Understanding ISO in Photography

ISO is one of those confusing topics in photography, so I am hoping to clear up a few things in this article.  I will cover what ISO really is, where it came from, how to say it, and how it affects our images.  

What is ISO in Photography?

We have all kinds of buttons, knobs, and settings on our camera.  They can be very intimidating as a new photographer…or even as an experienced one!  As technology advances it seems the features keep piling up. 

When it comes to exposure though, the basic principles don’t really change.  If our photo is too bright, we’ve overexposed and need to stop down.  If it is too dark, we’ve underexposed and need to open up.  It is pretty straight forward. 

There are several tools available on our cameras to help us get a good exposure.  We have the built-in reflective light meter that gives us a good starting point.  It basically tells us if we are going to be over exposed, under exposed, or properly exposed based on the light in the scene and our current exposure settings.

Along with our light meter we have different metering modes.  These allow us to control where and how the meter evaluates light.  Some common metering modes are multi-segment, center-weighted, and spot.  Metering modes act almost like rules for our meter.

In addition, we have the histogram, that uses actual data to tell us if we got or are going to get the desired exposure.  It’s a great tool that lets us know if we are clipping our highlights and/or our shadows.

Of course, the most important tools we have when it comes to exposure, are the exposure controls themselves.  There are three settings that allow us to control the brightness of our photographs.  The Aperture, Shutter Speed, and ISO.  

Often referred to as the Exposure Triangle, these three controls all work together to get us an exposure.  It’s very much a give and take relationship between the three, however, it is important to understand that only the Aperture and the Shutter Speed have any effect on the actual light entering the camera.  

The Aperture is the opening in the lens.  The size of this opening is determined by the position of a set of aperture blades inside the lens.  The blades can be opened or closed by changing the f-stop setting.  The aperture is a physical control that allows light to enter the camera.

The Shutter Speed is also a physical control (in most cameras).  The shutter is a set of curtains that opens for a specified length of time (Shutter Speed) and allows light in to hit the camera’s sensor.

Again, both the Aperture and Shutter have direct control of the light.  ISO in photography is a bit different as it really has nothing to with the actual light entering the camera.  For now, we are going to over-simplify what ISO is by saying it is the sensitivity of the image sensor.  Meaning, as we increase the ISO setting the brighter our exposure gets. 

It’s All in the Name

Ok, so we have these three different exposure controls or settings.  The terms “Aperture” and “Shutter Speed” make sense and are good descriptions of these controls.  The word “Aperture” means “an opening, hole, or gap.”  And that is exactly what it is in relation to photography.  The opening in the lens is called the aperture and is measured in f-stops.

“Shutter Speed” is also a descriptive term that makes sense.  The shutter curtain(s) open and close for a specified period of time.  The speed is how long the shutter is open and exposes the sensor to light.  Shutter speeds are measured in fractions of seconds and whole seconds.

Alternatively, we have the term “ISO.”  If we continue thinking in terms of the camera’s sensitivity, it’s not a very descriptive name.  In fact, it gives us no indication of what this setting does!

So, where does this term “ISO” in photography come from and what does it mean?  “ISO” is derived from the acronym “International Organization of Standardization.”  It originates from the days of film photography when different films had varying sensitivities to light.  Back then, various film manufacturers had their own sensitivity standards.  To standardize these sensitivities and make it easier for photographers to understand and compare different films, the International Organization of Standardization (ISO) developed a set of standards for film sensitivity.  

What they did is assign numerical values to different film speeds.  Lower numbers represented low sensitivity to light (slower films) and higher numbers were assigned to higher sensitivity films (faster films).  For example, ISO 100 is less sensitive to light than ISO 800, which makes ISO 800 a better choice for low light conditions.

When digital cameras came about, many of the same terms were carried over from film, including “ISO”, making it easier for photographers to understand the concept.  Now, instead of ISO relating to film speed (sensitivity) it represents sensitivity in digital cameras.  

Many people that photographed with film may remember that the ISO setting on the camera was called “ASA.”  The ASA was the American standard for film sensitivity.  ISO and ASA are essentially interchangeable in meaning.  The ISO standards, which are now global, replaced the ASA standards.

How do we say it?

As I said earlier, “ISO” in photography is derived from the acronym “International Organization of Standardization.”  So…if it’s an acronym, shouldn’t it be “IOS?”

This is what the actual Organization has to say on the matter:

“Because “International Organization for Standardization” would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalization), our founders decided to give it the short form ISO.  ISO is derived from the Greek “isos”, meaning equal.  Whatever the country, whatever the language, we are always ISO.”

I guess that clears that up, right?  It’s not really an acronym at all, rather a word derived from the Greek “isos.”  So if it’s a word, that tells me the proper way to say it is eye-so.  With that said, there seem to be some strong opinions amongst photographers on the topic.  Many will insist it is eye-ess-oh and others eye-so.  Bottom line…both ways are perfectly acceptable. 

What is ISO Really?

Typically, we refer to ISO in photography as the sensitivity of the camera’s sensor to light. So, does that mean we can actually change the sensor’s sensitivity by just spinning a dial or changing a setting?  The short answer…no…not really…well kind of…

 The camera’s sensor sensitivity to light is a constant and we can’t just change it when we need more light.  What we can do is turn up the volume so to speak. Here’s a very brief explanation of how our digital cameras work.

When we take a photograph, light from the scene passes through the camera’s lens through the aperture.  The amount of light that passes through the aperture is based on the f-stop setting, which is just the size of the aperture opening. 

The light then falls onto the image sensor, which consists of millions of tiny photosensitive elements called pixels or photosites.  The job of the photosite is to collect light.  For the entire duration the shutter remains open (shutter speed) the photosite is collecting light.  During this process the photosites create an electrical charge.  The amount of electrical charge at each photosite is directly proportional to the intensity of the light hitting it.

When the exposure is complete (shutter curtains close), the accumulated electrical charges at each photosite are converted into a digital signal by this thing called the Analog-to-Digital Converter (ADC).  The ADC measures the strength of the electrical charge at each photosite and assigns it a corresponding digital value.  The digital values from all the photosites together form the digital image data, representing various shades of colors and brightness levels in the photograph.

Now this is where ISO FINALLY comes into play!  The ISO setting adjusts the sensitivity of the image sensor to light.  If we increase the ISO setting, we are effectively amplifying the electrical charge read by the ADC.  In other words, we are boosting the signal from the photosites, which is akin to applying an electrical gain.  This makes the image sensor more responsive to light.

So yes, we are increasing the sensitivity of the camera’s image sensor to light, but we are doing it by cranking up the electrical gain.

ISO and Image Quality

On the surface, ISO seems like a great thing!  If we need to increase our exposure in low light situations, we can just crank up the ISO right?  Of course we can, but it comes at a price.  That price is in the form of image quality.

So, here’s the deal.  The only reason we ever absolutely need a higher ISO is in the event we need a faster shutter speed.  Whether that is because we are handholding and want to avoid camera shake or we want to make sure we are freezing any motion in the scene.  If we get to a point where we’ve gone as wide as we can with our aperture and we still need a faster shutter speed…this is the time we increase our ISO.

The reason we want to be careful with the ISO setting is because as we increase it, we start to introduce noise into our image.  As mentioned earlier, this is where our image quality starts to take a hit.  This is not the cool retro film grain look some of us crave.  Digital noise is ugly.   And I should be careful here when I use the word “introduce.”  Increasing ISO does not “introduce” noise, it just amplifies what is already there lurking in the shadows.  Simply put, noise exists because there was not enough light collected by the photosites, which is why it exists in the shadows particularly in low light conditions.

The best analogy I’ve heard on this topic is that of a radio signal.  Let’s say we are in our car listening to music on the radio.  When we are driving through a city or town where the radio station is being broadcast from, we get a really good signal. We can crank up the tunes fairly loud and it still sounds pretty good.  There is a good signal to noise ratio (SNR).  SNR is calculated by comparing the strength of the signal to the noise present.  A higher SNR indicates a stronger, cleaner signal relative to the noise.  In this case, better sound quality. 

Back to our radio analogy.  As we drive farther away from where the radio station is transmitting from, the signal starts to fade and it doesn’t sound as good.  When we turn up the volume, the louder we go, the worse the sound quality gets.  It gets distorted and hissy and just doesn’t sound good as there is now a lower signal to noise ratio.

The exact same thing happens in photography!  Just think of the radio signal as light.  When there is enough light, we have a higher SNR.  This means there is a stronger, cleaner signal relative to the noise, resulting in higher image quality.  The opposite is true when there isn’t enough light, having a lower SNR, which results in lower image quality.  Thus, as we increase the ISO we are just amplifying that noise that exists due to the lower SNR.

Should we be scared of ISO?

Does that mean we shouldn’t increase the ISO?  Absolutely not!  The potential for introducing noise into our images can make us really nervous about increasing ISO, but there are many situations where we just can’t avoid it.  

As I stated earlier, we should increase our ISO any time we need a faster shutter speed.  For example, if we are photographing indoor sports, we should fully expect to use a high ISO.  It’s a combination of low light and fast action.  We are going as wide with our aperture as possible to allow us to use a fast shutter speed.  In these situations, our aperture just won’t go wide enough to get us enough light where we can use a fast enough shutter speed to stop the action.  We need a faster shutter speed…we increase the ISO.

It is more important to get the shot without motion blur and have noise than it is to get an unusable blurry photograph.  Noise can be dealt with in post-processing.  Honestly, most editing programs these days do a very good job of removing noise especially in the age of artificial intelligence (AI).

On a very important related note, it is better to get a good exposure in camera using a high ISO than it is to under expose it and try to fix in post-processing.  If we under expose an image because we want to keep our ISO low, then go into Lightroom and bring up the exposure, we are going to see all kinds of noise.  It is going to be much harder to fix than if we had just used a higher ISO and got the proper exposure.  Any time we try to increase the exposure in the shadows during post-processing we are going to enhance the noise and make it more obvious.

Bottom line don’t be scared!  ISO is an exposure tool that is available to us and plays a vital role in us getting those low light photographs.  Every camera is different when it comes to ISO and digital noise, and every photographer has different tolerance levels of ISO thresholds and noisiness.  As technology advances, the sensors just keep getting better and better as does the de-noise capabilities of the post-processing software out there.

If you are new to all of this and want to learn more about exposure, check out our online Intro to Manual Exposure course!


  • Rob Gappert

    I love my family, I love Montana, and I love photography. And according to my wife, they are not always necessarily in that order… I enjoy sharing my experience through my photographs. I primarily shoot nature and landscapes, but I’ll pretty much a take a picture of anything that catches my eye. You will see by looking at my work that I like bold, vibrant colors. I try to photograph, what I would consider, “untouched nature”. I struggle to incorporate people in my images, it’s a problem I’m trying to overcome. I love to travel, but I don’t believe vacations are for rest and relaxation. I’ve found that the great majority of tourists tend to sleep in, so it’s really not that difficult to find solitude in the most beautiful of places. I have always been and always will be a follower of light.