What is ISO?

To better understand what ISO really is, let’s recap exposure.

The light of your scene passes through the aperture and once the shutter opens, the sensor is being exposed to that light.
When the shutter closes after a certain time (exposure time = shutter speed), the process of exposure has ended.

After the process of exposure (the sensor is not exposed to light anymore), the data the sensor measured is sent to the processor in your camera. And according to your ISO setting, it adds more or less gain to that data to artificially brighten it.

So: ISO is the gain.

Common Misunderstandings Regarding ISO

1. Exposure Triangle

Oh wait, did I just say ISO is “added” after exposure?
The Exposure Triangle lists it as part of exposure?

Unfortunately it does – and that’s is the single biggest issue that causes confusion in photography.
Exposure is first of all  –  no first of all  –   F I R S T    O F    A L L – about light.
Remember, the sensor has to get exposed to the light in order to record data. But light is completely missing in the Exposure Triangle.

Instead of that light (remember, it is called scene luminance) the Exposure Triangle lists ISO, which doesn’t have to do anything with exposure because it is added after the shutter was already closed.

So most beginners learning the Exposure Triangle get confused and don’t seem to understand that image brightness is first and foremost depending on the amount of light, or in other words, the brightness of your scene.
The other settings are a reaction to that light. Aperture can actively change that amount by reducing the size of the opening. Shutter speed can actively increase or decrease the amount of light by increasing or decreasing exposure time.
ISO can’t do either, because once it enters the stage, the light has already stopped exposing the sensor.

2. ISO Changes the Sensitivity of the Sensor

Changing ISO doesn’t change the sensitivity to light as many people want to make you believe. That would require the sensor to physically and electronically change. That’s not the case. The sensor stays the same no matter what ISO you set*.
In fact ISO doesn’t even sit within the sensor, but in the processor. When you double ISO, all the camera does, is just think: “oh, let me just cheat a little. I’ll just do as if I counted twice as much light particles and double the brightness of what I just recorded.
Well, but isn’t that basically what you do when you bring your (RAW!!!) image into Lightroom or any other editing software and raise the brightness?
Yes it is. So basically, raising ISO is the first step of image editing. More on that in the more advanced part further down the page.

ISO is the gain that is applied by the processor of your camera

*Very few cameras have two native ISO settings, but the rest of the ISO settings on these cameras is still only gain.

Understanding ISO Numbers

While the ISO units with 100, 200, 320, 400 ISO are not as confusing as aperture numbers are, they are still way more confusing that they need to be. There are two ways of making ISO numbers easier to understand.

See ISO as a Factor

If you remove the two zeros after every ISO number, ISO “100” becomes “1”. See that as the gain factor. A doubling of the gain factor (2x) means the camera processor doubles the original light that it measured.

The easiest way is to remove the "00"
ISO 100 becomes gain factor 1x
ISO 200 becomes gain factor 2x
ISO 400 becomes gain factor 4x
and so on...
Gain Factor
ISO 1001x
ISO 2002x
ISO 4004x
ISO 320032x
ISO 640064x

OR: See ISO as Percentage

See ISO 100 as 100% so you take 100% of the light that is available without adding anything to it. With ISO 200, you raise the gain to double the brightness that your sensor recorded to 200%. ISO 400 doubles that again and increases the brightness the sensor recorded to 400% and so on.
Some cameras even have ISO 50, meaning that the light recorded by the sensor is artificially reduced to 50%.

The effects of ISO

1. Image brightness (not Exposure!!!)

You surely realized that ISO adds brightness to your image.

But while scene luminance, aperture and shutter speed control exposure and therefore add brightness in a natural way, ISO just amplifies the outcome created by the others.

Why is that difference important?
For two reasons. First it has a major downside, which we will discuss in a minute, and second it keeps people from understanding that the three camera settings aperture, shutter speed and ISO are not a magic mix you have to meet to get the perfect image brightness.
What they are is your (in case you use manual exposure) or the cameras (if you use any auto-or semi-auto mode) reaction to the light that is available in your scene. If there is a lot of light, then you have a lot of options. If there is little light, you need to make compromises.

Doubling of Image Brightness AGAIN!

Remember the factor above? For the fourth and last time (after scene luminance, aperture and shutter speed), we see that doubling of brightness, or in other words the concept of a “stop” or “EV”. So doubling ISO will double image brightness.

ISO graphic in bold letters


sample shot of aperture set to f/11
ISO graphic in bold letters


sample shot of aperture set to f/8
ISO graphic in bold letters


sample shot of aperture set to f/5.6
ISO graphic in bold letters


sample shot of aperture set to f/4
ISO graphic in bold letters


sample shot of aperture set to f/2.8
ISO graphic in bold letters


sample shot of aperture set to f/2
ISO graphic in bold letters


ISO graphic in bold letters


You surely remember the very same set of images from aperture and shutter speed. I raised ISO and compensated with lowering one of the other settings. If you can get the same image brightness each time, but with different camera settings, it means that…

...Stops are Interchangeable

In photography these “stops” and with it the connected doubling of image brightness is super important. It means that all the settings are interchangeable. If you want to brighten your image by one stop (double it) you can choose whether you want to do that with aperture, shutter speed or ISO.
If you want to raise one – let’s say the aperture-number to get a bigger depth of focus, you need to compensate for that with one of the other (shutter speed or ISO) in order to get the same image brightness.
I will discuss which setting to chose over the others, on the next page.

Just to water your mouth: I have created a cheat sheet for exactly that purpose.

The ISO Range

Depending on your camera, ISO often ranges from 100 to 6400 (or even lower for older cameras), which is 6 stops, or in extreme cases from 50 up to 102400 (currently), which is 11 stops. Smartphones often start at 25 and only go up to 2000, which is also roughly 6 stops.

The advantage of cameras with a max ISO of 102400 over a camera with a max ISO of 6400 is 4 stops, so 2x2x2x2, which means that after applying the gain, your image can be 16x as bright. That is particularly important when you are shooting in very low lighting conditions, for example photographing the milky way, and you need the amplified image to focus on the stars. But also in a low-light environment like a school gym. See my comparison of available light scenarios. Some people forget about that fact and get frustrated. In our Photography Beginners Group on Facebook, I often read “I am bad at low light photography”. No, you are not, it’s the camera that will perform better at high ISO in combination with lenses that let you set lower f-numbers, that make all of a difference here. See my video “can you be bad at low light photography” on Youtube.

aperture range in stops

The numbers in between the bigger numbers are third stops and let you choose smaller increments to give you finer control over the image brightness (as explained here: smaller increments of exposure). Be aware though that some entry-level cameras will not let you set these smaller increments, but will rather only let you choose the ones in bold letters above (e.g. Canon t7, t7i,…).

The effects of ISO

2. unwanted noise ("Grain")

In a nutshell (a more in-depth explanation can be found below), higher ISO levels create noise in your images. For those of you who can’t really imagine what difference that makes and what noise really looks like, here are two 100% crops of images. The left shot by “creating” the brightness with a longer exposure time (which means you use actual light to brighten the image), the other was shot by just raising the gain to “fake” the brightness.

low ISO high ISO

How to Avoid ISO Noise

That’s pretty simple: see the gain as the ambulance. Whenever possible, use the other settings to get the brightness you want. But call the ISO-ambulance in case of an emergency.
That doesn’t mean you can avoid ISO noise all together. Since
photography is all about light and light levels often are really low,
you have to see noise as a compromise. In cases where the light level is low, you may have to raise ISO to get the shot. The only other option would be to not take the image at all.

BUT!!! Don’t let anyone tell you, that you have to use a high ISO for night scenes, a low ISO for daylight and so on. If you have a tripod at hand, or even just rest the camera on a table or chair, you could just as well raise the exposure time (shutter speed) instead of using a higher ISO (remember, they are interchangeable).
On the other hand, let’s consider you are outdoors in bright sunlight and want to photograph a hummingbird. You know by now which shutter speed to use for birds in flight. Using 1/2500sec shutter speed and an aperture of f/6.3 (most tele zoom lenses won’t let you choose a lower f-number), you will get you a pretty underexposed image if you don’t raise the ISO.

A second option to avoid ISO noise is to add light to your scene. That starts by going outdoors e.g. to take pics of your dog and ends with using a flash or a speedlight instead of raising ISO. But do the latter with caution for reasons I will explain in the more nerdy section below.

Note: the following paragraph contains a referral link. If you purchase topaz software from that link, you can get a 15% discount, using the code “wolfamri“. At the same time you will help me improve this website and my free youtube photography course. Just FYI: I would never for the life of me recommend anything that I wouldn’t use and like myself!

How to Remove Image Noise (For Non-Nerds)

Why for non-nerds? Further down in the advanced ISO knowledge section, I’ll explain techniques used by enthusiasts that require taking several images and stacking them. Here I will explain easier and much faster techniques for everyday use!
First and foremost: here is my preferred way of removing ISO noise: Topaz Sharpen AI. It’s a software and believe it or not, you read right. “Topaz Sharpen AI”. There is also a software “Topaz Denoise AI“, both are rather similar, but I prefer sharpen AI, because it does a better job at getting  a perfect balance between sharpness and noise reduction. The result is much better than what you’d get with for example Lightroom. Find a comparison of noise reduction in Lightroom vs Topaz Sharpen AI below.
Talking about Lightroom. Since it is the most widely used software for image editing, it can of course also reduce noise to a certain extent, I have done a video about the best practice: secret trick of removing noise in Lightroom).

Adobe Lightroom Topaz Sharpen Ai

The effects of ISO

2. Lower Dynamic Range

While I want to mention that effect here to make you aware that ISO has more side effects than noise, I want to explain further down in the advanced knowledge, because it is not all that easy to understand and compared to noise, it is also not that prominent in your final images.

Wen you shoot an image with the same brightness, but twice the ISO, the camera actually only records half the light.
Let’s do a short experiment. Put your camera in shutter priority, set your shutter speed to 1/100, ISO to 100 and take an image. Then raise ISO to 200 and take the same shot. Do you think the camera now captured more light, or the sensor was more sensitive to light?

What is dynamic range. Dynamic range is the amount of luminance levels (EV) that can be recorded by a sensor in a single image.

Why is a higher ISO reducing that dynamic range?

The actual dynamic range of a sensor starts at the noise floor. The noise floor is a constant flow of “noise” caused for example by camera electronics. These “mix” with the signal that is coming in. If the signal is just as high as the noise, you couldn’t tell if it was signal or noise. So to be measurable, a signal has to be higher than the noise floor.

advanced ISO knowledge

The nerdy stuff

If you are a photography beginner, skip this section and and practice what you have learned so far.
Return, once you feel comfortable to learn more about shutter speed.

What Does Signal to Noise Ratio Mean?

When talking about advanced ISO knowledge, I need to mention the more sophisticated – some will say the scientifically correct – term for ISO noise, “signal to noise ratio“.
In a nutshell, the signal is the pure and unchanged “original”, which is the incoming light and noise is any deviation of the pure signal. So anything that is different than the incoming light, caused by camera electronics and other “issues”.

An image with a high signal to noise ratio has less noise than an image with a low signal to noise ratio.

  • Letting more light onto our sensor, changing the exposure, so adjusting aperture and shutter speed, will improve the signal side of the ratio.
  • A higher gain (ISO) does not change the signal, but will increase the noise part, so decrease the signal to noise ratio.

In other words: If you can use exposure to brighten the image, don’t use gain.

How Does ISO "Work"?

To understand that, we need to go back to where I explained how light exposes a sensor. A sensor is divided into rows and columns of millions of tiny pixels.

Let’s look at two neighbouring pixels from the side and  think of them (simplified) as cavities in our sensor that collect the incoming light particles.

  • The left pixel counts a lot of light particles. 
    Every cavity can only collect a certain amount of light particles until it is full. If it is 100% full, like the left pixel, the sensor knows that this part of the image is really bright – so it sets the brightest value possible, which is 100% white.
  • If the sensor gets hit by more light particles than the cavity can hold, that wouldn’t change anything because it cannot get any brighter than white. So the additional light information gets lost. That’s when we typically talk about over exposure – or clipping.
  • The right pixel on the other hand doesn’t get hit by a single light particle. The sensor knows that this part of the image is extremely dark and therefore sets the darkest value possible, which is 100% black.

Between white and black, there are obviously a few other brightness levels. The amount of brightness levels a sensor can capture in one image, is called “Dynamic Range”

As you know by now, ISO is not a part of exposure. Exposure is simply determined by the amount of light that hits the sensor being a result of light intensity x duration. The intensity is determined by scene luminance and aperture (the bigger the opening, the more light it will let in) and duration is determined by the exposure time (remember, that’s the correct term for shutter speed).
Our cameras count the light particles that hit the sensor while the shutter is open. Modern cameras can differentiate 16.348 different brightness levels per pixel and colour (that’s 14bit – an easy to understand explanation what that means can be found in my video RAW vs JPG). So the measured light per pixel is “classified” into one of these 16.348 different levels. Level 1 would be extremely dark, so all black, and level 16.348 would be extremely bright, so all white. Everything that is brighter than white (over exposed) will also get assigned to that highest level and the same happens for the lowest level with everything that is under exposed.

Let’s consider I am photographing the milky way. Before adding the gain (ISO), the sensor has recorded the pure exposure, which is the one on the left. Then the camera adds the gain and creates the image on the right.


What causes Noise?

There are many different kinds of noise but they can be basically classified in two categories: photon noise and read noise. The latter is “more important” in photography because it creates more noise than the other and it’s also the one we can much better improve on. So let’s start with that.

1. Read Noise

Read noise is “created” in your camera. Due to a number of reasons, the number of light particles neighboring pixels count, can differ, despite being hit by the same amount of light particles.
Let’s say, a few neighboring pixels are hit by 10 light particles. The logical result would be that they all count the same amount of particles – namely 10 – and output the same brightness.

no iso read noise

But due to for example heat issues, electronic interference, etc., one pixel makes a mistake and only counts 9 particles. So one pixel will be darker than the others, even though it should have the same brightness.

closeup graphic of ISO read noise

Another example for such an error would be a reset failure. Let’s say you take an image, and the sensor counts the light particles. Before you take the next shot, the camera has to reset the count of every pixel to zero. But that doesn’t always work reliably. So one pixel might “reset” the count to 5 instead of 0. In our above example of 10 light particles hitting, that one would get a result of 15 instead and create a much brighter spot than the neighboring pixels. So additionally to the above dark pixel, we get a bright pixel.

bright and dark pixels due to read noise

Imagine that happening thousands of times in your sensor that has millions of pixels and what was meant to be an exact representation of the light in your scene, suddenly has quite a few  “flaws”. The brightness differences between these pixels may not be all that visible if you take the readout as is  and don’t add any gain.

But adding gain, these differences are duplicated (ISO 200), quadruplicated (ISO 400), octuplicated (ISO 800),….. whateverplicated 🤣 or 1002x (ISO 102.400). At one time the difference between neighbouring pixels becomes very visible. And what we start to see is ISO noise. Or scientifically correct: a lower signal to noise ratio, because the signal was perfect to start out with and the noise was added by the camera.

graphic: more noise after raising iso

Read noise is actually subdivided further, for example into front-end read noise and back-end read noise. A little more on that further down when I talk about ISO invariance.

2. Photon Noise (Shot Noise)

Photon noise is very hard to explain or understand for non-physicists. But I will try to explain it briefly.
Photon noise is “natural noise”. When you photograph a scene, there is never a constant flow of light particles, even if the light doesn’t change. Instead there are tiny fluctuations that are not visible to our human eye. Camera sensors however, who have nothing else to do but count these light particles, are much more sensitive in this regard, and they record these fluctuations.

Interestingly, these fluctuations depend on the absolute number of photons emitted by the particular part of your scene, or in other words: the brightness of the part of your scene. Darker parts have more relative fluctuation than brighter parts. And so darker parts will have more noise than brighter parts. Like in our example above, these fluctuations are not all that bad if you don’t amplify then by adding gain. But once you do, they become more prominent. Particularly in the darker parts of an image.

While shot noise only contributes rather little to the noise in our images, it needs to be mentioned when talking about sources of noise.


Why is ISO noise better visible in uniform areas?

That’s pretty easy to explain: if two neighboring pixels are supposed to record the same brightness, but they don’t, you will see the difference pretty quick. If on the other hand you are shooting a very complex surface like sand on a beach, it will be hard to differentiate noise from the natural brightness difference of neighbouring sand particles.

Why is ISO noise better visible in dark areas?

let’s say in a bright area of your image, the sensor counts 1.000 light particles for a certain pixel. If that pixel is inconsistent and only counts 999 light particles, you will hardly realize at all because the difference will be 0,1%.


graphic comparing a brightness difference of 0,1%

 If in a dark part of your image, the sensor only counts 2 light particles for a pixel, and due to an inconsistency, 1 goes missing, the difference will be 50% – so pretty massive.


graphic comparing a brightness difference of 50%

I know these are simplified examples, but they should give you a good understanding why ISO noise affects different parts of your images differently.

(Why) do Bigger Sensors Create Less Noise?

Full frame, or even bigger sensors are usually better in low light than smaller conterparts. Particularly when you compare sensors of the same generation. Sensor tech evolves quickly, so comparing a 10+ year old bigger sensor to a very new smaller sensor might not give you the same result.

But why do bigger sensors have an advantage?
That’s rather easy to explain. You know that the aperture lets more light in, the bigger the opening. Well, and a bigger sensor has bigger pixels (considering that both sensors have the same pixel count – e.g. 24megapixels) and every single pixel is like the aperture. So basically, the bigger it is, the more light it can capture. I could go more into detail on pixel denisty and pixel pitch, but this might lead too far for this ISO page.

How to Remove Noise (For Nerds)

Long Exposure Noise Reduction

I’ve already talked about long exposure noise in bulb exposures on my page about shutter speed. Below you see a typical example of an image shot with an exposure time of 200sec. Despite the use of ISO100, you get a lot of extremely bright and colored pixels in the darker areas of the image.  This kind of noise can be reduced or removed by using the cameras built-in Long Exposure Noise reduction function. Where it basically takes another image with the same settings (so 200sec) but closed shutter after your real image. Then it compares the two and “edits” out the noise using interpolation of neighbouring pixels.

Dark Frames

Dark frames

ISO Invariance

For a camera that is ISO invariant (or better said were, because for now cameras just come close, but aren’t truely ISO invariant), it wouldn’t make a difference if you set a higher ISO in camera or leave the ISO at 100 and later edit your RAW (!!!) files in Lightroom. Or in other words, an image shot at ISO 100 and later pushed by 3 “EV” in Lightroom would look exactly like an image shot at ISO 800.

Why would that even be desireable? Because it would give you much more control over the dynamic range in your image. You could for example only brighten the shadows and leave the highlights untouched, while raising ISO in camera, you would always have both, brighter highlights and brighter shadows.
The closer cameras are to ISO invariance, the more leeway they give you for brightening shadows and lowering highlights in Lightroom or other editing tools.


Do ISO invariant cameras exist yet?

To some extent, every camera is ISO invariant. See the topic “Hi and Lo ISO settings” below. For these settings, every camera is ISO invariant. Other than that, some modern cameras get really, really close to being ISO invariant, while others are pretty far away. You can check how good your camera is in this regard by visiting dpreview’s awesome comparison tool.

"Hi" and "Lo" ISO settings

Having the option of shooting at ISO 50 can be a big advantage and in most cameras it neither adds noise, nor does it remove dynamic range. It’s just simply halfing the photons measured at ISO 100.
On the other end, the Hi settings are really nothing more than image editing. Just simple digital amplification (after the A/D conversion). You can just as well do that in Lightroom – just with the option of keeping the highlights better under control.

So you learned two things:

  • Longer exposure time, brighter image
  • Longer exposure time, more blur


A sensor basically counts light particles.
Light particles are like snowflakes.
The longer it will snow on a dark surface,
the brighter it will get

Interesting ISO-related Links:

Do you want to shoot images like these?

These images were all shot by me. My name is Wolf Amri, I’m a professional photographer and film maker.

On my youtube channel, I explain not only exposure, but everything else you need to know about photography. Explaining in a video is much easier than written because I can show you images, animations and examples on the way. So let me recommend watching my youtube beginner photography course.

Leave a Reply

Your email address will not be published. Required fields are marked *