What is ISO, Understanding The Basics

January 8, 2024

What is ISO


  • ISO- International Standardization Organization
  • Noise- unwanted random signals (in this case photons) of light striking your sensor

Lets start with something right away. ISO means International Standardization for Organization (ISO). It is an organization that creates standards that are universal across brands. In 1979 they created a standard that measured the reaction time in which the chemicals on a film strip reacted when exposed to light. ISO was the named dubbed and it stuck. Now it refers to how sensitive a camera is to light. Lets talk about that for a minute so you have a better understanding (and me too).

What did ISO mean back then?

The lower the number the slower the reaction that occurred on the film strip, the higher the number the faster the reaction. Simple and straight forward. This gave reference to the speed of film. Fast films, those that developed quickly, had a higher number and vis versa. So if you subject wasn't moving it was generally better to use a slower film but if they were, you used faster film. This standard is still used in film but the name for it has been coopted for digital cameras today.

ISO Today

There are no chemical reactions occurring in your camera, so the chemical reaction standard created by ISO is technically a name hold over. Photoelectric effect is not nearly as easy to say as ISO and does not have a history with photographers making the transition to that phrase not particularly useful.


ISO today is the name given to how sensitive a camera is to light. Since I have a science background, seeing the word sensitive, I instantly wanted to know what this actually meant, because having the right word has power. In this case, it probably refers to how effective a sensor (a semiconductor) transfers light energy into electrical energy. The better the sensor the better the process is. Now lets explain what I just referenced in greater detail

When light enters the camera it is focused onto your sensor. Your sensor on your camera is covered with pixels which can also be called photo sites. The light strikes the photo sites (your pixels) and when it does the light energy is converted into electrical energy that denotes both color and brightness. The longer the light source strikes these pixels the more electrical energy is built up for these values.

At the end of your exposure, the camera stops collecting light and measures the electrical charge built up on the sensor and assigns a value to each pixel, the brightness and color value to that pixel. I assume its numerical and the higher the value the brighter the output is.

Now one thing to note here, the electrical signal is actually electrons moving around. This is a physical thing... and computers only read in digital (0's and 1's). In order for the camera to actually read the electrical charge, it must be converted by something called an amplifier. When this amplifier is converting all this electrical data to digital data it can be more or less sensitive in this process. Before I go further lets talk about noise which directly relates to what we will discuss next.

What is Noise?

All around you there are photons of light bouncing around. Your eyes are only sensitive enough to see the main "signal" coming in but those random photons are there. When I say signal, I mean the actual image of your world, the things you are looking at. For cameras it is the light bouncing off the thing you are photographing into your camera.

Cameras picks up on the main signal, the things they are pointing at, fairly easily. Then there are random photons coming into the camera as well. The sensor of your camera still detects these photons and registers them as an electrical signal. These random photons create random patterns within the image because they too are converted from a photon to an electron and created an electrical signal. You can see that in the start image to the left. If you look close you can see the random patterns in the sky and shadows where it should be just black.

These random patterns are noise.

There are other reasons for noise including imperfections in sensors and their structure, electrical wobbles in the current that create patterns and so forth. But those are not what I want to focus on for this topic.

You will observe noise most easily in the shadows of your image, because the signal is weakest there and does not over power the random photons coming into your camera and the imperfections of your sensors.

In general we refer to this all as the signal to noise ratio. If you have a strong signal it over powers the random noise that is always present. If the signal coming in is weak, it will not over power the random noise present and thus you get noise in the shadows of your images no matter what ISO you are at.

Returning to the amplifier and conversion process

Now lets get back to that amplifier and its conversion process. If you set the amplifier to be more sensitive (increase your ISO), it will be more sensitive to this noise and reveal it. It amplifies the entire signal even the noise. This results in those random patterns of light and dark splotches showing up across your image in the shadows. The shadows have less signal than the highlights, thus the random noise present becomes more obvious. When the amplifier is less sensitive it has a tendency to not increase the noise generated by random photons coming into your camera.

Bringing this all together.

On your camera you have your ISO dial or switch or setting or what ever. When your ISO is low your camera processes light "less sensitively." It focuses primarily on the main details and does not amplify the noise within the image. When you increase your ISO, the camera processes all light coming in "more sensitively" and amplifies the noise resulting in more noise in your images.

One more thing- ISO and Dynamic Range

As I have read up on the ideas found in ISO you come across this idea that as you increase your ISO you decrease your Dynamic range. Now There are not a lot of good explanations for this, but the best I could see was called the Dynamic Floor and Highlight range. At lower ISO the camera can process shadows better, thus at lower ISO's shadows are clearer and can be deeper. Now As you increase your ISO the "floor" or the lowest point in the shadow details that can be easily detected actually goes up. In other words the camera is not as effective at seeing into the dark. Now the upper end, the highlights, are not effected. So as you incrase your ISO the darker areas that can be perceived clearly are lost but you still have the same roof in your cameras ability to see bright points. As your iso goes up the difference between the Dynamic Floor and the Highlight Clipping point get closer together and thus you loose dynamic range.

This is a good article that reveals this.

ISO in your photography.

Low ISO means higher dynamic range, cleaner images, but means your camera needs to expose for longer to increase the signal to noise ratio. Remember the higher the signal the less noticeable the noise is. As you increase your ISO your camera processes light more sensitively reducing the amount of time you need to take an image, but it decreases the signal to noise ratio and reduces dynamic range and creating more noise in your image.

One Final Thought

Higher ISO does not technically mean more noise. If you adjust your f-stop and exposure time you can increase your signal to noise ratio and still over power the noise present and have decent images. So on bright days with high ISO you still can have decent detail in your highlights.

The towers of the Virgin in Zion National Park with a cluster of fall colored trees on the left side.
Towers of the Virgin In Blue

The fall leaves in Zion have always enticed me and many other photographers. I was hoping to get out to Zion in 2023 and explore and see what the views were like in a different spot. Along the way I found some old pioneer debris and some views I had not photographed before. In the end I left this image a bit cool because I liked the feeling of the image. Not every image needs to be warm.

Posted in Education.