The exposure of pictures captured by cameras (digital or film) is done through a combination of the shutter speed and f-stop that fills a sensor with light. Most amateur photographers have a basic understanding of shutter speed, but many don’t quite “get” f-stop, the numbers used to designate the various settings and the combinations of shutter speeds and f-stops. This brief explanation is designed to help expand that understanding.
F-stops
The f/stop is a ratio between the diameter of the aperture (the hole in the lens, created by a series of blades, that the light comes through) and focal length of the lens. If your lens has f/stops ranging from 2.0 to 22, the 2.0 setting corresponds to the opening with the most area, while the 22 opening has the least area through which light can enter.
Each f/stop setting allows in approximately ½ the amount of light of the preceding setting. So if your camera has an f/stop sequence of 2.0, 2.8, 4, 5.6 8, 11, 16, then the 4 setting lets in about ½ the light of the 2.8 setting, or said another way, going from 4 to 2.8 is a doubling of the light let in. Huh?
Consider a 100mm lens, an f/2.0 indicates that the lens opening (diameter) is 50mm. The f/stop calculations is 100 divided by 50 (100/50), or 2.0. The next calculation needed is the actual area of the aperture opening. In the above example, the diameter is 50mm. The radius is one half the diameter, or 25mm. The formula for the area of a circle is pi*r2, or 3.14*502, equals 7850. Using the same lens (100mm) and the next f/stop of 2.8, and completing the same series of calculations, you will get a value of 4004. The reason that this value is not exactly ½ of 7850 is due to the rounding error with the value of pi used. But regardless, you can see that it is very close.
Shutter Speed
Shutter speeds are measured in fractions of a second and are based on a halving or doubling like the f/stops. For example, a shutter speed of 1/60 is twice that of 1/125 but half of 1/30.
Putting it all Together
As photographers know, lower numbered f/stops (say f2.0 v f2.8) allow for greater depth of field (what is in focus ahead of and behind the subject of the photo). And the shutter speed determines, greatly, whether the camera can stop the action (prevent blurry photos). The cost and weight of the lens increases dramatically, especially as the focal length increases, for those with low numbered f/stops. That’s why you typically see telephoto lens with a lowest f/stop of 4 or 5.6.
