The Graduated Neutral Density (GND) filters

In the previous article we discovered that our eye sees much better than our camera, especially when it comes to dynamic range.
One of the most used methods to solve this problem in photography is the use of GND filters (Graduated Neutral Density).

What is a GND filter

A GND is a filter made of two distinct parts: a completely transparent one, and a darker one.
Setting the darkest part of the filter in correspondence of the brightest portion of the scene, we can reduce the exposure difference in the frame.
To reduce the exposure difference is to reduce the dynamic range of the scene, and thus allow our camera to simultaneously capture the detail in both bright areas and dark areas of the scene.
Basically, to make an analogy, the GND filters are sunglasses for our camera.

Types of GND filters

The first distinction should be among the screw-on filters and slot-in ones. As you will see more ahead, however, if someone tries to sell you a GND screw-on filter, just run!
The GND filters are typically distinguished by the type of transition that exists between the transparent area and the dark area of the filter. For this reason, we can identify three families:

1. Hard Edge filters, that are characterized by a clear boundary between the transparent and dark area. They are therefore used when the separation between the bright and the dark areas of the scene is very defined, such as the horizon at sea.
2. Soft Edge filters, that are characterized by a soft transition, and therefore are used where the transition between light and dark areas is not so clear. A classic example is a shot in a mountainous area.
3. Reverse filters, that are nothing more than Hard Edge filters with the dark area that fades away the more you move from the line of separation to the upper border of the filter. Basically they were invented to better manage sunrises and sunsets, where the light is more intense on the horizon line. If you love seascapes like me, this filter will be one of your best friends forever!

 

Another distinction is given by the constructive material. The higher quality filters are made of resin or optical glass. When you choose your filter, remember that putting in front of a lens of thousands of dollars a few dollars filter is not a great idea! If possible, prefer glass filters.

Finally, the filters are distinguished by gradation, or in other words for the ability to block the light through the darkest area.
Normally in landscape photography this difference is from 1 to 4 stops during sunset and sunrise, depending on the particular weather conditions. This is the reason why on market you will find almost exclusively these gradations!



How to use the GND filter in the field

The use of these filters in the field is very simple: try to make an exposure reading in the darkest area and one in the brightest area of the scene (usually the sky). The exposure difference will indicate the intensity of the filter to be used. Let’s assume that the light meter reading for the sky is 1/250” and the one for the rocks in the foreground is 1/30”. The difference between those readings is 3 stop, so to balance the exposure you must use a 0.9 (3 stop) GND. At this point, just take the filter and cover with its dark side the brightest part of the scene. This is the reason why a GND screw filter does not make sense: you would not have the possibility to align the dark area in accordance with the scene!

To avoid having to hold the filter with your hands (that could be a problem if you are going to use the GND filters together with other filters) you can buy an Holder system, that once mounted in front of your lens will do the job for you.

At this point the limited dynamic range of your image will be just a bad memory!
But you know, the more you get the more you want.. And if now I tell you that we can go even beyond the human eye using another kind of filters?

Stay tuned for the next episode!