Aperture

Given a scene, there are 3 main settings you can change to control the exposure of an image: Aperture, Shutter and ISO... these 3 are generally referred to as the exposure triangle.

Before getting into the details, I would like to give a rough though not perfectly accurate analogy, just to get the feel of these 3.

Imagine you wish to fill a bucket( volume= x litres) with water, under a running tap. Your aperture control is the knob of your tap. Shutter speed(duration) is the amount of time you keep the tap open. ISO is k/x, where k is some constant. Close the tap partially(analogous to making the aperture smaller) and you have to increase the time for which the tap is kept open(shutter speed) in order to fill up the bucket.... Or you could instead use a bucket with less volume (increase the ISO).

Now coming to Aperture. It is basically the size(diameter) of the opening of the lens. The aperture size is controlled by aperture 'blades'.

On cameras, the aperture value is not stated as a diameter size in mm. Rather, it is stated as an F-number, like f/2.8 (P.S. F2.8 means the same as f/2.8, dont let notations confuse you!). The way F-number is calculated is

F-number=(Focal Length of lens)/(Diameter of Aperture)

for example, if I have a 50mm lens and the diameter of the aperture is 20mm.

50/20= 2.5

therefore the F-number is F2.5 or f/2.5

This means, the LARGER the aperture, the SMALLER the F-number.

Also note, as amount of light varies as the area of the opening, an increase in the diameter by square_root(2) times(~1.4 times) will double the area and hence double the amount of light entering.

f/1.4 lets in double the amount of light as f/2.0

similarly, f/4 lets in 1/4th the amount of light as f/2, not half!

The other important aspect associated with aperture is the Depth of Field-> the range of distance where objects are 'acceptably sharp' or perceived to be in-focus.

A large aperture(small F-number, like f/1.8) will have a very shallow depth of field compared to a small aperture (like f/8, f/16, etc )

The image examples illustrate the effect of aperture on DoF. Both images were shot with my EF 50mm f/1.8 lens(this says that the largest aperture possible on this lens is f/1.8) focussed on the grills. See the change in sharpness of the background!

So if you want to blur the background(especially useful for portraits), use a large aperture (small F number)!

Last, but not least, unless absolutely necessary, do not stop the aperture down below (i.e. don't make it smaller than) f/8 on a normal digicam, f/16 on an APS-C DSLR (entry level DSLR-> one that is not full frame). The reason is, although DoF increases, due to diffraction of light, your pictures will lose sharpness.(I could write a whole page on diffraction and the effect of the sensor size and pixel density, but it would bore almost everyone to death! :p )

Now turn your camera dial to Aperture Priority and experiment with different aperture values!