What Does ISO Actually Do? (A Simple Explanation for Beginners)

What Does ISO Actually Do?

If you've ever explored the menus on your camera, you’ve probably come across a setting called ISO.

For many beginners, this is one of those camera settings that feels a bit mysterious. You might see numbers like 100, 400, 1600 or even higher, but it’s not always obvious what changing them actually does.

Don’t worry if ISO has always seemed a bit confusing. Once you understand what it’s actually doing, it starts to feel much more logical.

The simplest way to think about ISO is this:

ISO controls how sensitive your camera is to light.

One way I often explain it when teaching beginners is to imagine a sponge.

If you place a dry sponge in water, it will soak up some of that water. But if you squeeze it slightly and make it more absorbent, it can soak up even more.

ISO works in a similar way.

A higher ISO number allows the camera to “soak up” more of the available light, which makes the image brighter.

Here's a simple example

Imagine you're taking photos outside on a bright sunny day.

Because there is plenty of light available, your camera doesn't need to work very hard. In situations like this, photographers usually use a low ISO number, such as ISO 100.

But if you're indoors or photographing something in lower light, the camera may struggle to gather enough light for a clear image.

Increasing the ISO makes the camera more sensitive to the light that is available, a bit like squeezing that sponge so it can absorb more.

In darker situations, you might see ISO numbers like:

• ISO 800

• ISO 1600

• ISO 3200

These higher numbers help brighten the photo when there isn’t much light around.

When might you increase ISO?

A higher ISO is often useful when there isn’t much light available.

For example:

• photographing indoors

• taking pictures in the evening

• capturing fast-moving subjects like children, pets or wildlife

In situations like these, increasing the ISO allows your camera to make the image brighter without needing to slow the shutter speed too much. (link this to previous blog post)

It’s one of the tools photographers use to help get a clear, well-exposed photo when light is limited.

The catch

If ISO only made photos brighter, it would be an easy choice to increase it all the time.

However, there is a small trade-off.

As ISO increases, you may start to notice grainy or speckled areas in the image. Photographers usually call this noise.

You may have noticed this already if you’ve taken photos indoors or in the evening. Because of this, photographers usually try to use the lowest ISO possible while still getting a bright enough photo.

Why ISO is only part of the story

Although ISO affects brightness, it doesn't work on its own. What Does ISO Actually Do

Every time you take a photo, your camera is balancing three important settings:

• ISO

• shutter speed

• aperture

These three settings control how light enters the camera, and they all influence each other.

Once you understand how they work together, photography becomes much easier to control because you can decide exactly how you want the image to look.

If you'd like to understand your camera properly

If your camera sometimes feels confusing or unpredictable, you're definitely not alone. Most beginners feel exactly the same way at first.

The Get Camera Confident course guides beginners through these camera settings step by step in a clear and friendly way, so you can start taking control of your photos rather than guessing what the camera will do.

FIND OUT MORE ABOUT GET CAMERA CONFIDENT

Sarah Lester is a professional photographer with over 20 years of experience behind the camera. She specialises in helping beginners feel confident using their cameras in a clear, simple and jargon-free way.

Her Get Camera Confident course has already helped more than 1,000 students start taking photos they’re proud of.