Variable ND lens filters differ from fixed ND lens filters in that you have wider control over light exposure. A standard ND filter has a specific light density or f-stop. However, a variable ND lens filter covers a range of light densities and f-stops in one single filter.
The f-stop ranges on a variable ND lens filter vary slightly, depending on which brand of filter you buy. For example, Gobe variable ND lens filters allow 1 f-stop (ND2) to 8 f-stop (ND400) light reductions. This is a good range for most photographers, giving you the light reduction effects you need for many scenarios. Do you require f-stop specs outside this range? The good news is that Gobe has plans to develop variable ND filters with wider specs, such as ND4-ND1000.
If you want to change the f-stop on the variable ND lens filter, you simply rotate the outer frame. This increases or decreases the level of light that enters the sensor on the camera, allowing you to achieve the desired results.
Many people assume that the markings on the frame represent the individual f-stops. This actually isn’t the case, since the markings aren’t calibrated. These markings do, however, give you a general indication of the different light ranges as you twist the filter. Image quality does tend to worsen the nearer you get to the extreme ends of the f-stop range. Avoid pushing the filter past the minimum and maximum markings. This can adversely affect the way the filter works and creates an undesirable cross or X mark.
By turning the filter and adjusting the stops of light exposure you can slow down shutter speeds to introduce motion or blur to your images, or widen your aperture to produce a shallow depth of field between objects in the forefront and background of your scene.
How many f-stops you reduce light by on your variable ND lens filter largely depends on how much daylight you are exposed to at any given time. A gloomy, cloudy day may require a different f-stop light reduction compared to a bright sunny day at noon or under snowy conditions. For example, shooting by the sea at midday using long exposures can give the water a really silky, smooth effect, particularly compared to later on in the day.
If you want a sharply focused photo, you need to allow as much light as possible to your camera, so if the light is dull, opt for a low f-stop to boost the aperture. To capture a shallow depth of field in an image, consider choosing around f/1.2 or f1/4, but for deep depth of field, especially in landscape photography, high f-stops in the range of f/16 or f/22 may be preferable.
Choosing which f-stop to use is often a matter of personal taste based on the end result of your image. Shoot the same scene using different f-stop ranges by turning the variable ND lens filter to see how they differ.