The Exposure triangle is made up of three things that measure your camera’s relationship with light. The three things are: Aperture; how wide or narrow the hole is that lets the light in, Shutter speed; how short or long the fraction of time the hole is left open to let the light in, ISO; how sensitive the film or sensor is to light.
The steps in the settings for all three elements of the triangle all measure the effect of light in terms of doubling it one way or halving it the other way.
Aperture is measured in f stops. From f/1.4 to f/32, with f/32 being the narrowest. Not all camera lenses have the full range of f stops. Why is too technical for me. Issue like the size and focal length of the lens, how much light can get through the amount of glass in a lens, the size and clarity of that glass, and other even more techie things.
Shutter speed is measured in fractions of a second. Cameras will vary in the range they provide. My old Canon AE-1 film camera which had a mechanical blind that shot across the aperture gave me shutter speeds from 1 second up to 1/1000 of a second. Bulb is a setting that lets you keep the shutter open as long as you want it to; for capturing meteor showers, stars, fireworks or a bunch of lightning strikes.
ISO is measured in 100’s no doubt to compensate for all the math in f stops. Think of ISO like skin. You know how a fresh crowd on a beach for the same length of time will end up with some sun-burnt and lobster red while others are fine. The extra sensitive ones are the high ISO numbers.
Film used to be commonly sold as rolls of 12 or 36 exposures at 50, 100, 200, or 400 ISO. Now you can set digital sensor values to well above 3200 ISO in some cameras. But the more you increase ISO the harder the electronic light sensor has to work causing the visual equivalent of static to become visible. Photographers call it noise. Cheap or small light sensors just like cheap or small loudspeakers will distort the signal if you try to turn the volume up too much.
And who cares what ISO means. One correct answer is that it is a combination of two older standards called ASA and DIN, from the days of film. Combined by the International Standards Organisation.
As I said already, the steps in all three elements of the triangle all measure the effect of light in terms of doubling it one way or halving it the other way.
You can easily see and work out the doubling and halving of values in ISO and shutter speed. But you’ll find aperture trickier to fathom. That’s because if you want the exact same amount of light hitting the sensor from a bunch of different lenses all set at f/8 you will need the diameter of the hole in each lens to be larger or smaller depending on the size of the lens. To get that standardisation you need ratios. And that’s what f stops are.
If you do a bit of math using the lens size (focal-length), pi and the f stop ratios you can have fun showing that each step in the f stop scale does equate to doubling the amount of light that reaches the sensor. Or halve it if you’re going in the other direction. This is as good a site as any where you can explore the math. http://www.uscoles.com/fstop.htm
But if you’re like me, you take the lens manufacturers word for it so you don’t have to do math.