I have wanted to use a Graduated Neutral Density filter for a while. To be honest, it is mostly for the cool factor, but I can definitely see what the benefits would be. I have a good amount of photographs where the sky is blown out, with the most recent ones happening when I was photographing the falls in Rawdon. This week I finally went out and bought one. With that said, I am writing this article based on researching the use and not by years of experience. This is the basic information; I’ll write more as I learn any new tricks.
Neutral Density (ND) filter.
Before I talk about Graduated Neutral Density filters I have to talk about Neutral Density filters. A ‘Neutral Density’ filter is a tinted piece of glass that mounts to the front of your camera. What it does is evenly block light from hitting your sensor and thus allowing for longer exposures or wider apertures on bright days/subjects. There are three basic grades of filters – ND2(0.3), ND4(0.6), and ND8(0.9). There are other options, but these are the basic configurations. These filters block from 1 to 3 stops of light in the following configurations:
- ND2 / 0.3 ND : 1-Stop
- ND4 / 0.6 ND : 2-Stops
- ND8 / 0.9 ND : 3-Stops
So if you are shooting at f/11 for 1sec, dropping on a ND4 filter (2-stops) should give you an f/11 at 4sec exposure (1-stop being 2sec).
There are two reasons why I see you would want to drop the stops – slower shutter speed or lower aperture.
Shutter Speed
The first is to slow down the shutter speed in a bright location. Being able to have a lower shutter speed will allow you to get that smooth flowing water effect on rivers and waterfalls, or it will give you speed trails on a fast-moving objects like moving trains or running children.
Aperture
The second is to allow you to shoot at a wider aperture. Sometimes there is so much light in the shot that opening up the aperture is simply impossible. Darkening the surrounding by 2 to 3 stops will allow you to go to the lower apertures to get that great shallow depth of field shot. If you are shooting a flower on a bright sunny day, shooting at f/2 will put the main subject in focus while the rest of the image is out of focus.
I own and ND4 circular filter that mounts to my camera and I played around with it a bit. Here is the small post I had about that.
Graduated Neutral Density Filters.
A GND filter goes from a Neutral Density State (e.g.:ND4) down to clear glass. If you put it up to the light, you will see that the top part is dark and the bottom part is clear. The most common type has a gradual change from dark to light (Soft Edge), but you can also get filters that have a stronger visual line between the dark half and the light half (Hard Edge).
If you have ever been in a situation where you had to decide whether to meter off the bright sky or the dark ground, you could have used a GND filter. Placing a GND filter so that the ND part goes over the sky and the clear part goes on the ground will even out the exposure needed for both areas – Leveling the playing field so to speak. With a GND, you should be able to get the richness of a beautiful sky mixed with the detail of the scenery.
How to Use a GND
Using a GND is pretty easy. The first thing you have to figure out is which one to use. In order to do this, you should meter off the sky, then meter off ground (no sky in the shot). The difference is stops between the two will tell you what level of GND is needed. f/11 at 1/4sec and f/11 at 1/15sec would be 2-stops and need a ND4 filter.
Once you have chosen the correct GND, mount it on your camera so that the darker part is over the bright sky and the lighter part is on the ground. The square filter system, like the one I have, allows you to move the filter up and down in the mounting bracket. This allows you to properly position the gradient to the scene you are shooting. You can purchase circular GNDs that screw onto the lens, but you have no control over the position of the dark and light areas without moving the camera.
Some filter systems allow you to mount multiple filters over each other. It all depends on the desired effects.
Some Key Points
- ND Filters are great for softening moving water or lowering depth of field.
- GND Filters are great for landscape shots where you want to make the sky even out with the ground to a certain degree.
- Meter off the sky and the ground to find out what GND to use.
- External mounted GND filters (square or rectangle) allow you to control the position of the filter.
- For more information, check out Wikipedia on the subject