Understanding ISO | Completing The Exposure Triangle

ISO is an enabler.

ISO enables you to have a faster shutter speed in low light. ISO is there for you when you need a narrow aperture in low light. ISO completes the exposure triangle.

Just what is ISO?

I like to think about ISO as film speed. The digital equivalent of buying a roll of film in days of old.  Shooting in low light? Grab a high number. Shooting in bright light? Choose a low number? Shooting in different situations? That’s where things got trickier in film days.

Digital ISO solves that frustration. Changing your ISO in each situation gives you digital flexibility.

Remembering to change your ISO becomes the big frustration.

Here’s my work flow to get you thinking about your own.

1. Set ISO by looking at the light I’ll be shooting in

  • Bright & Sunny? 100-200
  • Outside but not so bright and sunny? 200-400
  • Inside well lit? 400
  • Inside not well lit? 800-1000
  • Indoor, very low light? 1000-6400

2. Set Aperture for my desired depth of field/creative look

3. Set the Shutter Speed for a correct exposure as indicated by my light meter (the needle thingy has to be in the middle ) *

4. Check the Shutter Speed to make sure it is fast enough for my situation.


  • Need faster shutter speed? Bump the ISO up to enable a bit faster shutter speed
  • Shutter speed super fast, more than I need? Set the ISO to a lower number for less potential noise.


5. Shoot

Remembering to set your ISO is all about getting into a routine when you pick up the camera. You will build the muscle memory and get faster at changing your settings. With practice you really will!

Let’s take a look at the “ISO what ifs.”

What if I can’t bump my ISO any higher without getting grain in my image?

Then it’s a trade off. You will need to:

  • Get more light in your scene, either artificial or natural.
  • Settle for a slower shutter speed – prop your camera on something or settle for motion blur.
  • Use a wider aperture (smaller f/ number).

What if I shoot outside with an ISO that is too high?

This happens all the time. You’ve set it high from the photos you were taking the night before and you forget to change it. Your photos may have some noise, but in bright light it won’t be quite as obvious. It isn’t ideal. Just remember to get in the habit of ALWAYS checking your shutter speed. If you are outside and it is around 1/2000 you know your ISO is HIGH!

Why can my friend set her ISO to 6400 and get beautiful images while I set mine to 1000 and have a noisy mess?

Here is where I have to admit, better equipment can create better photos (sometimes).  Newer camera models and models toward the pro-sumer to professional end of camera lines have more sensitive digital sensors capturing sharper, less noisy images at higher ISOs.

Does that mean you need a new camera? Not at all. Know the limits of your camera. Take the same photo through the full range of your available ISOs. Compare your results. Look for that spot where you begin to see noticeable noise that can’t be corrected through noise reduction software. Keep that number in mind when you shoot.

What if I just set my  camera to AUTO ISO?

It is an option. A great option for those of us who forget it each and every time. The drawback? You know the situation better than the camera. It is always going to default to a higher ISO to get a photo. You may not always want to deal with the potential noise/pixelation a higher ISO gives you.

One trick some newer cameras have is to set AUTO ISO limits. A handy little trick to tell your camera DO NOT go any higher than X or higher than X. Nice little compromise setting. You will need to dive into your camera’s menu to change the setting.

So we’ve made it all the way around the exposure triangle of shutter speed, aperture and ISO. They each work together taking cues from the others.  It’s a tight relationship.

What tricks do you use to remember how they work together?

No tricks? Still stumped by how they work? Where do you get frustrated with the three?