So there were a couple of comments on this post (actually all the comments!) related to GND and ND filters so I thought a quick post devoted to explaining the two might be appreciated by some readers. Here goes…
GND, or Graduated neutral density filters (also known as just graduated filters) are different from other filters in many ways, one of which is that they are often rectangular rather than circular but the functional difference is that part of the filter is a darker (neutral) colour that fades into the clear sections. The transition from neutral colour to clear can be either abrupt or gradual but the function is the same, reducing the difference in brightness between the ground and the sky. Therefore they are used mostly for landscape photography. GND filters also have numbers which indicate how many stops of light they reduce the brightness by.
ND or neutral density filters reduce light from all wavelengths passing through the lens across the entire filter (there is no clear part of the filter). They have three uses, firstly to reduce the brightness of light, secondly to allow the use of a larger aperature and finally to allow the use of a slower shutter speed. Although they are also useful for landscape photography they are also handy for other outdoor photography.
And in terms of the basics that’s it! Simple eh?