4x5.65 Nd Grade S/e N3

Graduated neutral density (GND) filters restrict the amount of light across an image in a smooth geometric pattern. These are sometimes also called "split filters." Scenes which are ideally suited for GND filters are those with simple lighting geometries, such as the linear blend from dark to light encountered commonly in landscape photography

Prior to digital cameras, GND filters were absolutely essential for capturing dramatically-lit landscapes. With digital cameras one can instead often take two separate exposures and blend these using a linear gradient in photoshop. On the other hand, this technique is not possible for fast moving subject matter or changing light (unless it is a single exposure developed twice from the RAW file format, but this increases image noise). Many also prefer using a GND to see how the final image will look immediately through the viewfinder or rear LCD.

GND filters come in many varieties. The first important setting is how quickly the filter blends from light to dark, which is usually termed "soft edge" or "hard edge" for gradual and more abrupt blends, respectively. These are chosen based on how quickly the light changes across the scene, where a sharp division between dark land and bright sky would necessitate a harder edge GND filter, for example. Alternatively, the blend can instead be radial to either add or remove light fall-off at the lens's edges (vignetting).

Placing the blend should be performed very carefully and usually requires a tri pod. The soft edge is generally more flexible and forgiving of misplacement. On the other hand, a soft edge may produce excessive darkening or brightening near where the blend occurs if the scene's light transitions faster than the filter. One should also be aware that vertical objects extending across the blend may appear unrealistically dark

A problem with the soft and hard edge terminology is that it is not standardized from one brand to another. One company's "soft edge" can sometimes be nearly as abrupt a blend as another company's so called "hard edge". It is therefore best to take these on a case by case basis and actually look at the filter itself to judge the blend type. Most manufacturers will show an example of the blend on their own websites.

The second important setting is the differential between how much light is let in at one side of the blend versus the other (the top versus bottom in the examples directly above). This differential is expressed using the same terminology as used for ND filters in the previous section. A "0.6 ND grad" therefore refers to a graduated neutral density filter which lets in 2 f-stops less light (1/4th) at one side of the blend versus the other. Similarly, a 0.9 ND grad lets in 3 f-stops less light (1/8th) at one side. Most landscape photos need no more than a 1-3 f-stop blend.