I am happy you're back for the last blog post of my little series on how to get started with manual mode!
Before the digital age, ISO used to refer to film speed, i.e. the sensitivity of film to light. You would go to the store and buy a roll and decide on a specific ISO number depending on how and where you were going to use your roll. If you wanted to take your camera to the beach on a sunny afternoon, you would most likely go for a lower ISO number than if you were planning on documenting your grandma's birthday indoors. As soon as you had put your roll of film in your camera, you pretty much had to stick with your ISO settings until the roll was finished.
Things have changed a lot with digital cameras!
Mainly, ISO does not exactly refers to film speed anymore but to some technical capacities that are built-in to your camera and can affect its sensitivity to light. The more professional the camera, the higher the ISO can be pushed in low-light situations.
Another major difference is that you can change your ISO number with each individual picture.
Both with film and digital cameras, a higher ISO implies more grain. Depending on how much light there is and your preferences, you can purposefully crank up your ISO to get more grain, or try to work with your other settings (aperture and shutter speed) to avoid it. When the grain is unwanted and unappealing to the eye, we usually call it digital noise.
Here is an example of a picture taken in very low light on a gloomy day. I had to set my ISO to 3200, which is very high. Can you notice the grain? At that level, I consider it digital noise.
The following black and white image was taken at ISO 1600. There is some grain, but less than the previous one.
The last image was taken at ISO 320 (there was enough light to go with a low number). It has barely any grain at all.
I always try to go for reasonably low ISO numbers if I can because it is easy to add grain in post processing. On the other hand, it is a little more difficult to remove it.
I hope this helped you understanding ISO!