Before we can discuss how to use Neutral Density (ND) filters it helps to get a little idea of what they are and how they help. For the most part ND filters screw on to the front of your lens. There are exceptions, particularly for larger lenses and for graduated ND filters. For our purposes we will assume you would use the screw on type.
When you screw this filter on your lens you are able to fool the camera into thinking the light is much less than it really is. For example, a 2-stop ND filter makes your camera think there are two stops less light than there really is. Therefore, with the filter in place you must change your exposure by 2-stops to get a proper exposure. The key here is proper exposure. Usually, you use one of these filters to slow down the shutter speed so you can show motion. Without them the only way to change the shutter speed and still get the proper exposure is to compensate with either aperture or ISO, and even then you have limits. Adjusting the aperture or ISO could have an adverse effect on the image, or not capture the effect you’re looking for. So it’s pretty safe to say that you use these when you want to slow your shutter speed.
When you buy one of these filters you will buy it based on the number of stops you want for adjusting the exposure. As mentioned, these filters are usually used to slow the shutter speed in a given situation. This means that you buy these, and use them, based on the number of stops you want to reduce the shutter speed. It’s difficult to know in advance the number of stops you will want to adjust for because you don’t know what the lighting will be. Therefore, you can buy an assortment of ND filters for differing number of stops. One thing to keep in mind is that the filters are stackable. This means you can stack a 3-stop ND filter on a 2-stop ND filter to obtain a 5-stop adjustment. Just remember that you probably will begin to push it when you get past 10-stops. Also, stacking these filters will multiply any deficiencies that may be inherent in adding any additional glass in front of the lens. I would recommend you start with a couple, maybe a 2-stop and a 3-stop. You can find these for about $50 each on the cheap end and about $200 each on the expensive end. The differences are primarily the lens coating and whether they are glass or plastic. You will probably want to do some homework about the differences in results each type will render before making the purchase. I own both and the more expensive ones definitely make a difference.
So when will you want to use these? Well, there are a few different ways to get some really good results. As I said before they are good for showing motion. This is because they slow down the shutter without making you change the aperture or ISO. So you can still get the sharp image of the stationary items, but water or clouds in motion can show a silkiness that can be really stunning. Here’s an image I took of the waves on the beach in Bar Harbor Maine. It was shot at F-22, 1/30, and ISO of 200.
This was at about 4:00pm in the afternoon on a fall day. The ND filter was a 3-stop filter causing me to change the shutter speed from 1/250 to 1/30. This allowed the moving water to have the silkiness you see.
It’s important to note the process. If your shooting in Manual mode you’ll want to get your meter reading before you screw on the filter. This is because the filters darken the view so much that you may not be able to see through the view finder. This is especially true for 3 or more stops. It’s not necessary if you are shooting in Aperture Priority mode because the camera will automatically adjust the shutter speed for you. One thing you will want to be sure to use is a tripod. Most of the time you’ll slow the shutter down to a point where hand holding is not an option. Since you’re wanting to steady the camera for a slow exposure you’ll also want to use a cable release, or a mirror up procedure. This further increases the possibility that stationary objects will be tack sharp while everything else shows motion. Finally, if you find yourself exposing for extremely slow speeds you may want to block the view finder. A piece of gaffer’s tape will do the job. This keeps extraneous light from finding it’s way into the exposure.
Below is another image. This one was taken in Big Sky, Montana just before noon. See how silky the falls look and how detailed the trees and ground are?
I stacked my 3-stop ND filter with the 2-stop ND filter to obtain 5-stops of difference. The shot was taken at F-22, 1/3, and ISO of 200. That means the original exposure measured a shutter speed of 1/100.
Finally here’s one I took of Sylvan Lake in South Dakota, just outside of Rapid City.
This was a fairly windy day with a bright sun above and a little left of the camera. In this case I didn't want to show motion, but to show a stillness in the water and a hazy look in the clouds. By slowing the shutter speed even more, than in the images above, I was able to go from a silky look to a very glassy look. The image was taken in the middle of the day with wispy clouds and a very bright sun. Landscape photographers will tell you that as a rule this is the worst time of day, but if you have a 10-stop ND filter you can do some very interesting things. Notice how the clouds look very milky and the water is very glassy and still. The conversion to black and white also added some creativity to really make it interesting. This image was taken at F-20, 15 sec shutter, and ISO of 200. The original exposure measured a shutter speed of 1/60. The ND filter slowed it to 15 seconds adding the glassy look to the water and the milky look to the clouds. Scott Kelby has a very good and short video on how he does this. Probably worth a look if you think you may want to try it.
One more thing I want to mention. It is important to note that if your objective is to show motion or stillness, as I have in these examples, then your camera should be set to either Manual or Aperture Priority mode. Otherwise, your camera software will adjust the exposure by making changes to the aperture and defeat the purpose.