I want to improve my photos of landscapes with high contrast (sunset/sunrise). What are good selections for GND filters and what are some techniques for using them correctly?
There are essentially 3 kinds of GND filter: Soft, Hard, and Sunrise/Set. All these come in various 'strengths'.
Soft GND filters have a gentle gradient from dark to transparent and so are good for landscapes with irregular horizons such as mountains, hills and to some extent buildings.
Hard GND filters have a more sudden change between dark and transparent and so suit landscapes with level horizons, like desert or seascapes.
Sunrise/Set GND filters have a band of 'dark' across the centre of the filter, leaving the upper sky and the foreground unfiltered. This allows you to pull back detail in the brightest part of the sky at sunrise and sunset.
Just because there are dedicated Sunrise/Set filters does not mean they will always be suitable for every sunrise or set. They are most suited to relatively clear situations: if you have a fantastic cloudscape lighting up the whole sky, they'll be no good.
The different strength filters reduce the amount of light in the dark part of the filter by a set number of stops. Most filters are sold with designations such as 'ND2'. These translate to stops as follows:
Determining which filter to use
The most accurate method for using GND filters is to make use of your camera's metering system. In Aperture Priority mode (you can do it any mode, but A or Manual is the most common landscape mode) point your camera at the foreground, or the 'land' in your landscape, and make a note of the exposure the camera calculates, e.g. @f16 1/60. Now point your camera at the sky (in the case of sunsets, at a point above or either side of the sun) and again note the exposure, e.g. 1/500.
Now you just need to calculate the difference between the two exposures: in this case a 3-stop difference (1/60: 1/125, 1/250, 1/500). So you need a 3-stop filter in order to get the correct exposure for the sky and the ground. You can then either re-meter for the ground and lock the exposure, or head into Manual and dial in the 'ground' exposure settings.
The next step is positioning the filter. Hard filters are easier to position as the difference in light is easier to see. Soft filters are harder, but the positioning is also less critical than the hard filter. If you have a Depth of Field preview button this can make the position clearer, or you can simply take a test shot and adjust. If you are using Manual mode you will also see if you have the filter too low in the scene as the light meter will start to show underexposure.
Choosing your filters
In terms of buying filters, as ever, go for the best you can afford: Lee and Hoya are respected brands. Personally I think there's little use for a 1-stop filter but if money is no object it won't hurt to have it. If you are on a budget I would go for 2 and 3 stop filters, soft and hard edged if possible, as well as a sunset filter or two, but it will depend on your location and the type of shot you plan on taking - if you're 300 miles from the sea, you might not want to bother with a hard filter for example.
If you are shooting digital, I recommend against GND filters. Shoot multiple exposures and composite them during post-processing. Although this takes more work, you are assured of better results - you are not limited by a linear gradient, nor restricted to working with a single take of the scene.
Here is an example of this method rendered from 3 images shot at +/- 1 stop.
In fact, except a polarizer, haze and dense ND (e.g. 5 stops) filters, there is little need for filters in digital photography if you are willing to make the adjustments in post-processing.