ISO and Noise; What They Are and Why You Need to Know

Unfortunately, all sensors have a limit to their sensitivity. Once crossed, the images begin to suffer from a phenomenon known as noise.

Have you ever taken a photo that looked grainy and lacked sharpness? Chances are your camera’s ISO was too high. If you don’t know what I’m talking about, that’s ok. In this article, we’ll review ISO, its relation to image noise, and how it impacts image quality. After the discussion I’ll walk you through how to recreate the experiment I used in this article. We’ll cover what it is, why it’s important, and how to find it for your own camera. This discussion will help you improve your photography, especially if you are interested in night photography.

Objectives:

1. Understand the relationship between ISO and noise
2. Understand the impact of noise on image quality
3. Understand how to find the limit for your own camera

What is ISO

For film cameras ISO is a measure of the film’s light sensitivity. Digital cameras inherited the term, but ISO doesn’t mean the same thing. For digital cameras, it represents the way you can brighten or darken the photo you are taking. The sensor of a digital camera records a certain amount of light per image. You brighten the image by increasing the ISO value and darken the image by decreasing the ISO.

In the days of film you would buy different ISO rated rolls of film for different scenarios. All the photos in that roll had the same ISO value. Today’s digital cameras have the advantage of being able to change the ISO value from one shot to the next.

How is ISO measured

ISO is measured by a series of numbers, ranging from 50 to 32,000+ (these depend on camera model). The lower the value, the lower the brightness output. Conversely, the higher the number the brighter the output.

ISO 100 is the lowest value for most digital cameras. While that is the norm, some camera manufacturers allow you to widen the ISO range down to 50 if you so choose.

Just like aperture and shutter speed, changes in ISO are referred to as stops. A stop of light is either doubling or reducing by half the amount of light for an image. In the case of ISO the math is pretty straight forward. In the following chart you can see eight stops of light between ISO 100 and ISO 25600.

***Note: the “0” values in the chart just mean that there is not a change because we are comparing the same value of ISO in the row and column.

Why is this important? 

Understanding how you can control the brightness of a scene with ISO is important because you are not always shooting in the same lighting conditions. Think about your camera sensor or roll of film like your eyes.

In the middle of the day you can see everything around you. However, if it is too bright, what can you do? Put on a pair of sunglasses, right? In this situation, your eyes are too sensitive. Putting on the sunglasses is akin to lowering the ISO.

Now, what about the middle of the night? Your eyes are not sensitive enough to light to see well in the dark. If you don’t have a light with you there are few options. One is to stay out long enough that your eyes become somewhat adjusted. Another way is to turn on your flashlight. The beam from the flashlight brightens the scene so you can see. This is like raising the ISO value on the camera.

Noise happens

Having the ability to change the ISO on your camera is great, but it does have limits. Even though your camera might be able to go up to ISO 50,000 you probably wouldn’t want to print the resulting image. This is because at some value of ISO, the image quality starts to suffer from a phenomenon known as noise.

Noise is the term that describes the graininess of an image. Noise happens because the camera sensor is unable to collect all of the information from a scene. The processor then starts to fill in the blanks but doesn’t really understand how. You’ll know this is happening when you start to see red, green, and blue dots popping up and sharpness reduction.

If what you just read doesn’t make sense, that’s ok. Another way to think about noise is static on a radio broadcast. Static comes through your radio when your antenna can’t receive all of the information from the radio station. The same thing happens to your camera’s sensor. Beyond a certain value of ISO not all the information is received. Thus, your image will start to lose sharpness and you’ll start to seeing red, green, and blue dots throughout. 

In order to better understand what I’m talking about, let’s look at some examples of noise. The first image below shows a series of exposures at different ISO values. The lowest, ISO 100, progressing up to ISO 25600. The second image highlights the difference between the noise levels of ISO 100 and 25600.

WOOF – THAT’S SOME NOISE, EH?

Because lower ISO values are less sensitive to light, the images produced will show little-to-no noise and appear sharp. However, as ISO increases, the greater chance your images become grainy, or noisy. This becomes a concern for printing and displaying photos because they will lose sharpness and clarity. Does that make sense?

That’s cool for you, but what about my camera?

How to find your own comfort limit

If you are wondering “that’s cool for you, but what about my camera?” Not to worry, I’m getting there. Let’s walk through the experiment.

What do you need?

• Some subject that doesn’t move
  • In this case a beautiful cactus
• Camera, lens, and tripod

Set up

1. Place your subject in a location with relatively even light
   1. I recommend keeping the background uncluttered, this helps when you are evaluating the transition in noise between different ISO values
2. Set up your camera and tripod to compose your scene
3. Manually focus on your subject
   1. This removes variation from your experiment
4. Switch your camera to aperture priority mode
   1. Set your ISO to 100
   2. Adjust the exposure to suit your taste
5. Set your timer to 2s
   1. Depending on the lighting in the area you chose there might be some shake introduced when you push the shutter button.
   2. Setting the timer lessens the chance of camera shake while taking the photo
6. Double-check your focus
   1. Measure twice cut once

Shooting

• Take a series of photos
  • Step from 100 up to the maximum ISO value for your camera
  • I would suggest starting with whole stops first, then refine around the area you start to see noise you don’t like.
    • 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200, etc.

Review images

• Upload to your computer
  • Review full-screen images
  • Decide your limit

That’s all there is to it. Well sort of…

Next steps

The best way to know your limit for sure is to experiment with your camera’s settings at night or other low light scenarios.  By taking the time to test your equipment you’ll gain the experience necessary to improve your photography.

Thank you

Thank you very much for stopping by to read this article. I hope it helped you understand what your camera is doing when you change the ISO setting.

If you are looking for a practical way to test your own equipment in the field, check out this article on astrophotography. Reaching a balance between ISO and noise is a very important component of working at night. It takes some practice, but when you nail your first image you’ll want to go back for more.

Until next time. Thanks for reading and I’ll see you on the trail.