Name: Do *you* understand ISO?
Uploaded: 2026-03-12T17:41:33.000Z
Duration: 11 min 36 s
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I thought it was just how sensitive a camera sensor is to incoming light, and

that using the lowest ISO setting possible would give you the nicest image with the lowest noise.

But I was wrong about both in a way that led me to do the exact opposite of what I should have been doing when taking photos or videos.

In fact, once you've set the shutter speed and aperture, the best practice is to use the highest acceptable ISO setting, not the lowest.

And based on how long into my photography journey it took me to learn this, I bet a lot of other people are confused too.

ISO in digital cameras is, in general, shockingly misunderstood.

At a practical level, ISO is just one of several settings that changes the brightness of an image, like the duration of the exposure, or the size of the lens's aperture, or brightening the image after the fact with image editing software.

But the ISO setting isn't like shutter speed or aperture, nor is it like digital brightening.

It has a unique place in the middle of the journey from light to image.

also a big reason to care about your ISO settings, is that images taken at high ISO settings often have a lot of ugly digital noise and related image quality issues that can be bad enough to render images unusable.

And because of the association between between noise or graininess and higher ISO settings, you'll often hear recommendations to try to keep the ISO setting as low as possible to avoid this noise.

But this advice represents a gross misunderstanding of what ISO is and how it works.

It is true that lowering the ISO setting will reduce the apparent noise in your photo, but

it will also darken the image along with it.

And in fact, in most cases, lowering the ISO setting will darken the image more than it reduces the noise, which means if you digitally brighten the image later, the noise will appear worse than if you had just used a higher ISO setting in the first place.

So don't blindly listen to recommendations to keep the ISO setting low, because a high ISO setting isn't what's creating the noise.

Also, you might have noticed I've been saying ISO setting rather than ISO.

It turns out that the setting labeled ISO in most cameras is not technically adjusting the ISO, but

I'm planning another video on the disaster.

That's the actual technical definition of ISO in digital cameras.

Look for that if you really want to dive into the weeds.

Okay, so here's what the ISO setting is doing, why it's special, and

why it works differently than many of us have been told.

Digital cameras do two fundamental things.

Digital turn photons of light into analog electrical signals and then turn those electrical signals into data.

More specifically, though still way oversimplifying.

After passing through a camera lens, light hits the photosites arrayed on an image sensor, which absorb the light and turn it into an electrical voltage for each pixel.

Those analog voltages are typically then amplified somewhat before being measured and converted irreversibly into digital data.

Data which is processed and stored in a file that can be digitally manipulated and displayed as an image on a device.

The ISO setting doesn't change the amount of light coming into the camera that's affected by what you point the camera at, what lighting you have, what kind of lens you use, how big the lens aperture is, and

And the ISO setting doesn't change how many photons are absorbed by the sensor, or how many electrons they knock free to generate voltage.

Even though for most of my photography life, I thought that's what ISO meant.

If you do want the sensor to be more or less sensitive to photons, you basically have one get a different sensor, which typically means getting a different camera.

But after the photons are absorbed and converted into a voltage by the sensor, and

before that analog voltage gets permanently digitized into ones and zeros, the that in between, time is the window where the ISO setting typically comes in.

For the vast majority of digital cameras, ISO simply means voltage amplification.

Essentially, it turns up the volume on the voltage coming out of the sensor so that it's easier to measure accurately and convert into digital ones and zeros, and

But why is amplifying the electronic signal different from just brightening the digital file afterwards?

In a perfect world, it wouldn't be different.

But the problem is the world isn't perfect, and

the process of taking analog voltages and converting them into digital ones and zeros, using what's creatively called an analog to digital digital converter.

This process can be a big source of the visual noise and inaccuracies you see in a digitized image.

If you brighten the digital file afterwards, you're brightening everything.

The image, the intrinsic noise from the sensor, and the noise added from the analog to digital converter.

In contrast, analog electronic amplifiers introduce much less noise, and

the position of the isosetting amplifier in the signal chain is crucial.

It brightens just the original image and the intrinsic sensor noise before the noise from the analog to digital conversion is introduced, so that, relatively speaking, the noise from the Digital conversion is less prominent in most most digital still and video cameras, even when shooting in raw.

Changing the ISO setting means changing the analog amplification, which allows you to make the signal going into the analog to digital converter as large as possible to minimize the effect of the analog to digital converter's noise.

Well, this is the case most of the time though, even within a single camera.

Sometimes changing the ISO setting changes the amplification and sometimes it just does digital stuff.

We can get into this at the end of the video in the Caveats.

Anyway, ISO is kind of like how if I stretch out the surface of this balloon, I can magnify the drawing on it before I take my photo and get way better results than I would by first taking the photo and then digitally amplifying or zooming in on it.

Or it's like if an audio signal is passing through a system that has a background hiss, it's best to turn up the volume on the signal before the hiss gets added, rather than after, when you'd also be turning up the volume of the hiss.

The reason so many people incorrectly warn against using high ISO settings is because images with bad noise are often taken using high ISO settings.

But this is a confusion of cause and effect.

Raising the ISO didn't create the noise, it was darkness that both required the use of high ISO and that creates a noisy image.

Images taken in darkness or with low light have noise for two main reasons.

First off, light is made up of particle like photons, and

if you have photons hitting a grid of pixels in a uniform but random way, just by chance, some of the pixels will receive more photons than others.

This is physically inherent noise in an image that you can't get rid of other than by adding more light so that the variation in the number of photons is small relative to the total amount of photons.

And second, sensors aren't perfect and there's some inherent variability in their pixel sensitivity.

Plus, pixels can spontaneously generate voltages even when complete darkness.

This is called dark noise or thermal noise, which you can see in a photo with a lens cap on.

Thermal noise can be reduced by cooling the sensor you have or using a different sensor altogether.

The end result is that a dark image coming off of a sensor will unavoidably halve noise, and

the analog to digital converter will unavoidably introduce noise.

And the goal is to brighten the image while brightening as little noise as possible.

The best way to do that without adding more incoming light is by increasing The ISO that is amplifying the voltages before the additional noise from the analog to digital converter is added.

Essentially, if you set the ISO to be very high, the image itself, together with the intrinsic noise from the sensor, will dominate over the noise from the analog to digital conversion and result in an image that's mostly the image.

But if you set the ISO to be very low, the digital noise introduced from the analog to digital conversion dominates.

So from an image quality perspective, it's almost always going to be better to use a higher ISO setting rather than leave it low.

If you have no other options to get more light, the darkest shadows of an image will have their noise reduced by turning up the ISO.

In fact, some people advocate for overexposing photos or videos by turning up the ISO as much as you feel comfortable and then darkening them afterwards to get the best image quality.

This requires both knowing how not to actually and irretrievably overexpose the image and how to correctly darken it after the fact, which is easy if you're recording RAW file types, but

it's more difficult or potentially impossible if you're not.

And there is one scenario where using a high ISO setting can cause an image to be too noisy.

If you set the ISO first, then adjust the aperture and shutter speed afterwards.

In that case, the high ISO makes the image look brighter, which incorrectly encourages you to change the other settings to compensate and reduce the amount of light hitting the camera's sensor.

And that low amount of light causes noise.

Okay, so what are the practical implications to actual real world photography and videography?

If your image is too grainy, here are the steps to take.

The first step is to add more light to reduce the intrinsic noise from the sensor capturing the image.

This could mean lighting your subject better, or increasing exposure time or the size of the aperture in your camera settings.

Once you've decided on the amount of light you're going to let hit the sensor, step two is to amplify the voltage coming from the sensor.

And to amplify that voltage as much as you can without making the image so bright that important areas become irretrievably overexposed.

I should be very clear that you don't want to just arbitrarily, arbitrarily set the ISO setting as high as possible.

Because even though you'll gain detail in the dark parts of the image, you'll irretrievably lose all information in the bright parts that overwhelm the analog to digital converter and are just recorded as pure white.

My favorite way to do this is to set the shutter speed and aperture manually, but

That way I can make the creative choices I want with shutter speed and aperture.

Maybe I want a fast shutter speed to capture a bird in flight or a small aperture so that more of the image is in focus and then the camera automatically adjusts the ISO as high as possible given those settings and the light available without overexposing the image.

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Do you understand ISO?

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