The lens directs light onto the film or light sensor and captures an image. Where it becomes complicated is that you need to control the amount of light that is sent through to the film or sensor, this is where the aperture and the shutter speed come into play.

Aperture is formed by a set of blades that interlock contained within the lens, a fundamental of photography that has seen little change. How open or closed these blades are control how much light is sent through to the sensor or film.

The shutter speed controls how long the sensor or film is going to be exposed to the light and in turn we have the fundamentals of correctly exposing a photograph.

You would think it was more complicated than this seeing as photography can be such a hard art form to master, although we only have two core settings it is the extent of the creative input that can be used that gives a photographer so many ways to reproduce a shot based on their own perception, camera settings and other creative inputs for example lighting effects, on lens filters & composition just to name a few.

A correctly exposed photograph in theory is to simulate what the human eye would perceive the light levels to be. An overexposed shot is too light; an underexposed shot is too dark.

The advantages of two settings:

Two settings gives you control over a wide variety of shooting conditions, allowing shots to be taken in very bright light all the way down to very dark. So if we have a small aperture combined with a fast shutter speed we are able to shoot in extremely bright light.  If we are now in complete low light situations we can open the aperture and set a long shutter speed to allow us to shoot in almost no light conditions.

Depth of focus: (depth of field)

I prefer to use the term depth of focus over depth of field, as I believe it is more intuitive. Apart from the two settings allowing us to shoot over wide varying lighting conditions aperture also affects our depth of focus. As an easy guide to the use of aperture the wider the aperture the shallower our depth of focus is. The simplest way to illustrate this is with portraiture; we want the subject to be in focus while we throw the background out of focus.  So in essence with a wide aperture less objects in the shot will be in focus while with a small aperture more objects will be in focus. Landscape photography is a good illustration for the use of small apertures, use a setting such as f/16 and your whole shot will be sharp as a tack.

Sharp images:

Aperture also affects the sharpness of your image this is more prevalent if you are using a dSLR camera. The perceived sharpness of a shot can be affected even in a perfectly focused photo. Stopping down (using a smaller aperture) will improve overall sharpness of the image this is because shooting at a wide aperture in turn uses the full surface area of the lens. Although the glass of most lenses is uniform and free of optical imperfections the centre of the lens still provides the highest quality. Most commercial lenses still have a degree of imperfections near the outer of the lens so using a smaller aperture stops light being refracted from these tiny imperfections and in turn reduces distortions and gives the sharpest image possible.

The f-stop:

Aperture is expressed in f-numbers or commonly called f-stops, it is easy to remember the larger the f-stop number the smaller the aperture and in turn the smaller the aperture number or f-stop the larger the aperture. Small aperture less light is passed through to the film or digital sensor and larger aperture the more light is passed through to the film or digital light sensor.

Photo credits

Intro photo by joiseyshowaa
Aperture examples by paulobrandao and mikebaird