You may have heard the term ISO, as it’s one corner of the ever-so-important exposure triangle.

But what is it exactly? And how can you best put it to use, set it for incredible image quality, and use it to achieve maximum story impact? We’re glad you asked because that’s exactly what we’re about to cover!

Before we get too far into its modern-day application, let’s take a little journey down memory lane and brush up on our film history.

The early days of ISO
The acronym ISO stands for the international organization of standards (except someone sneakily flipped the O and the S). The term arose in the chemical film days, and refers to the numerical measure of the sensitivity of film stocks. The short of it is that a lower ISO — for example, 100 — is less sensitive to light, and a higher ISO — like 400 and above — is more sensitive to light.

You may also see ASA thrown around as well, which stands for the old American Standards Association. ASA numbers are essentially the same as ISO numbers, just … more American? (It’s a long story, which you can Google.) But you’ll see ASA show up mostly in the motion picture film industry because they like to play it old school.

OK, now that we’ve got the terms down, let’s take a closer look at how it all operated. Back then, lower ISO film used finer, more closely packed film grain, which produced sharper and cleaner images. But these tightly packed grains needed a bigger punch of light to react (either a brighter scene, wider aperture, or slower shutter speed). These are referred to as slow film.

The higher ISO films used larger, more loosely packed grains and reacted to light faster — thus known as faster film — which allowed for faster shutter speeds, closed-down apertures, and the ability to shoot with less light (like the great indoors). But this gave less sharp, clean results, and the grain was more visible in the resulting image. Sigh!

So that’s the basic background. A+s all around. Now, it’s time to switch classes, and head to electrical engineering.

ISO goes digital
There is obviously no film in DSLRs, cinema cameras such as the Canon C100, Reds, Alexas, and so on. In its place is a sensor, which is not only handy since you can adjust your ISO setting on the fly (instead of being limited to the stock that was in your camera), but they also allow you to film in just about any lighting condition.

ISO in digital cameras increases the sensor’s sensitivity to light, both by pumping more power into their sensors and by using the signal processors to brighten the image. What’s interesting is that a similar thing happens to your image quality with your digital sensor as did with film stocks. Lower ISOs will give you a cleaner image, while higher ISOs can start to look grainy and noisy.

For a digital sensor, the noise occurs because the sensor pixels start to erroneously give an electrical response to light that isn’t there, essentially giving mini errors. Once you go to a high enough ISO setting on your digital camera, you’ll start to see some yucky digital noise that makes your image look not so pretty.

If you have a higher sensor resolution and more pixels packed into one place, you’ll need more light to get a clean image. Boost the ISO of a 40-megapixel camera sensor, and you’ll get a much noisier image than a 12-megapixel sensor of the same size.

Still with us? Good! We’ve got one last techy point before we get into the fun stuff: how it all impacts story.

Native ISO
Each sensor has its own idiosyncrasies (even ones from the same manufacturer!). For example, every camera sensor has a native ISO setting, or a specific ISO value that the sensor was built for. Setting your camera to that specific ISO will give you the cleanest image possible with the most dynamic range, or the amount of detail you can capture from the darkest darks to the brightest brights. It’s kind of like the days of pushing/pulling film. While it can help with ‘adding’ or ‘removing’ light, it can also result in some pretty muddy imagery thanks to a lack of contrast and tonal range. Native ISO values vary drastically, from 100, 200, or 800 (or even 3200 such as the case with the Sony a7s). Your best bet is to Google your specific camera model’s native ISO to find it.

OK, now the fun part.

ISO = image quality + story
You can look at ISO strictly as a measurement of light. The lower the ISO, the darker the image. The higher the ISO, the brighter the image.

Here we keep our ISO nice and low at 320 and use the rest of our exposure triangle settings to balance out the scene.

And you can look at ISO strictly as a measurement for image quality. Native ISO produces the cleanest image with the most dynamic range. With non-native ISO, you get a less than perfect image with reduced dynamic range.

Now we bump up the ISO to 850, the Canon C100’s native ISO.

Or you can look at ISO as a combination of the above with the addition of the ever-so-important story. That means you want to balance exposure requirements with image quality and add in how you want the specific shot or scene to feel.

For example, if you want a clean and polished story, you could set your ISO to the native camera setting. But when your video calls for grit, you could artificially bump up your ISO to introduce noise into the image, even if you technically don’t need more light in your scene.

And here in this example we bumped the ISO up to 8000 on the Canon C100 to introduce additional grain into the image.

It’s a little tough to see the differences in compressed web images, so here’s a 200% crop of the ISO 850 and 8000 stills from above.

In our recent ‘On Set’ filmmaking courses, both Ryan Booth and Joe Simon (unbeknownst to each other) intentionally shot various scenes in their films at 1600 ISO and 3200 ISO with the combination of an ND filter to cut light.

Why did they do so, you ask? To introduce natural grain into the images to make them feel less perfect and more organic. In other words, they approached the ISO setting at not just a technical decision, but a story one.

In the end, ISO, like shutter speed, frame rates, and all other technical decisions you make are also opportunities to infuse story into your choices. Next time you go to set your ISO, think about how to get the most value from it, one that will allow you to accomplish your ultimate vision and serve the message of your film.