In landscape photography, it is helpful to have three specific lens filters in your camera bag. These are the circular polarizer (CPL) filter, the neutral density (ND) filter, and the graduated neutral density filter (GND).
What is a Graduated Neutral Density Filter?
A Graduated Neutral Density filter, or simply a GND filter, is a very helpful tool for landscape and outdoor photography.
A very common issue that most landscape photographers will face out in the field is to balance the exposure between a (sometimes) very bright sky and a darker foreground.
A GND filter will allow the photographer to balance the luminosity of the scene between the bright sky and the darker ground, for example when shooting during a sunrise or sunset.
GND filters consist of glass that is divided into two parts, the upper side is darker and acts like a regular ND filter (available in different ND strengths) while the bottom part is clear glass that has no effect on the image.
The concept is to put the sky, which is usually much brighter than the scene below, in the top part of the filter and reduce the incoming light and let the ambient light of the ground be captured through the clear glass.
Types of GND Filters
Now let’s analyze the types of graduated ND filters.
Soft Graduated ND Filter
The soft graduated filter (GND-S) features a soft transition effect between the darker and lighter zones from the filter top. It is probably the most widely used type of graduated filter that can be used in almost all scenarios, no matter if the horizon is even or uneven, i.e. in an urban setting.
Reverse Graduated ND Filter
While the reverse graduated (R-GND) filter still has the darker part on the top as the soft and hard types, the main difference is that the transition from the darker zone to the lighter is inverted, resulting in there being a darker part close to the middle.
This provides a lighter effect than the soft graduated filter and it is ideal for scenarios in which the Sun is close to the horizon.
Hard Graduated ND Filter
The hard graduated filter features a very fast transition effect between the darker and lighter zones from the filter top, which translates into a very short transition and the density of the filter is constant over almost half of its surface. This type is mostly used when the horizon is straight and is visible in the bigger part of the frame, i.e. on a seascape.
Center Graduated ND Filter
The center graduated filter is a filter that can be used to adjust exposure on a very precise area, as it only has a very thin area that is darkened.
This filter can be used in scenarios where a specific part of the image has to be darkened, like for example when you are shooting at the seaside and the Sun can be seen close to the horizon, but the sky is full of clouds.
The Proper Way to Calculate the Correct Exposure
Although the new digital viewfinders and LCD screens of the latest cameras can help us understand on the fly if we have achieved a balanced exposure for our shot, still there is a proper way to set the ideal exposure when using a graduated ND filter.
The first step is to change the camera shooting mode to manual to make use of the camera light meter. We will use it to take a reading from the foreground/terrain while filling most of the frame with it and noting it. You may want to use spot metering instead of matrix/multi metering when taking the reading to have an even more accurate result.
Then we take another reading for the sky, having a big part of it in our frame.
Meter for the ground
Meter for the sky
By comparing the two separate readings, we can find the luminosity difference (in f-stops) between the sky and the ground to know what ND strength should have the GND filter for that specific scene.