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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Exposure is based on three elements. shutter speed, aperture size and the ISO rating of the film being used (or sensor sensitivity). Assuming we are using an ISO rated film or sensor of 200. Lets say our light meter reading is 1/125 @ F11. This will give us the correct exposure. 1/250 @ F 8 will give us exactly the same exposure. Which exposure setting we use depends on other creative considerations. Such as whether we want to freeze the action or show movement. Do we want a shallow depth of field or have everything in focus? Lets say we want to freeze an action shot with no movement visible at all. We would need to select a shutter speed of 1/500 or faster. Using the exposure example given above.( 1/125 @ F11), we would need to open the aperture up to F5.6 if we wanted to use a shutter speed of 1/500. This would balance the exposure. By speeding the shutter speed from 1/125 to 1/500 we are giving the exposure 2 full stops of less exposure. To ensure the same amount of light is passing though onto the film or sensor, we need to open or increase the aperture to allow more light through. Opening the aperture by 2 stops from F11 is F5.6. see the table below. All these setting give us the same exposure.