Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the film or sensor. The size of the opening is controlled by an adjustable diaphragm of overlapping blades similar to the pupils of our eyes. Aperture affects exposure and depth of field. Just like successive shutter speeds, successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step as shown on this diagram.Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These “relative apertures” are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.We just learned that the next aperture will have a diameter which is 1.4 times smaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. “Stopping down” the lens from f/4 to f/5.6 will halve the amount of incoming light, regardless of the focal length. You now understand the meaning of the f/numbers found on lenses:Because f-numbers are fractions of the focal length, “higher” f-numbers represent smaller apertures.Maximum Aperture or Lens Speed

The “maximum aperture” of a lens is also called its “lens speed”. Aperture and shutterspeed are interrelated via exposure. A lens with a large maximum aperture (e.g. f/2) is called a “fast” lens because the large aperture allows you to use high (fast) shutterspeeds and still receive sufficient exposure. Such lenses are ideal to shoot moving subjects in low light conditions.
Zoom lenses specify the maximum aperture at both the wide angle and tele ends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies that the maximum aperture is f/2.8 throughout the zoom range. Such zoom lenses are more expensive and heavy.

The next post will discuss how the aperture effects Depth of Field

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