What is Aperture?Aperture is a measure of the size of the opening within the lens that controls the amount of light that reaches the film or sensor. It is usually given is a strange sounding unit f-stop
f-stop = focal length * diameter of openingThe strange sequence of numbers it is quoted in f/1.4 (f/2) f2,8 (f/4) f/5.6 (f8) f/11 (f/16) f/22 might make more sense when you realize that all are approximate multiples of √2(the square root of 2) and the amount of light reaching the sensor is proportional to the area of light on the sensor (ie πr²) Thus each f-stop is equivalent to doubling or halving the light. The bracketed numbers in the sequence above are the half stops.The lower F-stops let in more light and give you a faster shutter speed for your best exposure. Its as simple as that.
In most camera the size of the opening is controlled by a bit of clever mechanism, usually called the diaphragm, where a set of thin blades are pivoted, connected around the a ring shape and sprung in such a way that as the ring is rotated the blades expose a larger or smaller gap in the center. This was the reason why the aperture control was on the lens (just at or behind the convergence point of the lens, Today most DSLR lenses have the same style of blade diaphragm but the control for setting the diaphragm opening is on the camera body, perhaps in a menu, perhaps as a separate wheel or disc. So you should find you are able to control the aperture setting even if your lens doesn’t have a ring to adjust it.
The Aperture setting is also one of the main factors in determining depth of field. the smaller the opening through which the light passes the greater the range of objects in your scene that will be crisply in focus on the sensor. This is why many camerphones with tiny lenses seem to have almost infinite focus. Whereas the larger openings (lower f-stop) will reduce the range of focus to a narrow band. Portrait photographers often use this to blur out distraction backgrounds but have the face just in focus