Blur is also a clearly identified syntactical aspect of photography and fits into an area the generic to the medium. Before the photographic process this idea was unknown, but with the first efforts to make photographs through the Daguerreotype and the Collotype around 1840, people began to understand the relationship between the time taken to make an exposure and any subject movement that might take place in the scene being photographed.

Now Blur in an image is something that any audience associates with movement, speed and time.

Marcel Duchamp's painting "Nude Descending a Staircase" was based on this idea of movement revealed through photography.

But while blur might be a generic factor, there are many types of blur that can be incorporated into a photographic image, and each is read in a different way.

Selective Subject Blur

This type of blur relies on parts of the scene to remain stationary and other parts to be moving

By placing the camera on a tripod and selecting a slow enough shutter speed, inert objects are recorded as sharp while moving subjects are recorded as blurred. If the moving subject is a car or person it tends to elongate in the direction it is traveling creating a visual distortion. The moving object visually differentiates itself from the back ground.

The speed of the subject and the length of the shutter speed determined the amount the subject will be blurred.

With the use of very long shutter speeds, objects that move very slowly can be made to look like they are traveling relatively fast due to the resulting subject blur. If the exposure is long enough some parts of objects can visually disappear altogether.

If there is only a small differential between the speed of the subject and the shutter speed, the amount the subject is blurred may be so small that the image is unconvincing looking more like unsharpness rather than blur.

If on the other hand the differential is too great and an object is moving too fast it may have moved across and outside the frame during the exposure and may not record at all in the image.

In this image from the Last River Song the exposure of the fast flowing river was about 1 minute hence the timeless, surreal quality that turns the water to vapour.

Detail from the Last Rivers Song Lloyd Godman 1984

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