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Lens Aperture

Introduction:
The lens aperture is the variable opening in the lens which allows a pre-determined amount of light to impact the sensor be it a piece of film or a digital sensor.
 
Aperture:
Aperture is measured in F-stops. These numbers are actually denominators of fractions rather than actual representative measurements. The lower the number, the larger the opening. For example, an F1.4 lens is considered extremely "fast" (wide open, letting in a huge amount of light). An F8 lens is considered "slow", its opening is small letting in a low amount of light and requiring longer exposure times.

An F1.4 (1/1.4) lens lets in twice as much light as a F2.8 1/2.8) lens. Every stop you close down for example going from an F5.6 to F8 cuts the light reaching the sensor in half.

Stopping a lens down lets you extend your shutter times for particular effects and to extend the depth of field.
 
Lenses:
Zoom lenses tend to have smaller minimum apertures that prime (fixed focal length) lenses. This is due to the limits in internal space and placement of zoom components. Prime fast lenses are especially useful when shooting indoors in low light conditions and for portraits due to their low depth of field when used wide open. Zoom lenses are more versatile but often lack the speed of primes.

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