Going Going GND
- June 2nd, 2010
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In the days of powerful image editing software such as Photoshop, Paint Shop Pro, GIMP, and others, some people may argue that things like filters are obsolete. Certainly, in some cases, with digital photography’s auto-white-balance, and shooting in RAW makes things like color correction filters not as useful as they once were, there are still some things that can’t be replicated as easily in an image editor. One of these effects is from the Graduated Neutral Density (GND) filter.
What is that, exactly? To answer that question, let’s look at the component parts of the name. A Neutral Density (ND) filter is an accessory that mounts onto the filter mount screw at the front of your lens. It cuts down the amount of light that will hit the sensor without distorting or discoloring the image (that’s where the ‘Neutral’ part comes in). Think of it as looking through tinted windows in your car. How much light it cuts down is dependent upon what type of ND filter you use. This can typically be determined by looking at the number beside the ‘ND’ in the filter. Below is a simple table that describes how many stops of light a particular ND filter will reduce:
| F Stop Reduction | Marking |
|---|---|
| 1 | ND2 / 0.3ND |
| 2 | ND4 / 0.6ND |
| 3 | ND8 / 0.9ND |
The ‘Graduated’ part of the name simply means that the filter (usually rectangular in shape) is coated with the specified ND strength in a graduated pattern (darker to lighter). As you can see from the photo below, the top part of the filter is quite dark, and gradually gets lighter in the bottom. The filter system by Cokin that I have comes in three parts: a ring that mounts onto the filter screws on the lens, the filter holder that slides onto the ring, then the filter itself, which slides into one of three tracks of the filter holder. Personally, I like this system a lot for a couple reasons: because it can be transferred between lenses of different sizes simply by purchasing the appropriate ring size, and also because it allows me to stack filters if I decide to (admittedly, I haven’t done that yet, but I like that I have the choice to when I do!)
So what can we do with this little contraption that we can’t do in our fancy schmancy image editor? Well, one of the most common things the GND is used for is to control exposure between the sky and the ground. Without the GND filter, we have to make a choice between having a well-exposed ground, but a blown-out white sky, or a nicely exposed sky, but a dark ground. For me, at least, I want it all! Have you ever looked at that sky and thought, if that sky was about 3 stops darker, the sky and the ground would look great together! Okay… If you’ve ever thought that, consider yourself addicted to photography! To solve our problem, all we have to do is slap on an GND8 and, based on our table above, we’ve dropped the sky down 3 stops, and now the sky and the ground can live together in perfect harmony in your frame. If I lost you in the geekiness of it all, the bottom line is that the GND8 will allow you to capture things like the the deep blue hues of the sky and the texture of the clouds. Without the GND filter, the detail in the clouds will be blown out beyond all recovery. Short of pasting in, or brushing in, a new set of clouds, the blown out patch of white will remain white. With the GND, you no longer have to compromise — you get beautiful tone throughout the image. Below is a comparison picture of the final result (GND on the left, no GND on the right). No Photoshop here, except for a mild saturation bump.

Well, that’s it for now. Hopefully, with the GND, you can graduate the level of of your landscape photos!



