What is ISO in Photography?

ISO in Photography

Back in ye old days when I had a film camera (crikey I sound old!), ISO in photography was to do with how sensitive the film was to light.

I generally used ISO 100, 200 and 400, for bright conditions, cloudy conditions and all round conditions respectively.

But if you wanted to change your ISO when using film you had to change the film!

It’s a little different with a digital camera’s ISO…

Digital Photography ISO

ISO is part of the exposure triangle along with Aperture and Shutter. By adjusting your ISO you’re changing the exposure of the image.

ISO stands for International Standards Organisation and it measures the sensitivity of the image sensor to light.

  • The lower the number the less sensitive to light the image sensor is eg. ISO 100
  • The higher the number the more sensitive to light the image sensor is eg. ISO 1600

DSLR’s have a huge range of ISO compared to film, for instance my DSLR camera goes from 100 to 1600. But some cameras have an even greater range  like a Canon 7D, which has an ISO range of 100-6400 (expandable to 12800).

Each time the number doubles and so does the sensitivity to light.

The lower the camera’s ISO the better the image quality of the photographs.

The higher the camera’s ISO the more chance there is of having ‘noise’ or graininess in the image (depending on your image sensor).

So what is noise?

The downside to this is that the higher the ISO the more noise or graininess you get in your image. Therefore if you chosen a high ISO, then the image sensor has become very sensitive to the light coming into the camera, and the result of this is an increase in graininess or ‘noise’ appearing in the image. So it can be a bit of a balancing trick because you don’t want too much noise.

The best thing to do is try out a couple of different settings to see which is best… and you can also do a few things in post processing to reduce noise.

Take a look at the zoomed in image below, the one on the left is taken at ISO 100 and the one on the right is taken at ISO 1600, can you notice the noise on the right image?

ISO comparison 100 vrs 1600

ISO 100,                                                                ISO 1600

How do I change it?

On my camera I have a little ISO button just next to the shutter button, but in some cameras it may be in a menu.

I can have it in Auto ISO setting, and let the camera choose the ISO for the image. It will try to choose the lowest one possible compared to the other exposure settings I have chosen.

Or I can override the camera and choose 100, 200, 400, 800, 1600 (see how the numbers are doubling).

In the auto ISO setting, the camera will automatically select a higher ISO when light conditions are low, but you may want to adjust this.

Why would I want use it?

So to put this into context – say you’re shooting a horse show-jumping event inside. You can’t use a flash because of the horses, but you still want a fast shutter speed to freeze the horse in action. So the only solution is to higher the ISO and make the image sensor more sensitive to light, to capture the event and take a decent photo.

This will increase the risk of noise in the image. But you can use post processing software to remove some of the noise afterwards.

So you can start to see how increasing your ISO will help you take better photos in certain situations. Like for indoor sporting events, Theatres, Concerts, Churches, Night photography or any other indoor gathering or celebration.

Have a play about and see the difference that ISO can make to your images. And take note what the camera chooses in auto ISO mode, to get a feel for the best settings and results. Here’s some basic guidelines:

  • ISO 100 – great for sunny outdoor photos
  • ISO 200 – great for dull days outdoors
  • ISO 400 ish – good for indoor with no flash
  • ISO 800+ – great for night photography


(This is an old post that I have come back to and updated as I’ve learnt more).

Comments 7

  1. Pingback: ISO digital camera experiment » The Photographer Blog

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  3. Thank you Mandy….I am a complete novice to taking photographs seriously…. I have taken lots over the years always promising myself to learn more about photography. All the techno jargon did put me off and there has to date not been anyone to explain in lay-woman’s speak….until you.. Thank you so much for this information and the comments from others on the blog. I look forward to learning lots and getting great pleasure taking photos in the future…… When i find which camera to start with!! 🙂

  4. Post

    Brenda – I’m glad you’ve found it helpful, I’m all for keeping it simple!

    As for choosing a camera, the best thing to do is go pick it up. The camera may sound great on paper but you may not like the feel of it?So see what it feels like to hold and look through, then you’ll know which one you want!

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