A stop in cameras is a doubling or cutting in half of light. Using money as an analogy, if I start with a dollar and my money increases by one stop, I now have two dollars. If I start with a dollar and it increases by two stops, I now have four dollars (not three). If I start with a dollar and it goes up by three stops, I now have eight dollars: One becomes two, two becomes four, four becomes eight. Reverse that when decreasing.
Cameras have three light-management tools: Shutter, aperture and ISO. Each is measured in different terms. Shutter is measured in seconds and fractions of a second. Aperture is measured in “f-number” or “f-stops”, which is the ratio of the focal-length to the diameter of the opening inside the lens (aperture). ISO (aka “film speed” for film) is measured in a scale commonly starting with a low of 100 through a high of 1600 to 12800 or higher.
Imagine the time before airliners could fly around the world. American pilots spoke English to American control towers, but what about those first flights to French or Spanish-speaking destinations? Today of course the airline industry requires that all pilots and control tower personnel be fluent in English, because English was declared the official language of the international air traffic industry.
Now imagine the days before digital photography and preview screens that let us “dial in” our settings by trial and error. Photographers had to calculate their settings before the shot, and then wait until the prints came back from the developer to find out how they did. What was needed was a common “language” that put all three controls on equal footing. That language was “stops”.
Let’s say that a film photographer knows that his settings are right for the conditions: If he then increases his shutter speed by one stop to avoid motion blurring, the picture will be darkened by one stop. He then can either use film that ‘s one stop faster, or increase his aperture by one stop. Either one will offset his shutter adjustment, and keep his original exposure correct. Of course the same is true for a digital photographer, except that instead of switching films, that part of the process is now a control on our cameras just as shutter and aperture are. In fact, you may have noticed that when you spin the dial that adjusts each of these three settings, you feel it clicking. Each of these clicks represents an adjustment of either 1/3 or 1/2 stop either way (see your manual for more).