ISO is one of the three components of the Exposure Triangle. Knowing what ISO is and why it is important is key to understanding how to take a properly exposed image. ~CB
Learn Photography: Understanding ISO
What is ISO?
To understand what ISO is, let’s start by talking about your camera sensor. The sensor is the electronic component in your camera that records the image after you press the shutter release. It is the digital equivalent to film.
Setting how sensitive the sensor is, determines how fast the image is captured and converted into an electronic file. Plain and simple, ISO determines how sensitive your camera sensor is to light.
Altering the ISO setting has an impact on both Aperture and Shutter Speed, as well as the quality of the image. This interaction between ISO, Aperture, and Shutter Speed is the Exposure Triangle in itself.
As a quick aside, notice we are talking about light again. That is not a coincidence because there is one truth, photography is capturing light.
We know now that ISO is how sensitive your sensor is to light. What does that mean? Let’s begin with an example before talking about the technical aspects of ISO.
One of the things I like to do in my free time (besides photography) is to go hiking. I started into photography as a way to get myself out of the house and onto the trails more. When I go hiking, I can alternate my pace, and it determines the amount of time that I am on the trail, as well as how far I travel.
It also determines what I see and experience along the way. If I am strolling along at a leisurely pace, I tend to see just about everything as I walk. I see each tree, the texture of the bark, as well as its leaves and shape. I see the scene in great detail, and I can tell you all about what I see.
All this comes at the expense of how far I will travel on my hike, however. If I quicken my pace to cover more ground, I will not have as much time to soak in the scenery and will miss some of the details. The faster I go, the more my head is down looking at the trail, and I see less and less.
Let’s Apply Our Example to ISO Now
This is exactly how I like to think of ISO. At the low numbers (50 or 100 on most cameras), your sensor is not as sensitive to light as it will be at the higher numbers. Since the sensor is less sensitive, it takes more time to process the light in the scene. With this comes more time to accurately soak in the scene and the sensor can easily process all of the information and yield a crisp photo.
Noise: Mistakes Made by Your Sensor
As the sensor becomes more sensitive, the time it takes to capture the image becomes less. However, because the sensor is working faster than before, you lose accuracy.
As you increase the ISO into the higher ISO settings, the sensor can encounter situations where it does not have enough time to capture all of the information necessary for the image. When this occurs, the sensor will guess and place a pixel (a dot of color) in these gaps. This is what you will hear other photographers call “noise.”
Noise first appears in the darkest parts of your image, appearing as odd colored dots, (typically green or red), that makes your picture look grainy. With some cameras, it can also appear as a consistent pattern of colored bands in the image as well. My Canon 5D Mark II creates a checkerboard pattern of green lines in real low light situations.
The good news is that there are excellent computer programs that can recognize and remove the noise from your images. (That’s a talk for another day.)
Take a moment and look at the ISO settings available on your camera. Most cameras start at 100, then progress along a doubling pattern, so 200, 400, 800, 1600, 3200, 6400, 12800, etc. (Depending on the make of your camera it may have additional settings between these values as well.)
I am pointing out this doubling because the sensitivity of your camera sensor will also follow this pattern as well. By changing your ISO from 100 to 200, the sensor is now twice as sensitive as before and will allow the image to be captured twice as fast.
A Balancing Act!
There is a clear tradeoff between increasing your ISO setting to allow for faster image capture and the noise that comes with it. Determining the correct balance is something that you will figure out over time. Shooting in many different situations at different light levels and ISO settings will help you determine exactly how your camera will behave.
As photographers, we are sometimes challenged by scenes that have less than ideal lighting conditions, often with low or little light available. Being able to adapt to those situations is the difference between an ok image and a great picture.
There is a rule of thumb in regards to ISO and how to use it. You should always shoot at the lowest possible ISO that the light in the scene allows.” That isn’t necessarily a “how to” manual is it?
I always start with my ISO at its lowest setting (100 in my case), then increase it only if I have to, to get a proper exposure with the shutter speed and aperture I want to use. I will go into more detail when I talk about Aperture, Shutter Speed and how it all comes together in the Exposure Triangle. More information is on the way!
When to Consider Increasing Your ISO
In the meantime here is a short list of situations that might need an ISO correction to be able to shoot a properly exposed image.
* Indoor shots, especially those lit with artificial light.
* Cloudy days.
* Indoor sporting or concert events.
* Subjects that are in the shade.
* Underwater photos.
Do you see the pattern yet? Changing your ISO becomes an option whenever you are not in direct sunlight.
My Challenge to You
Take your camera and shoot in low light. Set your ISO at the lowest setting. Then shoot the same frame, but increase your ISO to the next highest setting. Do this for all of your available ISO settings. Study the resulting images to see how the quality of the picture changes with as you increase ISO settings.
Wrapping it Up!
This wraps up our first step in talking about exposure and how ISO, Shutter Speed, and Aperture (and their interaction) determine it. As I wrote this post, it became apparent to me just how intertwined that these three components are, and how difficult it is to explain one of them, without leaning heavily on the other two.
Next, we will move onto Shutter Speed. I will explain it, as well as point out how ISO interacts with it as well.
As always, if there are any questions or comments you might have about today’s post, please add them to the comment section below, or you can email me. You can also find me on Facebook, and Instagram.