digital sensor. For example, an ISO setting of 800 will generally look horrible on a compact camera, decent on your average DSLR, and almost unnoticeable on a full-frame sensor.
Additionally, darker colors will show your ISO noise much more prominently than lighter ones. This is something to keep in mind when shooting in dark environments or if you purposely underexpose your image.
You can remove the noise later in post process, but it’s always best to have none to begin with as most noise removal processes will only soften your image, giving you a less-than-sharp final product. However, it’s always better to have a photo with noise than no photo at all, which makes ISO a great asset to have for those less-than-optimal situations.
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