What is ISO Invariance?

All digital camera sensors produce digital noise. That noise is at its minimum when you use your camera’s base ISO. As you increase ISO, digital noise increases as well. With ISO invariant sensors, you end up with the same amount of noise whether you:

• Increase ISO to brighten your exposure.

• Keep ISO the same, underexpose the image, and then brighten it later in post.

If that sounds confusing, don’t worry! To really understand ISO invariance, we need to start with some core principles of how exposure works in digital photography.

Before I dive in, I want to emphasize that I’m not an engineer, sensor designer, or technical expert on camera electronics. My understanding of ISO invariance comes from a lot of research and testing. I’ve done my best to explain it accurately, but in practical terms that anyone can understand (at the risk of oversimplifying certain details).

To learn more, watch the video below.

https://www.youtube.com/watch?v=DV3Bi6c9-ig

If you have any trouble watching the video above, you can also watch it here. 

How Exposure Works

On a basic level, exposure is made up of several variables working together:

• The brightness of a subject or the ambient light. For example, a landscape is brighter when the sun is out and darker when a cloud passes in front of it.

• Aperture. This determines the physical size of the opening that allows light to pass through the lens to the sensor. Smaller apertures let in less light, while larger apertures let in more.

• Shutter speed. This controls how long that light is allowed to hit the sensor. Faster shutter speeds mean less light reaches the sensor, while slower shutter speeds allow more light in. (Remember that sensors record light cumulatively, which means that even if the scene you are photographing is dark to the eye, with a long enough exposure the resulting photo can end up looking very bright).

Together, all three of these variables determine how much light reaches the sensor. This matters because the more photons that hit the sensor, the less digital noise you’ll see in the final image.

That’s why photographers often recommend exposing to the right. This means giving your image as much exposure as possible, pushing the tonal data toward the right side of the histogram, without clipping highlights. Doing this gives you the cleanest possible image file. Just remember that it's important to avoid clipping important highlights because when highlights are overexposed ("clipped"), you can't recover detail.

Most of today's digital cameras do a good job at minimizing noise, so these days you don't have to be as slavishly devoted to exposing to the right as photographers used to be. And, when I am shooting subjects with particularly bright highlights, I'll often nudge my exposure value more to the left as an insurance policy to make sure I don't accidentally clip important highlights (like, for example, when photographing a bright white egret). But, generally, you'll end up with a cleaner image if you expose to the right.

Where ISO Fits In

There’s a fourth variable that affects exposure value, and that’s ISO. ISO is different from aperture and shutter speed because it doesn’t change the amount of light hitting the sensor. It's simply a measure of how much the camera is amplifying the light that the sensor is recording.

Every camera has a base ISO, which is where the sensor performs best. This is typically ISO 100 (but not always; for example, Nikon cameras often have a base of ISO 64). At base ISO, the sensor produces the least amount of digital noise and also has the highest dynamic range. Please note that many newer cameras have two base ISOs; this is called "dual gain." With dual gain sensors, performance is optimized at both of the two base ISOs (although the lower base ISO typically offers slightly better performance than the second, higher base ISO). So, for example, a dual gain camera might have 100 and 640 as its two base ISOs.

When you increase ISO, it acts as an electronic amplifier that boosts the signal (light) captured by the sensor to make the image brighter. Raising ISO increases the brightness of the image but also amplifies existing digital noise, resulting in a grainier, lower-quality image.

So, when you're increasing ISO, you're not actually adding more light. You’re just electronically amplifying the light the sensor is capturing. And the penalty for this is increased digital noise (and reduced dynamic range). With cameras with a single base ISO, performance will steadily decrease as you use progressively higher ISOs; with dual gain cameras, performance decreases as you move higher than the first base ISO, and then when you hit the second base ISO you'll see an improvement, but then it is all downhill after that.

What Does ISO Invariance Really Mean?

ISO invariance comes into play when you can’t change the amount of light hitting the sensor (for example, when your aperture and shutter speed are locked in and can’t be adjusted, and you can't control the ambient light or subject brightness). If light levels aren't sufficient to get a proper exposure, the only way to make the image brighter is by increasing ISO.

In this situation, you have two options:

• Increase ISO to brighten the image, which increases noise.

• Keep ISO the same, underexpose the image, and then brighten it later in post, which also increases noise.

With an ISO invariant sensor, the resulting noise is theoretically the same either way. This doesn't mean that there’s no penalty for underexposing your images. There’s no penalty only if increasing ISO is the only way to increase exposure. If you can adjust aperture or shutter speed to let in more light, then underexposing and pushing the file later will absolutely increase noise.

The key point is this. You reduce digital noise by giving the sensor more light without clipping your highlights. If you aren't increasing the light, you're going to end up with more noise. It's just that with an ISO invariant camera, the increase in noise is the same whether you increase your ISO, or leave your ISO where it is and increase the exposure in post.

Which Cameras Are ISO Invariant?

Many newer digital cameras are ISO invariant. Older DSLRs generally are not. Keep in mind that ISO invariance isn’t something you can turn on or off. It’s not a button or a feature you can enable. It’s simply a technical characteristic of the sensor. You'll want to verify whether your camera is ISO invariant or not before making any changes to your workflow (a quick Google search can easily answer the question for you). And also note that not all cameras that claim "ISO invariance" are perfectly 100% invariant.

Should You Change the Way that You Shoot if Your Camera is ISO Invariant?

Understanding how an ISO invariant sensor works can be helpful for advanced photographers, but taking advantage of it would require changing the way you shoot. For most photographers and most shooting situations, that’s not something I recommend, for several reasons.

• First, ISO invariance is confusing, and even some advanced photographers might find it a bit daunting.

• Second, if you're relying on ISO invariance and avoiding increasing ISO past your camera's base, you will end up with a bunch of dark images. This makes it hard to review images in real time to make sure you haven't made any technical or artistic mistakes, and it makes editing and processing photos more challenging as well.

• Third, built into the definition of ISO invariance is the fact that you end up with the same amount of noise whether you increase your ISO or not. Which means that for most shooting situations, increasing your ISO to get the exposure value you want isn't actually imposing a significant penalty on your images (see below for an important caveat to this statement). So, for most photographers, my advice is to ignore ISO invariance, and simply increase your ISO whenever you need to.

When ISO Invariance Actually Matters

ISO invariance becomes important when you’re photographing high-contrast scenes, or subjects with bright highlights that you need to preserve. When you increase ISO, you’re typically reducing your sensor’s dynamic range. Not by a huge amount, but the reduction is still there. By keeping ISO at its base level and underexposing, you can protect highlight detail and then push the shadows later in post-processing. I sometimes take this approach when doing astrophotography in order to protect highlight details in bright stars. Also, even if you don't need the extra dynamic range, if you are shooting a subject with bright highlights in dynamic and challenging lighting conditions, you might prefer to strategically underexpose as an insurance policy to make sure you don't accidentally clip your highlights. I might take this approach with subjects like the white egret example I offered above.

So, when using an ISO invariant camera in such situations, if it gets dark and you need to increase your ISO to get a proper exposure, you're better off shooting at your base ISO instead, which will result in underexposure. You’ll still get noise, but you’re less likely to clip your highlights than if you had increased ISO.

If you decide to take advantage of ISO invariance, you should shoot at your camera's base ISO for optimal performance. If your camera has two base ISOs, then you should use the one that is closest to (but below) the ISO you would need to get the proper exposure value.

Here's an example to clarify what I mean. Let's say you have a dual gain camera with base ISOs of 100 and 800. If the ISO you need for a proper exposure is between 100 and 800, then you should shoot at ISO 100. If the ISO you need is 800 or higher, then you'd switch to the second base ISO. Of course, different cameras have different base ISOs, so you'd want to make sure you know your camera's base ISOs and adjust this formula accordingly.

A Real-World Example of ISO Invariance

One situation where I’ve used this approach is underwater photography off the coast of Baja, Mexico. I was photographing high-contrast subjects with very bright highlights, such as orcas. I needed a specific shutter speed to capture the action and a specific aperture to maintain sufficient depth of field, so those settings couldn’t change.

Because my subjects were moving quickly, there was no practical way to fine-tune my settings for every lighting scenario. Instead, I set my camera to the higher of my base ISOs (for my Sony a7R V, this was ISO 320), which with my aperture and shutter speed settings gave me a proper exposure for the brightest, highest contrast situation I expected to encounter. My goal was to protect the highlights in those conditions. That meant many of my images were underexposed, since the animals were often moving quickly back and forth between brighter water near the surface and darker water down below. But, by using one of my base ISOs, overall I maximized dynamic range, and the darker resulting exposures ensured that I preserved important highlights in the water and on the orcas.

The Bottom Line

ISO invariance is confusing and complicated, and for most photographers, it’s not something you ever need to think about. For most shooting situations, it won’t give you any significant advantage, and there are practical downsides to using it. Just remember that you should increase exposure value using shutter speed or aperture (or changes in ambient light or subject brightness) whenever possible, but if you need to raise ISO to get the proper exposure, by all means go for it. For advanced shooters who understand ISO invariance and are comfortable dealing with underexposed images, then it can be a useful advantage when dealing with certain high-contrast subjects.