The principles of using the aperture is probably the most confusing topic for a beginner in photography. Every time I come to this topic in my classes , I see a bunch of worried faces and by the time i finish the class most of them (i am sure) would have decided that photography is too complicated for them.
The truth is that Aperture and Depth of Field are confusing because of the various terms involved, many of them are just duplicate names for the same concept. But before we get to that, lets first understand the what the aperture does to your photograph
Aperture or Iris is a mechanical device that opens and closes everytime you press the trigger. Unlike the shutter, The aperture is found on the lens and it works only when connected to the camera (older manual lenses had apertures that could be manually opened and closed). The video shows how the aperture opens and closes in sync with the shutter.
The extent to which the aperture is open determines the amount of light coming in. This is denoted by a f-stop number. (see picture). The size of the aperture opening can be set before taking the shot. When the trigger is pressed, the aperture will open and close automatically to that setting
The minimum f-stop number in your lens can vary. In this picture it is f1.8. It can be anything: f 2.8, f 3.5 , f5.6. Similarly the maximum f stop can vary ; f16, f22, f32...
Depth of Field
Aperture not only controls the amount of light reaching the sensor , it also plays a very important aesthetic role. It controls the degree to which the subject and the background are in focus.
A more open aperture (f1.8, f2.8 etc) has shallow depth of field in which very little is in focus.
In the photos below, The left top image was shot with a more open aperture. Only the rose is in focus and the background is blurred. This is an example of shallow depth of field.
The right bottom picture was shot with the aperture at its maximum value, everything is in focus in that image