Shutter Speed, Long Exposure & Movement

If Aperture refers to how wide the camera's aperture diaphragm is opened, then shutter speed is the amount of time it is open in order to give a correct exposure. Put very simply, when we have a wide aperture, the shutter does not need to be open for as long to ensure the image sensor (made more sensitive by increasing the ISO setting) receives adequate light to expose the chosen scene. On the other hand, when a narrow aperture is chosen, the shutter needs to be open for longer to allow the light to enter. A popular diagram to explain how long the shutter needs to stay open at different aperture settings is shown below. This is generally regarded an easy 'at-a-glance' guide, and usually found in many a beginners booklet and introductory website (I've tried to select the most basic-looking one, but there are many different versions).

You can tell from this basic bar graph that, the larger the aperture (lower the number), the faster the speed of the shutter (which is to say it doesn't need to be open for as long). Also, at the top, it gives a guide for ho much depth of field is given by choosing said aperture value. And the bottom shows the amount of 'motion blur' that occurs at different shutter speeds.

Motion Blur

When we speak about motion blur, initially we think of it as a negative occurrence as a result of choosing too slow a shutter speed. It is said that the point at which this blur is most likely to occur is anything slower than 1/60th, although I have taken perfectly decent architecture (and even landscape) images at 1/40th: it depends on how steady you are at shooting handheld. However, in some shooting situations we actually desire this blur for creative effect to show movement in an image. So, as illustrated in the table above: Motion blur is to Shutter Speed what Depth of Field is to Aperture, in that this is the element that determines the creative effect. In the diagram below, you can see how much movement is present at various shutter speeds:

The above diagram shows the effects of using different shutter speeds from 1 whole second (slow; gives the water a milky texture) to 1/800th of a second (fast; practically a 'snapshot' of the water).

The shutter speed/exposure time allows the photographer to be as creative as he/she wants. I like to think of it as playing with light and time itself. When you have the shutter open for a certain amount of time, anything that happens in the scene- any light that happens to be falling upon objects (be they moving or stationary) is recorded by the image sensor. The resulting image will always be a snapshot, but what this snapshot contains is determined by how long the shutter is open.