In a single sentence, aperture is the opening through which light enters the camera. This is the first measure that is used to determine how much light should enter the camera. There are other uses of aperture control. We’ll come to that later.
In camera this opening can be controlled. It is made of a diaphragm which consist of several element. It acts like the iris of the eye, i.e. by controlling the movement of the elements the opening at the centre of the diaphragm can be increased and decreased.
In photography aperture is specified as f number. It is the ratio of the focal length of the camera lens and to the diameter of the opening. Hence, if the opening diameter of the aperture is 25 mm for a 50mm lens, then the aperture value is 2. It is written as f/2.
You don’t need to calculate the opening of the aperture every time you pick up your camera. Just remember that the lower the value of the aperture the more wide opening it has and hence more light will enter your camera. There is one misconception among the beginners. That is if wide aperture is required, they tend to keep the aperture value to f/16 or f/22. This confusion comes as the number is higher than f/1.8 or f/2.0. It is not wide aperture. It is the narrower. Every time photographers do not talk in numbers. If you are asked to get the aperture wider, then please go to the lower aperture value not the higher.
We have already told that aperture is used to control the amount of light entering the camera. There is another use of it. That is to control depth of field. We all know that when light rays can travel in parallel paths or in random path. In general condition, there are multiple sources of light, some direct and some indirect. As a result light rays travel in various paths. Now, when the aperture is narrow, the light rays that are travelling in parallel to the axis of the lens enters the lens easily and most of the light rays travelling in other direction gets blocked. As a result the light rays that enters the lens are highly collimated and they make a sharp image at the sensor of the camera. If the aperture is wide then uncollimated rays enters the camera lens and they makes sharp image for only rays having a certain focal length. In the later case rest of the portion becomes blurred. So, if the aperture is narrow then depth of field is more and if the aperture is wide then the depth of field is less. Low depth of field is used for macro photography where you want to stand out the subject in the whole image. I shall post photographs explaining this in next post.
There are many more interesting experiments that you can do with aperture. We’ll come to them after the basic topics are covered.
There is one thing I would like to ask you. You need to know the basics of optics to understand the technical things of camera and lenses. Hence, please get your optics revised.
For those who want to know more technical things about aperture, here is the Wikipedia link http://en.wikipedia.org/wiki/Aperture