I have had some queries about the filters that I use for my digital SLR Nikon camera.
I use a Cokin P121M graduated grey filter. It is a rectangular filter which is dark grey at one end and clear at the bottom. It fits into a circular-shaped holder which goes onto the lens, and the filter slides into this. You can then look through the lens of the camera, and adjust where the line of grey goes by sliding the filter up and down in the holder, swivel it around, and generally adjust the contrast of the picture you are taking.
You can also add more than one lens, and have coloured lenses to give, for example, a sunset effect, or a blurred edge. However, I'm happy with just the one.
This is the difference with a graduated grey filter...
With the filter
(Images are from the Cokin website)
Here's the website (and no, I don't get commission!)