This shot is another of Posforth Gill Falls.
It was taken with a neutral density filter which made the exposure longer than it would have been otherwise, hence the motion blur in the water.
Neutral density filters allow photographers to lower the amount of light striking the film/sensor. Essentially the ND filter is a piece of glass or other transparent material which is darkened to allow a specific amount of light to be stopped from being transmitted through the lens. The idea is that all wavelengths are affected equally so no colour cast will show (not always the case for the strongest ND filters).
Why would this be useful? Well for a given scene you might want to open up the aperture size or reduce shutter speed. For instance if you wanted to achieve a shallower depth of field you might want to stop down from an aperture of f11 to f5.6 but maintain the same shutter speed. Or perhaps the scene is too bright for the shutter speed and aperture settings you have available on the camera/lens – an ND filter can reduce the exposure value required. You might also want to slow the shutter speed down in order to get some motion blur into the image. An ND filter allows you to maintain the same aperture and increase the shutter speed for a given scene.
The amount an exposure will be affected is indicated in the number of the filter: ND2 reduces exposure by one stop. An ND4 reduces the exposure by a further stop. The Wikipedia link below has the full detail.
http://en.wikipedia.org/wiki/Neutral_density_filter
The picture was taken with a ND64 filter which reduces exposure by 6 stops. So, the exposure was measured as EV9 (ISO 100), which would equate to f11 at 1/4s. Maintaining the aperture setting and increasing the shutter speed to 30s should do the trick!
This is the same scene without the ND Filter:
However, there is something else that needs to be considered when making long exposures, Reciprocity Failure. In photography Reciprocity refers to the relationship between the amount of light falling on the film/sensor and the reaction of the silver halide/sensor to that light. There is a inverse relationship in normal circumstances between the intensity of the light source and the duration for which the film is exposed. So, if we let half the amount of light hit the film (reduce the aperture one stop) we need to double the length of time to get the correct exposure. We see this all the time when we calculate exposure or our camera meter does it for us.
This relationship breaks down though when the intensity of the light is too low. Films will need more than double the exposure for a halfing of the intensity of light in these circumstances. The rate varies from film to film. In this case the exposure should have been 30s but the exposure required was actually 120s. More info on this here: http://en.wikipedia.org/wiki/Reciprocity_failure#Reciprocity_failure
