What Are F/stops and How Do You Define a "Fast" Lens?

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Aside from the lens barrel, a camera lens is composed of two main components, the clusters of individual glass elements and an iris, which is a circular set of blades that open and close to allow controlled amounts of light to enter the lens and travel through to the imaging sensor to create pictures. (The pupil in your eye works the same way, by opening up wider in the dark or smaller in bright light, to allow the proper volume of light to enter your eye for a “proper exposure.”)

The iris opens and closes at specific settings called f-stops, which work in conjunction with the camera’s shutter to allow the correct amount of light to create a proper exposure.

(Note: The f-stop of a lens is determined by dividing the focal length of the lens by the diameter of the size of entrance pupil, i.e., the lens opening.)

Just as shutter speeds double (or halve) the time the shutter remains open as you adjust it up and down, each time you change the f-stop you either double or halve the amount of light entering the lens, and together shutter speed and f-stop create exposure settings (or the ratio of time and the amount of light entering the lens).

To best illustrate how shutter speeds and f-stops work together, think of filling up a glass of water. If you only open the faucet slightly (i.e. a small aperture) it takes longer to fill the glass, as compared to opening the faucet to full volume (i.e. the widest aperture), which fills the glass in correspondingly less time. Shutter speeds and apertures operate in a similar fashion. The wider the aperture (f-stop), the shorter the shutter speed, and vice versa. The size of the glass and the amount of water it will hold―i.e. the proper exposure―is a fixed quantity. The only variable is how fast or how slowly you want to fill it.

If you’re shooting in “Program” or “Auto” mode, your camera does this automatically. As you learn more about your camera you can make adjustments to each of the camera’s exposure modes (Program, Aperture Priority, Shutter Priority and Manual). You can control the f-stop and shutter settings to achieve different visual effects. To learn more about the exposure settings of your camera you should refer to your camera’s manual.

Note: The “A” setting on your shutter-mode dial does not always mean “Automatic.” Unless your digicam has separate settings for both modes, the “A” frequently stands for “Aperture Priority,” a mode in which you set the f-stop and the shutter speed is automatically set to a speed that complements the chosen f-stop. “Shutter Priority” is the reverse mode in which you set the shutter speed and the f-stop is set automatically.

So, lens speed refers to the maximum aperture―or f-stop―to which your camera lens can open up, and the “faster” the f-stop, the easier it is to shoot under low light and freeze fast-moving subjects.

Another lesser-known benefit of fast lenses is that the wider the maximum aperture, the more responsively your camera’s autofocus system will perform, especially in lower lighting. Ditto, the camera’s metering system.

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This info was very helpful for me you gave more info than my manual thanks

the glass of water analogy is a bit loose i think.. Light wont travel faster when you have wider apeture like opening a tap.. the term "fast" seems misleading for what actually happens also.. it should be referred to as a "volume lens" or something which reflects quantity not velocity.