When I first started out in photography, ISO was SO confusing to me. I didn’t really understand what it was, what it did, or why in the world it was important to me. Once I got it, though, I realized that ISO is really quite simple.

Sensitivity to Light

If you’ve ever shot film, you’ll remember that there were different film speeds you could purchase. You would get 100 or 200 speed film if you were going to be taking pictures outside, or 400 or 800 speed film if you’d be shooting indoors. ISO is basically the same thing. It’s just a measure of how sensitive the camera’s sensor is to light. So 100 ISO is less sensitive to light then 800 ISO.

Full sun, ISO 200 (could have actually been 100 even):

Dark room at the zoo, ISO 3200:

A good rule of thumb is that when you’re outside, you should have a lower ISO and when you’re inside you will need a higher ISO.


The other thing about ISO is that it controls how much grain is in your image. Grain, sometimes called noise, is generally unwanted in images, unless the artist is looking for a very specific look. Higher ISOs have much more grain than lower ISOs, so in most cases, you’ll want to use the lowest ISO possible for a correct exposure. This will limit the amount of grain or noise in your image, resulting in a cleaner picture.

Here are two 100% crops of the images above. You can see that the 200 ISO is completely crisp and clear, with absolutely no grain/noise. The 3200 ISO is less sharp and has a lot of grain in it.

Cheats (Aka Starting Points)

These are sort of my go-to ISOs for different situations. I start with these numbers and adjust if needed.

  • Outside, full sun: ISO 100
  • Outside, overcast or shade: ISO 400
  • Inside, natural light: ISO 800
  • Inside, dark with artificial light: ISO 1600

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