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How to Understand ISO on Your Digital Camera

In this article we’ll look at how to use and understand ISO on your digital camera better.

Do you use auto modes?

Are you the type of photographer who shoots in Manual mode? Or do you go to the other extreme and use one of your camera’s fully automatic exposure modes, such as Program? If you tend to go the fully automatic route then it’s quite possible you’ve never paid much attention to your camera’s exposure settings – ISO, aperture and shutter speed.

Aperture, you probably already know, controls the depth of field. Shutter speed affects the way moving subjects are recorded by the camera. But what about ISO? ISO is a remarkable setting in that it enables you to take photos in any scene from bright sunlight to candle light. It’s thanks to ISO that your digital camera is so versatile.

How to Understand ISO on Your Digital Camera

So let’s take a closer look at ISO and what the choice of ISO setting means for your photos.

What is ISO?

In simple terms, ISO is a measure of the sensitivity of the camera’s sensor to light. The lowest ISO setting of most digital cameras is 50, 100 or 200. At this setting, the camera’s sensor is least sensitive to light. At higher settings, like 3200 or 6400, the sensor is more sensitive to light.

Note: yes it’s more complex than that – this is the simple explanation for those who are new to this setting so they can understand it better.

Where does ISO come from?

The letters ISO stand for International Standards Organization (more correctly known as the International Organization for Standards). The International Organization for Standards lays out the criteria that camera manufacturers use to calibrate the ISO settings on their cameras.

The idea is that different camera and lens combinations all produce the same results at the same aperture, shutter speed, and ISO settings.

In other words, a photo taken at ISO 400, f/5.6 and 1/500th of a second, such as the one below shot with my Fujifilm X-T1, should look the same as one taken at the same settings on your camera, whatever it is.

How to Understand ISO on Your Digital Camera

Having a universal standard is important when photographers use light meters. For example, if a studio photographer sets up the lights and uses a flash meter to work out that the required exposure is, say, f/11 at ISO 100, then it’s important to know that these settings work for any camera.

In practice, there are often variations in the accuracy of ISO settings between different camera models. But for the most part, these are minor and nothing to worry about.

How to use ISO

ISO is part of the exposure triangle. It works with shutter speed and aperture to (hopefully!) give you a good exposure for the ambient light level of your scene. One of the benefits of digital cameras is that ISO is a variable that you can change from shot to shot if necessary.

The advantage is that you can use your digital camera in just about any lighting situation. When light levels are low, you have the option of raising the ISO, in addition to using a larger aperture or a longer shutter speed, to help you obtain a good exposure.

But you need to be aware that raising the ISO has a side effect – it increases the amount of noise in your photos, especially in the darkest tones. This is not the problem that it was 10 years ago as modern sensors are very capable (amazingly so) at high ISO settings. But you do need to be aware of it.

I used a low ISO of 50 for this landscape photo to help obtain a slow shutter speed (to blur the water) and for optimum image quality.

How to Understand ISO on Your Digital Camera

I used a high ISO of 6400 for this photo (below) as it was taken indoors with a hand-held camera in low light conditions.

How to Understand ISO on Your Digital Camera

Avoiding Auto ISO

If you use Program, Aperture Priority, Shutter Priority, or Manual exposure modes on your camera you can set the ISO yourself rather than let the camera decide what it should be. I encourage you to do this as it makes you think about the relationship between ISO and image quality.

This is more relevant in low light. For example, let’s say you find yourself in a situation where you are hand-holding the camera (therefore can’t use a slower shutter speed) and need to either raise the ISO or open the aperture to obtain the correct exposure.

If you open the aperture, you’ll have less depth of field. If you raise the ISO, you’ll have more noise. You have to make a choice. What’s more important, noise or depth of field? You have control, not the camera.

For example, I made this photo indoors with a hand-held camera. I needed a shutter speed of 1/60th of a second to avoid camera shake, so I couldn’t change that. I decided to shoot at ISO 3200 and f/8 to give good depth of field. Alternatively, I could have used settings of ISO 200 and f/2 to make a photo with much less depth of field. The choice is yours!

How to Understand ISO on Your Digital Camera

Full-frame versus crop sensor cameras

Generally speaking, digital cameras with full-frame sensors create images with less noise at any given ISO setting than crop sensor cameras (that is, the sensors in APS-C and Micro Four-thirds cameras).

But, as ISO performance has increased, the gap between full-frame and crop sensor has narrowed. Image quality (noise) is not the only reason why you might buy a full-frame camera instead of a crop sensor one, but it’s no longer the major consideration it once was.

For example, the photos taken at high ISO with my newer Fujifilm X-T1 camera (APS-C sensor) easily match the quality from my older EOS 5D Mark II full-frame camera. The high ISO performance of modern crop sensor cameras is more than good enough for most photographers.

ISO for landscapes, architectural and studio photography

The best quality images (i.e. those with the least noise) are always made at the lowest possible ISO setting. You can use ISO 100 quite comfortably on a hand-held camera in bright sunlight, but it’s more difficult in low light conditions, for example at dusk or indoors.

For those situations, you can use low ISO if you have a tripod to support your camera. The tripod allows you to use long shutter speeds without having to worry about camera shake. For that reason, low ISOs are ideal for landscape and architectural photography, where it’s normal for photographers to use tripods.

Low ISOs are also good for studio photography, as most studio lights are powerful enough to give good illumination at ISO 100.

I used ISO 50 for this photo and used a tripod to prevent camera shake.

How to Understand ISO on Your Digital Camera

Don’t be afraid of high ISO

Having said that, there is no reason to be afraid of the high ISO on your camera. The key is to test your camera at each major high ISO setting (1600, 3200, 6400, 12800, etc.) to see how much noise you get in your images and what your personal tolerance is for noise with your camera. You might, for example, find that you are happy with images taken at ISO 6400, but not at 12,800. Once you’ve established that then you can work within those limitations.

Subjects such as natural light portraits, or photos taken indoors, often require high ISO settings, especially if they are done when the ambient light levels are low, such as dusk. The high ISO settings of modern cameras are a great benefit in low light conditions because they let you experiment with taking photos hand-held that years ago you could only have attempted with a tripod (and slow shutter speeds) or by using a flash to light the scene.

How to Understand ISO on Your Digital Camera

I used ISO 6400 for this photo made in a temple in China. I had no tripod and the light was so low that I couldn’t use a lower ISO.

Another type of photography that high ISOs enable is astrophotography. High ISO settings are required to get a good exposure of the night sky that captures the stars without the trailing effect created by using shutter speeds longer than 20 seconds.

How to Understand ISO on Your Digital Camera

This photo was made at ISO 6400 and a shutter speed of 20 seconds.


Recent advances in camera technology mean that many photographers can get great results from their cameras at ISO settings up to 6400 and beyond. It’s a revolution that has changed the way some photographers work by opening up the possibility of working creatively in low light conditions. But it’s also important to understand that sometimes it’s best to use low ISO settings for the best image quality.

Questions? Let me know in the comments!

Want to learn how to get perfect exposure on your digital camera? Then check out my new e-book, Mastering Exposure and say goodbye to all your exposure problems!

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Andrew S. Gibson
Andrew S. Gibson

is a writer, photographer, traveler and workshop leader. He’s an experienced teacher who enjoys helping people learn about photography and Lightroom. Join his free Introducing Lightroom course or download his free Composition PhotoTips Cards!

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