Using Neutral Density Graduated Filters
Neutral Density Graduated Filters, or "ND Grads" as they are more usually known are used mainly for landscape images where the photographer is looking to even out the exposure between the bright sky and the darker land, retaining colour and detail in both. Without a ND Grad filter, it might be difficult to achieve a single image where the exposure is correct in both land and sky, which is why you see so many photographs where the sky is white and lacking any detail whilst the land is perfectly exposed, or the land is dark and featureless whilst the sky is perfectly exposed. The filter effectively covers the brighter area of the scene and cuts down on the light from that area as it enters the lens, evening it up with the darker area and resulting in a much better over all exposure. The "Neutral" means that the filter should be colour neutral and not cause colour changes to the image. Many of the cheaper Nd Grad filters are actually Gray Grads" and are not really colour neutral, which can cause colour changes in your images, notably giving the sky a slight magenta tone. Graduated means that the change from dark to light is gradual, although some manufacturers give you the choice of "Hard Grads", where the change is more abrupt and hard, or "Soft Grads" where the change is more gradual and softer.
The above image illustrates the effects of using an ND Graduated Filter
Without the filter we can see that the sky loses much detail and colour being over exposed. The land is a little underexposed losing details in the shadow areas of the image as the camera struggles to even out the average exposure between the two areas of the image. Where the filter is in place, the sky/land exposure has been balanced out to give detail in both areas of the image.
ND Grad filters are usually found in the form of rectangular plastic or resin form, which then slide into a filter holder. You can get round screw in ND Grad filters, but these are not recommended at all due to the fact that you cannot alter their positioning to suit the scene. You will need to have a filter holder, where the actual filter slides into, and an adapter ring which screws into the thread at the front of the lens and which you then clip the filter holder to. Different lenses require different size adapter rings to fit, so you can buy them in the all the most common lens sizes. (Most lenses have the required size in mm printed by the thread somewhere, ie 77mm, 67mm, etc etc). Some manufacturers, notable Lee Filters, offer a choice of filter adapter rings with special "Wide Angle" rings designed to sit lower on the lens to help minimise "vignetting" ... where part of the filter holder is visible as a dark ring around the edge of the image.
This shows the component pieces of the filter system
This shows all the pieces put together and mounted on a lens.
ND Grad filters are available in different strengths that correspond to one F stop of light, two F stops of light, or 3 F stops of light. Different manufacturers have different ways of indicating the strengths, ie Lee Filters use ND 0.3 for one stop, ND 0.6 for two stops and ND 0.9 for three stops. (For those that don't understand what a "F stop" is just regard it as a unit of light, so a one stop grad filter cuts down 1 unit of light at maximum strength, two stop cuts down 2 units etc.) Cokin use ND2 or ND2X for one stop, ND4 or ND4x for 2 stop, ND8 or ND8X for 3 stop filters. Of all of them a two stop filter will be the most useful for those who only wish to buy one filter.
How do you know what strength filter to use? Ideally you should find the difference between the exposure for the land and the exposure for the brighter sky. This can be done by filling the viewfinder mainly with land and noting the exposure, then filling the viewfinder with mainly sky and again making a note of the exposure. The difference between the two will tell you what strength filter you will need. Once you have decided which filter you need, set the camera to manual exposure, set the exposure for the darker land value, and cover the sky with the darker area of the filter. Eventually, with experience, you will get more used to guessing which filter you will need making the whole job much quicker.
Once everything is set up take an image and check the preview and Histogram on the back of the camera, sometimes some fine adjustments are required to get the perfect exposure.
Don't be afraid to mount filters at a slant to follow sloping horizons, or to create a dramatic effect on clouds, or to mount more than one filter in the holder to increase the effects, although sometimes using more than one ND Grad filter can lead to some strange colour tones in the image. You can use the camera's preview screen to experiment with what works and what doesn't!
Links:
Lee Filters
Hitech Filters
Cokin Filters
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