F-stops are relative measurements of light. When I say relative, I mean, that there must be some context for a comparison. Saying f/4 for example, means nothing without mentioning what we are referring to. Most of the time we are talking about a particular lens. “I shot this picture at f/4″ implies f/4 on the lens used to take the picture.
Ok, so what is an f-stop relative too? It is relative to there being no light restriction coming through the lens. If there was no restriction, we would call this f/1. A “stop” is one half of the amount of light, compared to the previous “stop”. Don’t worry, it gets much more confusing…
To reduce the aperture to allow only one half the previous amount of light the aperture is reduced in diameter by 
, not one half. is approximately 1.4. Therefore, if the first f-stop is f/1, the second f-stop would be f/1.4. The formula for finding an f-stop is 
Below is a chart that shows the f-stops, 1/3, half, and 2/3 stops. The numbers you see on your camera or your lens are probably rounded up or down a little bit from this chart.
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