An introduction to Shutter Speed and Aperture:

Both of the above variables help control the amount of light that reaches the film (or in a digital camera, the CCD sensor), however they have differing affects on the resultant photograph.

The shutter is a sheet that moves to uncover or obscure the film or CCD and normally obstructs the passage of light. The aperture is a hole in an optical diaphragm which can be varied in size to alter the amount of light entering the lens (a lot like the pupil and iris of a human eye).

In general to maintain a good exposure, if you lengthen the shutter speed (allowing more time for light to reach the film or sensor) you must decrease the aperture size (to allow less light into the lens in the first place) and vice versa.

Mathematical Relationship:

In photography and other branches of optics, the aperture size of a given lens is often described as an F-ratio or F-stop number. This is the ratio of the aperture diameter to the focal length of the specific lens.

This is expressed as NF = F / DA

Where:

NF = F-Number

F = Focal length of lens

DA = Aperture Diameter

As such the mathematical relationship between shutter speed and aperture diameter is directly proportional (when the shutter speed gets higher, the aperture must get larger to maintain correct exposure) and the mathematical relationship between the shutter speed and F-stop number is inversely proportional (as the shutter speed decreases, the F-Stop number must increase - meaning that the aperture diameter is decreasing to avoid over exposing the image). This is known as a reciprocal relationship. However, when the shutter is slowed down beyond a certain point or the effective shutter speed is made extremely fast (via the use of strobe lighting), the purely mathematical relationship fails. This is known as reciprocity failure. Rather than explain it here, search for the question "What is reciprocity failure" (hopefully, no one changes the wording of the question).

Their use in photography:

Fast shutter speeds will "freeze" the object in the frame whereas slow shutter speeds will cause moving objects to blur (which can be a very effective technique when photographing flowing water / waterfalls). A very low shutter speed will actually cause moving objects to disappear from a photograph totally and is a common technique used by architectural photographers to ensure that moving people or vehicles do not appear in photographs of buildings.

A small aperture (higher F number) will create greater depth of field, which is the distance in front and behind the point of interest which is being focused on. This is useful in landscape photography where you wish to include detail in the whole of the image. A large aperture (lower F number) will greatly reduce the depth of field, meaning that less distance in front and behind the point of focus will be sharp. This is very commonly used in portrait and wildlife photography where you wish to isolate the subject of interest from the potentially distracting background.

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