What is ISO in Photography? Digital Sensitivity Explained

The concept of ISO is probably the most commonly misunderstood aspect of digital photography. 

Would you say ISO takes part in defining the exposure triangle and that by increasing the ISO value you record more light in the same amount of time? If you think so, then you are wrong, but don’t be sad: you are in good company.

So, what is ISO in photography? In the heydays of film photography, ISO was referring to the light sensitivity of film, but today, with digital cameras, ISO is simply an amplification of the sensor’s digital signal, so to mimic the effect of an increased sensitivity.

If you often photograph in low light conditions, you know how important ISO is, and this article will let you understand what ISO settings are and how to use them.

iso settings

What Is ISO And What Does It Stand For In Photography?

In photography, ISO refers to the light sensitivity (or speed) of a film roll. Higher the ISO, the higher is the sensitivity of the film.

In digital photography, sensitivity to light in both film and digital sensors depends on the way they are made. With digital sensors to increase the ISO in digital cameras means to amplify the sensor signal, thus only mimicking an increase in light sensitivity.

Without going too technical, the ISO system is a scale used to indicate the sensitivity to light of a photographic film roll.

Kodak Color Negative Film with 400 ISO
The Kodak Color Negative Film PORTRA 400 has a speed of 400 ISO.

ISO stands for International Standard Organization. The same organization behind the famous quality standard ISO 9001 and the system was adopted in 1974 when it replaced the American ASA (American Standard Association, now called ANSI) and german DIN (Deutsches Institut für Normung) classifications.

Explanation Of The ISO Numbers And Range

The ISO system defines both an arithmetic scale, the same as ASA, and a logarithmic scale, the same as DIN.  

Commonly, though, the logarithmic scale is omitted the ISO are expressed using the familiar ASA classification: 50, 100, 200, 400, 800, 1600, …

While explaining the meaning of those numbers is beyond the scope of this article, let me say that every time the light sensitivity of the film is doubled, the ISO is also double.

How Does ISO Affect Your Images?

Iso And The Exposure Triangle

The most obvious effect of changing the ISO is on the exposure. In film photography, ISO values are part of the exposure triangle, together with aperture and shutter speed.

exposure triangle
The exposure triangle, linking together lens aperture and ISO (technically only true for film rolls).

Because my partner is a musician, I often do stage photography.

If I were to shoot in a theatre with film, I would choose a roll of film graded ISO 1600 rather than one graded ISO 100.

Since the film ISO 1600 is 16 times more sensitive to light than the ISO 100, for a given aperture, I can lower my shutter speed by 4 stops of light, making it 16 times faster. 

For example, let’s say that I’m using a film ISO 100 and that with an aperture f/2.8, my shutter speed is 1/10th of a second. By switching to a film ISO 1600, my shutter speed can now be 1/160th of a second. 

photographing a scene from a play
Shooting at f/2.8, I had to use ISO 2500 to reasonably freeze the actors dancing.

The ISO In Digital Photography

Until now, I have discussed ISO in film photography, and you may think the same applies to digital photography.

But you will be wrong: in digital photography, the ISO is only an analogy of the ISO in film.

To present a technical and rigorous discussion on ISO in digital photography is not the goal of this article, so let’s try to keep it as simple as possible. 

Here what is essential for you to know:

  1. In the digital world, the sensitivity to light of a digital sensor never changes. 
  2. The specific sensitivity to light of the sensor is called native ISO, and it may not be the lowest ISO you can set in your camera.
  3. By changing from ISO 100 to ISO 200, you are not making your camera sensor as twice as sensitive to light. What you are doing is amplifying the digital signal coming from the sensor. These ISO settings are called amplified ISO.
  4. Finally, to further extend the range of available ISO, there are the simulated (or extended) ISO. Rather than physically amplifying the sensor signal, the effect is simulated in-camera using specific algorithms.
native ISO and Extended ISO values
For my Olympus OM-D EM-5 Mk ii, ISO 200 is the native ISO, and Extended ISO values begin below ISO 200 (LOW) and above ISO 5000.

Because a sensor has only one native ISO setting, technically speaking, ISO plays no role in determining the exposure of a digital image. 

Yes, you can object that also film has a single ISO value, but back then it was cheap to replace a film roll with another one of increased sensitivity. Something not (yet?) possible in the digital world.

While we can ignore the subtle difference between real sensitivity and signal amplification in most everyday photography, it becomes crucial in astrophotography.

How To Adjust The ISO

Where To Find The ISO Settings On A DSLR Camera

Every digital camera is different, but usually, you can change the ISO settings in a Menu or via a dedicated button or dial.

ISO menu settings on a Sony RX10
Adjusting the ISO of my Sony RX10.

If you are unsure, it is best to have a look at your camera user manual.

Manual Or Auto Settings?

Because images taken at high ISO are noisy, ISO is often set manually.

Auto ISO is used mostly in those situations where light conditions change very fast, such as during a concert. 

The idea behind Auto ISO is simple: concentrate on your shutter speed and/or aperture, and delegate the choice of the best ISO value to the camera. You can set a max ISO value for Auto ISO.

Should You Adjust ISO Before Or After Aperture And Shutter Speed Adjustments?

The artistic vision of the photographer should have priority. 

Since that depends mostly on the use of a particular shutter speed and/or aperture, I will set those parameters first and use whatever ISO value gives me a proper exposure. 

ISO And Image Noise

One of the first things you learn when starting with photography is that high ISO settings mean noisy images.

grainy noise example from a image
Crop from unedited RAW from my Olympus OM-D EM-5 at ISO 2000. The grainy noise is evident.

When it comes to ISO, in fact, you are trading light sensitivity for noise, but that should not stop you from using high ISO if you need to. 

As a rule of thumb, it is best to have a noisy but sharp image than a clean, but blurred one. 

Fast films with high ISO give noisier images than slow films with lower ISO because they use larger silver halide grains in the emulsion affects, giving to the photo the classic grainy look.

With digital cameras, the amplification of the sensor’s output boosts both the real signal (the scene you are photographing) and the different kinds of digital noise.

However, not all sensors are created equally. 

In general, the modern sensors perform nicely at high ISO, particularly full-frame sensors, because they have larger pixels than those found in smaller sensors.

Luminance Noise And Chrominance (Color) Noise

When photographing at high ISO, two kinds of noise affect your image: luminance and chrominance noise.

To avoid getting lost in technicalities, loosely speaking we can say that:

  1. Luminance noise is monochromatic. It has a grainy look, and it is typical of underexposed images and in a photo taken at high ISO. You can see it as a random variation of pixel brightness.
  2. Chrominance noise is typical of digital photography and looks like randomly colored patches.

The image below was taken with my Sony RX10 at ISO 12800. Both luminance and chrominance noises are evident.

oversaturated shows chrominance noise
I oversaturated the image to readily show the chrominance noise, while luminance noise was untouched.

How To Reduce ISO Noise

ISO noise can be reduced in different ways: in-camera, during editing, or with a multi-frame noise reduction technique.

Reduce ISO Noise In-Camera

If you shoot JPEG, you can turn on the in-camera noise reduction filter.

The image below compares the same photo taken at ISO 12800 in JPEG format when HIGH ISO Noise Reduction is enabled and disabled.

effect of in-camera high ISO noise reduction filter
The effect of in-camera high ISO noise reduction filter available on the Sony RX10 for JPEG.

In-camera noise reduction is not available if you shoot in RAW.

The best way to create clean images in-camera is to Expose To The Right (ETTR). This means that it is better to slightly overexpose the image. Having the histogram peaking towards the right edge.You will have less noise by darkening a bright image in post-processing rather than brightening an underexposed image.

Reduce ISO Noise In Post Production

Every image editing software out there has some basic ways to reduce luminance and chrominance noise.

Luminance and Chrominance noise reduction setting
Luminance and Chrominance noise reduction in Adobe Camera RAW.

Dedicated and more sophisticated plugins, such as Noise Ninja, Neat Image, are also available for more precise control on the noise reduction.It is important to remember that a strong noise reduction will also remove a lot of details, making the image smoother and less defined. Use these techniques with moderation.

noise reduction will also remove a lot of details
The strong noise reduction created an image with far less details in it.

Reduce ISO Noise With Multi Frame Noise Reduction Technique

Multi Frame Noise Reduction (MFNR) is the only way to reduce noise while boosting the details. In astrophotography, the process is also known as image stacking.

The idea is simple: take several images of the same scene with the camera on a tripod, and average all the images.

Because the ISO noise is random, by averaging the images, the noise is reduced, while the details are boosted.

effect of stacking 32 astrophotography images
The effect of stacking 32 astrophotography images: you can see how the random noise is reduced while the real faint stars get more defined.

What Is The Best ISO Setting For Low Light?

There is not a single answer to this question: it depends on what you are photographing, how you are doing it, and what kind of result you want to have.

Use Low ISO For Static Scene From A Tripod Or For Creative Effects.

I like to walk around in Brussels at night and take nocturnal cityscapes. 

In this case, I use a tripod, and I have no need for a fast shutter speed. Therefore I prefer to use the native ISO of my camera to get cleaner images.

Atomium at night
For this photo of the Atomium at night, I used ISO 100 to get the cleanest image possible.

A benefit of taking long exposures is to empty the streets from moving people and, with traffic, I get interesting light trails.

Use High ISO For Handheld Photography And With Fast Moving Subjects

As I said, I often photograph in a theatre for plays and concerts. Since the use of flash is not allowed, I have to raise my ISO even if I use fast lenses.

For better images, consider taking close-ups, as the size of the grains due to the luminance noise are small in comparison to important details. 

using a telephoto lens
With telephoto lenses, the details in the image are larger than the ISO noise.

Close-ups are easier to clean than wide shots, and the cleaned image retains more details.

wide scene of a play
In this wide scene, the noise is hiding a lot of details in the actors.

How Does ISO Effect Night Photography And Astrophotography Images?

In astrophotography and night photography, ISO noise is well managed by shooting for image stacking.

The main problem with increasing the ISO in astrophotography is not the noise, but the reduced dynamic range the sensor can capture.

The dynamic range, DR, is the maximum difference in exposure values, EV, between the brightest highlight and the darkest shadow of a scene. 

Why? Because when you increase the ISO, you are amplifying the signal for all tones, and pixels can “saturate,” creating the infamous highlights clipping problem.

By increasing the ISO, the blacks are pushed toward the shadows, the shadows towards the midtones, the midtones towards the highlights, and the highlights clipped to pure white.

effect of increasing the ISO
The effect of increasing the ISO. Here, a pixel can record values between 0 and 100 before saturating.

Increasing the ISO reduces the usable DR, and in astrophotography, this means the stars and the bright core of galaxies and nebulae can easily be clipped to pure white, thus showing no details and no colors.

bright stars forming the Orion’s belt
In this image, the bright stars forming the Orion’s belt and the core of M42 are clipped to pure white.

If you track the sky, the best is to use the smallest usable ISO and expose longer or stack more images together.

ISO Invariance

The smallest usable ISO is the smallest ISO for which your sensor is ISO invariant

To find what is the best usable ISO, do this test at home:

  1. On a tripod, take a photo of a library, or a paint, at high ISO so that you have the scene adequately exposed (0EV). Make sure you shoot in RAW. This is your image of reference.
  2. Halves your ISO: you will underexpose by 1EV. Take a photo.
  3. Continue halving your ISO and get photos that are underexposed by 2, 3, 4 EV, or more.
  4. In your editing software, increase the exposure of the underexposed to bring them to the proper exposure (0 EV)
  5. Eyeball the noise: the lowest ISO that produced an image that, when brightened up, does not show more noise than that in the image of reference, is the lowest ISO invariant setting and the one you should use.

This great video explains more in detail how to take advantages of ISO invariance in astrophotography


Because of their analogy with ISO in film photography, ISO settings are often misunderstood, but now you know what they are and how you should set your camera for the best results.

About Andrea Minoia

Andrea Minoia works as a researcher in a Belgian university by day and is a keen amateur astrophotographer by night.

He is most interested in deep sky photography with low budget equipment and in helping beginners along their journey under the stars.