Updated March 2012.
Some photographers do not see the point of using graduate filters when taking the shot because the effect can be replicated in Photoshop afterwards. Not only that but in software the effect is very controllable and any colour can be applied.
Never the less, there are many photographers who enjoy the craft of getting it right in-camera and more power to their shutter fingers. And, it still looks better done at the time of shooting.
What Is A Graduate Filter?
These filters are a darker tint or even a colour at one end and fade to clear at the other. A Graduated ND Filter, for example, is grey at one end and by placing this part over a sky that's appearing over exposed, you can reduce the brightness difference between that and the foreground to give a more even exposure. These filters come in various strengths and the overall effect will change depending on what strength filter you use. They can also be used to add emphasis and darken stormy skies too.
Generally, the advice is to manually meter without the filter/s in place and then add them. If you do not do that, the camera meter can be fooled by the darker filter area and overexpose the foreground so it looks wishy-washy.
However, it is worth trying your favoured autoexposure mode too because the latest advanced metering systems are remarkably precise and can take into account the darker filter over the sky and still give the correct exposure.
Using Multiple Grads
Sometimes, it is tempting to use two or more graduate filters in combination – with one filter colouring the foreground and the other darkening the sky, or perhaps two on just the sky. That is probably just okay, but be careful of flare. Also, if you have a protection filter on the lens, it might be worth removing it. Too many extra air-glass surfaces will degrade image quality and make flare and ghosting a greater risk.
On wide-angle lenses, the filter holder might cause cut-off at the corners of the frame when a protection filter is in place. Either take the skylight or UV filter off or buy a slimline filter like those available from Hoya.
Speaking of flare, because the filters are relatively exposed non image-forming light can suffer from flare and image quality can suffer. You can use a lens hood or you can just use your hand or a sheet of card to stop stray light hitting the filter surface – take care not too get your hand/card in shot, though.