When it comes to the exposure triangle, ISO is perhaps the most technical of the three facets. It is important to know how it affects your images and why.

When it comes to exposure, ISO can be complicated. It becomes especially complex because its use and misuse varies from camera to camera and from sensor to sensor. There is no rule of thumb when it comes to ISO because every camera responds to its ISO functions differently. Some cameras can still create usable images at 25600 ISO, and others can barely reach 1600.

However, hope is not lost. Once you learn the core concepts behind ISO, you can apply that knowledge to specific cameras and quickly determine the best method for each system.

In the following video, I take a look at why you should use ISO, why you shouldn’t, and how (and why) it affects your images.

ISO is unfortunately not a magical “make brighter” button. (Although, one could argue that the more that camera technology advances, the closer we get to that being a reality.)

When you alter your ISO, it’s important to know that (most of the time) you’re losing something — whether it’s dynamic range, detail, or an overall clean image.

How Does ISO Work?

How To Properly Expose An Image: Understanding ISO — ISO

Once the light that will expose your image has passed through the iris, made its way through the shutter, and hit your sensor, an electrical signal is created. Each sensor contains millions of tiny light receptacles called photosites. These photosites then measure the red, green, and blue light and create electrical signals that your camera then interprets to create a colorful image.

Working with ISO is basically taking that electrical signal and either amplifying it or reducing it. This has multiple effects on your image. One of the main ones, and the benefit of the feature altogether, is that it increases your camera’s sensitivity to light and, therefore, the brightness of your image. However, this also increases your camera’s susceptibility to noise and digital artifacts.

Signal To Noise Ratio

How To Properly Expose An Image: Understanding ISO — Signal to Noise

Turning up your ISO is like turning up the gain on your guitar amp — taking the clean signal from your guitar and intensifying the signal to the point that it begins to distort.

This brings us to something called the signal-to-noise ratio. The signal is the pure light hitting the sensor and getting captured by the photosites, and the noise is the digital artifacting and anomalies that result from the process of creating and interpreting the digital signal itself.

Turning up your ISO is like stretching that signal or zooming into it in order to intensify the output. Therefore, you’re going to intensify the noise along with it, making it more or less visible in the image.

What ISO Setting Should You Use?

How To Properly Expose An Image: Understanding ISO — ISO Setting

There are many good reasons to alter your ISO setting. Most of the time, you’ll find that you need quite a lot of it in low-light situations. But you can also just squeeze a bit more light here and there to customize certain shots regardless of the light level.

For instance, you may find that you like the exact depth of field you get on a certain outdoor shot if you leave your aperture setting at an f10. Only problem is that your shot becomes too dark as you close your iris. Well, this is a great time to use your ISO. It’s a great way to make slight alterations without vastly affecting the image quality. However (obviously), the noise-level of your image is something to pay close attention to.

Another thing to consider is that all camera systems have a “native” or “base” ISO setting. This means that your sensor was created for use at a certain ISO, and altering that setting will either amplify it or reduce the signal from this setting. So, if your camera has a native ISO of 800, then if you shoot at a 400, you are reducing the signal. This doesn’t mean that you should always shoot using the native ISO, but generally, your camera will create the most-detailed image it can at that setting.

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