Uses of Graduated Neutral Density Filters
March 8th, 2011
GND filters, also known as split filters, limit just how much light comes through to an image. These are most needed when taking shots using natural lighting commonly encountered when taking landscape photos.
Before the use of digital cameras, these filters were necessary to create dramatic lighting in landscape images. When using digital cameras, one could use two separate exposures and combine the two images using a linear gradient in Adobe Photoshop or other photo editing software. This technique however cannot be used with fast moving subjects or changes in light unless in single exposure shots that are twice developed from RAW file format. This method inadvertently will increase noise in an image unlike if you were to use a GND filter. This also allows you to immediately view what a photo will look like using the camera’s viewfinder or LCD. 
There are two types of settings in GND filters; one has a soft edge for a more gradual blend and the other has a hard edge to get a more abrupt blend. Choosing which to use depends on how fast the light transitions in the scene. For example, a distinct change of lighting in a scene between the ground and the sky would require a GND filter with a hard edge. An example would be a landscape which includes dark trees and fields while the sky is too bright. With the use of a hard edged GND, the available light would be let in the lower half of the frame where the trees are while less light is allowed in the section of the sky. The result is a more even lighting.
The blend can also be radial to add or eradicate light fall-off at the edge of the lens’ edges, or more commonly known as vignette.
Blending requires careful precision with the aid of a tripod. Soft edges have a tendency to be more accommodating with errors in misplacement. Hard edges can create excessive darkening or brightening in the area where the blend occurs if light transitions faster than the filter. Remember that vertical objects stretching across the blend will appear unnaturally dark.
The soft and hard edge blending technique varies depending on what ty
pe of GND filter you use. One company’s soft edge can sometime be as abrupt as another company’s hard edge. It’s a trial and error when using the filter to determine the precise outcome.
Keep in mind the differential in the amount of light allowed in one side of the blend compared to the other end. In a previous article, we included a chart that lists how various companies designate the filters’ light reduction. A ‘0.6 grad’ is a graduated density filter resulting in 2 f-stops less light which is ¼ of light at one side of the blend compared to other. The same goes for a 0.9 ND grad, which allows 3 f-stops less light which is 1/8 on one side. Almost all of landscape images require a 1-3 f-stop blend or less.
