Once upon a time, back before the words “digital” and “photography” went together, photographers had to really think through what they wanted to do before buying a roll of film.

Back then, you had to know what ISO you needed for a given photo shoot because the film was rated for different “film speeds,” a.k.a. ISOs.

With the advent of digital photography, it has become easy to change camera settings to different ISOs.

So now there no longer is a need to worry about film sensitivity.

But it still can be helpful to photographers to know more about this setting and how it works.

What is ISO?

What does ISO mean in the world of digital photography?

Well, let’s first take a look at what ISO means in general.

ISO stands for the International Organization for Standardization, which is an international governing body that standardizes sensitivity ratings for camera sensors — along with many, many other things.

You might be wondering why the acronym isn’t IOS… Well, to quote directly from the International Organization for Standardization website:

Because “International Organization for Standardization” would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek “isos,” meaning equal. Whatever the country, whatever the language, we are always ISO.

Now on to more important matters. It is essential to have a standard definition of ISO because it means that no matter what camera you buy — Canon, Nikon, Sony or whatever camera producer is your choice — the ISO settings always equal out to the same thing.

So what does this mean to you?

How does ISO work?

On a practical level, by setting a different ISO, you are rendering your camera’s sensor either more or less sensitive to light.

ISO is pretty straightforward. Most cameras start at approximately 100 — which is a low sensitivity — and go up from there. Depending on the camera, the ISO ability can go into the hundreds of thousands.

Like aperture and shutter speed, ISO works in stops, with each stop equaling a doubling of the sensitivity from the previous stop.

While the expansion of sensitivity has its bonuses — mostly the ability to shoot in extremely light-deprived situations — there can be negative side effects, as well. Generally speaking, higher ISOs result in images with more “noise” or “grainier” images.

Lower ISOs also produces sharper colors and better contrast when it comes to capturing details in the highlights and shadows.

Why adjust ISO?

Keeping the ISO as low as possible in any given situation most likely will produce the best possible images.

However, sometimes a higher ISO is necessary, such as when you are trying to capture an image of a fast-moving subject.

ISO also works in conjunction with aperture and shutter speed.

Much like Newton’s third law of motion, any change to ISO results in a matching change to either aperture or shutter speed in order to achieve the same exposure.

Bottom line, the lower the ISO setting, the more light you need to expose the photograph properly.

A higher ISO setting means you’ll need less light to achieve the proper exposure.