elm54 wrote:
And had gotten the impression that filters weren't necessary with digital cameras, ...
That is correct, most of what photographers used to do with color filters can now be done in the image editor. But then, this is not a color filter we are talking about.
Graduated neutral density filters are rectangular, and they fit into a filter holder. They must be rectangular, since you need to be able to slide them up and down in the holder to situate the transition in the correct place over your scene. Some such filters have about 1 stop of darkness, some have 2 stops, and some have more. Some have a hard transition, meaning abrupt. Some have a soft transition and are gradual over most of an inch. Otherwise, one end of it is dark and one end is clear.
Let's take an example. I'll just use some arbitrary numbers here. Suppose your sunset scene gave you an average meter reading. Then suppose within that, the colorful sky was +4 and the dark foreground was -4. If you situate the graduated neutral density filter correctly at the horizon, you might "darken" the sky to be only +2 and leave the foreground at -4. You can see that it is easier to get the whole brightness range into the shot now. The trick becomes choosing a metering point, and that comes from experimentation.
---Bob Gross---