You might notice, that somewhere on your camera is a little button that says ISO. You may have played with it a little bit and noticed that it does, in fact, have an effect on your images. But what is it actually doing? What is ISO and how does it affect your photos?

When you first start photography, exposure is one of the first things you learn about. Getting the right (or at least close to right) exposure is key to taking beautiful photographs. Sure, in the digital age we have some wiggle room, but the closer the exposure is to correct, the easier it will be to create a beautiful final image. ISO plays a big part in getting correct exposure. It can be confusing at first trying to figure out where ISO fits in with all of your other settings, so here are 7 things you need to know.

What is ISO anyways?

ISO referred to the sensitivity of film back before digital photography became popular
1/20 sec at f5.6 ISO 1000

ISO stands for International Organization of Standardization. You can read about the Organization itself here and the history of ISO in photography here. Before the digital age, ISO referred to the sensitivity of film. You could get film in ISOs such as 200, 400 or 800 and this would determine how you needed to set your aperture and shutter speed when shooting. Films with lower numbers such as 200 were great for shooting in well lit scenes, such as outdoors. Films with higher numbers like 800 or 1600 were good for shooting in low-light scenes, such as indoors or at night. There is a tradeoff however. The higher the ISO the more grain there would be.

When digital photography took off, the concept of ISO stuck around. Low ISOs in digital photography are still great for scenes with great light, while high ISOs are often used for situations without a lot of available light.

This is where ISO gets confusing… AKA Sensor Sensitivity?

Many people in the photography world like to think of ISO as how sensitive your camera’s sensor is to light. That’s what it meant in the film days, so it makes sense that it would mean that now. However, this is not technically correct. Digital cameras have a base ISO. This is typically the lowest numbered ISO your camera can go to. On my Nikon Z6ii this is ISO 100. This base ISO is where your camera will perform its best. Any ISO higher than this is essentially just artificially lightening the image. And you’ll be able to tell too. After a certain point you’ll start to see grain in your photos.

When to Worry About Your ISO

While ISO isn’t technically a component in exposure it does affect how you calculate your exposure. Typically you will want to use the lowest ISO possible for the lighting situation you’re in and the kind of photos you’re taking. This will help prevent your photos from becoming unnecessarily grainy. When you need to worry about ISO is when you can’t achieve the shutter speed or aperture (or both) that you want to capture a scene.

Shutter Speed and ISO

2 seconds at f22 ISO 100

Shutter speed refers to how long your camera’s shutter is open and allowing light to hit the sensor. This is one component of exposure. A faster shutter speed freezes motion while a slower one shows it. Your ISO setting affects your shutter speed by allowing you to change the brightness of the image without having to slow down or speed up your shutter speed depending on the situation. Okay, that’s a bit confusing, here’s an example: Let’s say you’re photographing a charity 5k. You (and probably the charity 5k organizers) want tack sharp photos of the racers. For this you will need a faster shutter speed, let’s say 1/1600. However, it’s a cloudy day and your aperture is set at f8 so you don’t miss anything focus-wise. Your photos are coming out dark. Now is when you would worry about your ISO. Bump that baby up until you’re getting the exposure you want.

Another example: Let’s say you’re photographing a waterfall. You want to get that gorgeous, silky-smooth motion blur as the water careens over the rocks. It’s golden hour so the scene isn’t too bright. You’re using a tripod so you don’t have to worry about camera shake, and your aperture is as narrow as you can make it (for me that’s f22). To get that silky-smooth waterfall, you’ll want a slow shutter speed – maybe 1 second or longer. However, the scene is coming out just too bright. Lower that ISO until you’re getting the exposure you want.

Aperture and ISO

1/320 sec at f2.8 ISO 400

Aperture refers to the tiny opening in your lens that allows light to enter your camera and hit your sensor. This is the second component of exposure. Aperture affects your depth of field. If you’re looking for a narrow depth of field where only your subject is in focus while the rest of the image is not, you’ll want a wide aperture such as f2.8 or f4. If you would like a large depth of field where everything is in focus you will need a smaller aperture such as f16 or f22. I know that aperture can be confusing. I shoot almost exclusively in aperture priority mode and have also written a full article about it. Adjusting your ISO allows you to use the aperture you need and still get the correct exposure.

An example: Let’s pretend you’re photographing a beautiful mountain landscape in the early morning hours. The sun is just starting to rise so there is some light, but the scene overall is very dark. You’re using a tripod so you don’t have to worry about camera shake, but you want everything in focus and there’s a breeze. To get everything in focus you will want a mid to narrow aperture, probably f11 to f16. The breeze means that there will be movement in leaves, branches, trees, flowers, etc. To compensate for this you’ll want a faster shutter speed, probably 1/500 or faster. Unfortunately, because it’s still pretty dark out, with these settings, your image is going to come out pretty dark. This is where ISO comes in. Bump that bad boy up by a few stops until you’re getting the correct exposure.

Putting the Three Together

1/125 sec at f2.8 ISO 200

We’ve figured out how ISO relates to both shutter speed and aperture. Now, let’s put the three together. Since this is an article called “ISO Simplified” here is how they work together depending on which setting you’re focusing on and what you’re trying to achieve:

Fast Shutter Speed: Freezes movement and works well in brightly lit situations. Requires a wider aperture and higher ISO

Slow Shutter Speed: Captures motion blur and movement (think silky-smooth waterfalls) and more light in dark situations. Often requires a narrow aperture and low ISO. And sometimes a tripod.

Wide Aperture (small f numbers): narrow depth of field which blurs background (or foreground) while keeping subject in focus. Requires a faster shutter speed and lower ISO.

Narrow Aperture (large f numbers): wide depth of field which keeps most or all of the image in focus. Requires a slower shutter speed and higher ISO and often a tripod.

Low ISO: great for bright situations, slower shutter speeds and wider apertures.

High ISO: good for poorly lit scenes, faster shutter speeds and narrow apertures.

The Tradeoff

Unfortunately, when it comes to bumping up your ISO there is a tradeoff. Grain. The higher your ISO the more grain you will start to see in your images. This can sometimes make your images look murky or muddled and not as sharp and clear as they should be. There is some good news, though. Many modern cameras, through technical things that I don’t fully understand, are able to mitigate this. For most ISOs you won’t really be able to see that there is much grain. Still it’s a good idea to shoot with the lowest ISO possible that still allows you to achieve the exposure settings you want to create your image.

ISO Simplified

ISO and how it affects your photography can be super confusing at first. It’s a powerful tool that allows you to conquer all sorts of lighting situations without having to compromise your exposure. It can greatly affect how you choose your shutter speed and aperture. By understanding how ISO, shutter speed, and aperture work in harmony, you’ll be well on your way to capturing breathtaking images.

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Hey there! My name is Leah and I'm a photographer, blogger and wanderer from north Texas. I've been doing photography for nearly a decade now and absolutely love it. My day job is real estate photographer and in my free time you can often find me at a park taking pictures of leaves and flowers. Outside of that my fiance and I love to travel. We spent nine months backpacking through Europe and now spend our free time attempting to plan and going on shorter trips both in the States and abroad.

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