What is ISO, ASA and Film Speed?

If you've just started film photography, or have been doing photography for a while, you've no doubt heard the term ISO, or ASA, or film speed. This is our guide to what these terms mean!

ASA dial on Minolta X-300
Photo by Mike Marrah on Unsplash​​

What does ISO, ASA, and film speed mean?

Firstly, all these terms are the same thing. They are just different terms for the exact same thing. 

ISO is the most common and the acronym you will find on your digital camera. It is your camera's sensitivity to light. 

ASA is the exact same thing, but is more likely what you will find printed on your film camera. It stands for "American Standards Association".

Film speed is, again, the same thing. It is how sensitive your film is to light.

OK, but what do they MEAN?

Put simply, this is how sensitive your film is to light. 

Each film has a film speed. The larger the number, the faster your film is, and the more sensitive it is to light. This means you can have faster exposures in lower light because the film is so sensitive to light. We will go into this in more depth later in the article. 

You set your film camera to match the number on your roll of film. This is so that the camera knows what level of exposure your roll of film needs. On your digital camera, you can change your ISO freely as it only refers to your sensor and not a physical roll of film, so it is more of a setting that you can play with.

How do I know what my film speed should be?

This is normally pretty obvious on your canister as the number is usually displayed or included in the name of the film.

For example, Portra 400 is a 400 speed film, which means your camera needs to be set to ASA 400; Kodak Gold is a 200 speed film; Ektar 100 is a 100 speed film.

You get the point!

Film canister
Photo by Héctor Achautla on Unsplash ​​

What does fast and slow film mean?

This is simple! The faster your film, the more sensitive to light it is. The slower your film, the less sensitive to light it is.

The bigger the number, the faster the film. The smaller the number, the slower the film. 

For example, Portra 800 is a fast film whilst Ektar 100 is a slow film.

The faster your film, the grainier your images will be. The slower your film, the less grain your images will have.

Ektar 100 on a desk
Photo by Caleb Woods on Unsplash​​

How does film speed relate to "stops"?

If you have no idea what we mean by "stops", read our article all about them.

"Stops" in film speed are easy to measure as each number divides in half or doubles. 

For example, ASA 200 is half of ASA 400. Pushing your film from ASA 200 to 400 is stopping up by one stop.

Now, we've opened another can of worms, which is pushing and pulling your film.

What is pushing and pulling your film?

Here is a quick explanation.

Pushing your film means that, for example, you have loaded a roll of film with an ASA of 200, but you have set your camera to ASA 400, which means your images will be exposed as if the film is ASA 400. 

Pulling your film is the opposite. So, for example, you have loaded film with an ASA of 400, but set your camera to ASA 200. 


ISO, ASA and film speed are all terms for the same thing.

You set your camera's ASA setting to the number on your roll of film.

Fast film is grainier and more sensitive to light. Slow film is clearer and less sensitive to light.

We hope this article helped you! Please consider buying us a coffee so that we can continue to dedicate our time to helping photographers.

Max, owner of Cameras By Max

Article written by: Max

Max is the owner of Cameras By Max. They work full-time repairing and refurbishing all the 35mm film cameras you see on the website. Their favourite camera (at the moment) is the Olympus XA, and their favourite city in the world is Edinburgh.

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