What are “Full Stops”?

Have you ever heard of a term “full stop” in photography? Each of the increments between ISO numbers is called “a full stop” in photography. For example, there is one full stop between ISO 100 and ISO 200, while there are two full stops between ISO 100 and ISO 400. How many stops are there between ISO 100 and ISO 1600? That’s right, four full stops of light. Why do you need to know about stops? Because you might see it in photography literature or photographer might mention stops and it is sometimes confusing to understand what it truly means. But the term “full stop” does not just apply to ISOs – the same concept is there for shutter speed and aperture. It is easy to remember full stops between shutter speeds, because you just start from one and divide the number by two: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, etc. Obviously, the numbers are rounded (starting from 1/15, which should be 1/16) to make it easy for photography. It is harder to memorize stops in apertures, because the numbers are computed differently: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.