Fair warning, details about ISO is more complex to explain than shutter speed or aperture.
When people bring up ISO, it has to do with how sensitive film was to light. The lower the ISO, the more light it took for the film to react to create an image on it. Now using the shutter speed you’re wanting, aperture (depth of field) you want, and amount of natural light, you choose your film according to its ISO rating to get a properly exposed photo. The ISO rating is due in part by how big the particles were that reacted to light. The smaller the particles, the higher the number of them were required, and the amount of light required was higher. As the particles grew in size, you didn’t have as many, and so the ISO rating was also increased (considered “faster” as it didn’t require as much light to expose the photo). Light meters were helpful (or just experience) to figure out what ISO you needed in your environment with the other settings you choose to get a good exposure in the photo.
Digital cameras don’t have particles, but instead have electrical gain and standards that pretty much match previous film ISO. So When you increase the ISO in a digital camera, you’re increasing gain to “brighten the image”.
But why wouldn’t you just use the higher ISO film or setting in your digital camera? Well, the effect between film and digital is similar but not exactly the same, but the premise is the same.
Lets take film, the smaller and more plentiful the particles, the lower the ISO. Now if you’re using ISO 100, with some of the smallest particles, you require a lot of light, but you’ll get a much more detailed photo as there are many extra particles/dots in the frame. You can almost consider it like increasing the number of pixels or dots in the photo. The more there are, the more details you get, and the more uniform the dots are. If you use a higher ISO in film and take a photo, you’ll more likely to see the individual particles as they are bigger and not as many. This is the actual physical difference between the different ISO rated films, the grain. Lower ISO, the less grain, but higher ISO, the bigger and more prominent the grain is.
Digital Cameras don’t have different size grain as it’s a fixed size sensor. Instead of actual grain like you get in film, increasing the ISO (increasing the gain, not grain) creates noise. Many people might have a better understanding of noise when it comes to audio capturing than photography/videography. The higher the gain when capturing audio, the louder things become, but you’ll also start to get artifacts that sound like static. The same happens with photography and a digital camera, but instead of calling it gain, they just kept with the film standards calling it ISO.
So the higher your ISO, the more noise you’ll get in your photo. The more expensive a digital camera, it usually has a better ISO performance at higher ISO levels with less noise at the same setting as cheaper/older models.
You are able to fix some of the noise in post production if you wish, but it will lower the details overall to help with removing/smoothing out the noise.
One of my favorite tools when comparing ISO and noise between cameras, is using the image comparison tool from DP Review at:
You can select multiple cameras, select RAW processing (not JPG) and set their ISO setting, and you can see a large difference between quality of noise between cameras at the same settings.
When it comes to how to utilize this in your digital camera, there is a lot of personal preference. I personally have a maximum ISO for my camera that I find acceptable and don’t go above it no matter what. My limits for the Nikon D5600 is ISO 3200/4000, Nikon D750 at 6400/8000, and my Nikon Z6II at ~12800.
In low light settings:
Set a certain shutter speed and aperture setting, then I start at ISO 100 in my camera, and can raise that until I get a good exposure.
Freezing Tango Dancers:
Shutter: ~1/100th of a Second | Aperture: F1.4-F2.8 (depends on lens) | ISO: Whatever it takes to get a good exposure (up to my maximum)
Realistically, I have some wiggle room in all the settings, but knowing how they work together I can adjust each accordingly.