ISO, like exposure and aperture, is one of the three fundamental components of the exposure triangle. Of the three, it tends to be the photography setting that most confuses people. This is a shame since it’s not a difficult concept to understand. In fact, the ISO affects two things:

The amount of light in a scene that’s required to produce a given exposure.
The image quality and amount of noise in the image.

By the end of this guide, you will have a solid understanding of what the ISO is and how to properly control it to obtain the best possible exposure for your images. Let’s jump right in!

The basics of ISO

How many of you have heard that the ISO is an adjustment to how sensitive your camera’s sensor is to light? Well, contrary to popular belief, this is not true! Your camera sensor will maintain the same light sensitivity regardless of your ISO setting.

ISO is actually the amount by which the camera amplifies a light signal. Your camera sensor is nothing more than an electronic device that reacts a certain way to light hitting it. Aperture and shutter speed both affect how much light hits the sensor. The aperture adjusts the size of the hole in the lens, and the shutter speed adjusts how long the light is allowed to hit the sensor.

When the light hits the camera sensor, it is converted into a digital signal. The ISO is a value by which this digital signal is amplified. By amplifying this signal, you’re essentially amplifying the amount of light information that’s in the scene (not really, but that’s a good enough explanation for this). This, in effect, is similar to making the camera sensor more sensitive to light, though it’s not the same thing.

This signal amplification is not without its downsides, however. Unfortunately, the more you amplify this signal, the more noise you’ll introduce into the image, and the more the image quality will degrade.

Increased ISO = Increased image noise

ISO and image quality

So, as we increase the ISO, we also increase the amount of noise in the image. The more noise in the image, the less overall image quality we have. That’s all well and good, but what kind of degradation in quality are we talking about here?

Well, it depends partially on the camera and partially on what ISO we are shooting at.

All cameras have a base ISO value. This is the lowest ISO that a camera can achieve. For many cameras, this is ISO 100, though some have a base value as low as 25 or as high as 250. The critical thing to realize is that the base ISO is going to give you the best overall image quality. Because of this, I try my best to shoot at my base ISO as often as possible. I’d say that I can do this about 95% of the time with the kind of photography that I do. This, of course, isn’t always an option and can be nearly impossible with some genres (e.g., sports or wildlife photography).

As you increase the ISO from this base value, you increase the amount of noise in the image and, thus, decrease the image quality. Let’s look at some examples.

Fairmount Falls, Louisville, Kentucky — Fairmount Falls in Louisville, Kentucky. Shot at a base ISO of 100.

A zoomed-in view of the above shot. Notice the lack of noise.

Shot at an ISO of 6400. Even zoomed out you can notice the decrease in image quality.

Zoomed in, you can really see the noise and decrease in image quality. This is why I have to stack my milky way images!

Every camera handles different ISO values differently. Therefore, I highly encourage everybody to experiment with their camera to see how it handles different ISO values. This is by far the best way to get a feel for this!

It’s also worth noting that, as a general rule of thumb, cameras with larger sensors have significantly better noise performance than those with smaller sensors.

When to increase the ISO

As I mentioned before, the base ISO setting on your camera will probably work great 95% of the time (at least for landscape photography). However, there are times when we may wish to increase our ISO to help us achieve the f-stop value and/or shutter speed that we want.

In general, there are three occasions in which we might want to increase our ISO value:

Occasion 1: To achieve a brighter exposure
Occasion 2: To increase the depth of field in the image
Occasion 3: To increase the shutter speed

Let’s take a closer look at all three of these shooting occasions now.

Occasion 1: to achieve a brighter exposure

This situation occurs when we want to achieve a brighter overall exposure in our image, but we don’t want to change our shutter speed or aperture. A great example of this situation is when you’re out shooting the Milky Way. Let’s consider this shot:

The Milky Way as shot from Chimney Rock in Kentucky’s Red River Gorge.

When shooting this, I knew I wanted to keep my aperture wide open and my shutter speed slow to let in the most possible light. The slowest I could take my shutter speed without getting star trails was 20 seconds.

This means that my aperture is locked at f/3.5 (the widest my lens would go) and my shutter speed at 20 seconds (the slowest I could go without getting star trails). I needed to increase my ISO to brighten the exposure to the desired level.

In doing this, I have increased the amount of noise in my image, but I can compensate for this by stacking the images.

Occasion 2: to increase the depth of field in the image

Sometimes, you want to increase the depth of field in an image (by adjusting the aperture) while also maintaining your overall shutter speed. This means that the only variable left for us to modify is the ISO.

Recall that decreasing our aperture (increasing the f-stop number) is how we achieve an overall higher depth of field in the image. Also, recall that this is going to reduce the amount of light that is hitting our camera sensor. As such, this scenario will require increasing our ISO to maintain the same relative shutter speed.

Do you need a refresher on the concepts of Aperture and Depth of Field? Check out my guide on the subject!

A deer in the Great Smoky Mountains, Tennessee

Consider the above shot of a deer I got in the Great Smoky Mountains. For this shot, I wanted to be able to shoot at f/8 to get enough depth of field to keep the subject sharp, but I also needed my shutter speed to be a minimum of 1/400 of a second to counteract any camera shake from the 400mm lens I was hand-holding. This meant that my only course of action was to bump up my ISO enough to make this happen (in this case, up to 400).

By increasing my f-stop, I was able to achieve the depth of field I wanted without having to adjust my shutter speed.

Occasion 3: to increase the shutter speed

This brings us to the final scenario in which we might want to increase our ISO, which is to be able to increase the shutter speed of our shot without changing the aperture (and thus, the depth-of-field). Increasing the shutter speed will, of course, result in less light reaching the sensor. As such, we would need to raise our ISO in this case.

This is a scenario that I honestly don’t encounter all that often with the type of photography I typically do, but let’s take a look at an example.

Consider this shot of an Orangutan from the Cincinnati Zoo. The Orangutan was moving across his enclosure when I took this image, which meant that I needed to increase my shutter speed to get the exposure I wanted. Adjusting my aperture wasn’t an option in this case, so I had no choice but to bump my ISO up to account for the faster shutter speed.

Where to go from here

Like so many other things in photography, the key to mastering the ISO setting on your camera is to get out and practice. Hopefully, you found this guide to be a good starting place on your journey!

You may also be interested in my guide on understanding aperture and how it affects depth of field or my other free guides to photography and wilderness skills.

I’ll leave you with a few more examples to study!