The most common use of neutral density filters is for shooting creative photos of nature. Landscapes, waterfalls or any scene where there is a difference in luminance across the frame, neutral density filters can help you balance the exposure and get a good images. But that’s not all they can be used for. Neutral density or simply ND filters are also often used to shoot at slow shutter speeds, for a very creative effect. But what are neutral density filters exactly and how do neutral density filters work? Read on to find out.
Neutral Density filters
Neutral Density filters are small pieces of optical accessory, they are usually made of glass, which have a sort of coating applied to them that stops a certain amount of light from getting through. How much light will be stopped depends on the light stopping power of the filter. These filters are used creatively to get slow shutter speed images at daylight. Their effect is most noticeable when shooting flowing water. This is how photographers get the “cotton candy” look of water in daylight. Neutral density filters can be rectangular or round, depending on the mechanism by which they attach to the front of a lens. There are different types of ND filters used in photography, such as variable, graduated, reverse etc. You can read about other types of filters in our “Using Lens Filters” article.
0.3, 0.6 vs. ND 2, ND 4: What are These Numbers and What Do They Mean?
ND filters are made with different light stopping powers. Usually when you go to buy a ND filter you would come across specifications such as 0.3, 0.6, 0.9 or even ND 2, ND 4, ND 8, and so on. Both variations actually mean the same thing. A 0.3 ND filter is actually a 1 stop filter, a 0.6 ND filter is a 2-stop filter and a 0.9 is a 3-stop filter. Tiffen, a very popular brand measures the light stopping power of their ND filters in this way. Similarly, a ND 2 is a one stop Neutral Density filter that basically halves the amount of light entering the sensor. A ND 4 would be a two stop filter and so on… Cokin, another very popular brand, uses this naming convention for their filters.
How to Use These Filters
Round filters screw-in to the front of a lens. When buying a filter make sure to take a not of the size of your lens (52mm, 58mm, 77mm etc.) and get the right filter size for it. If you plan on using many lenses of various sizes, get the filter for your largest lens and buy step down adapters for others. Rectangular filters are attached via a filter holder that attaches itself to the front of a lens with the help of an adapter. Different filter manufacturers use varying attachment systems, which may or may not be compatible. Cokin’s P system is a very popular filter holder / adapter system, however most LEE filters will not fit into them. So when buying a filter / filter adapter system make sure to double check the specifications.
The mechanism for using these filters is very easy, the effects, however, can be quite dramatic to say the least. Let’s say you are photographing a small brook on a bright day. You want to capture a slow moving, smooth effect of the water, which can add more drama to the image. The problem is the brightness which forbids you from using a slow shutter speed. This is where a ND filter comes into the picture, literally.
The ND filter will stop a specific quantity of light, that will help you use a slower shutter speed. It’s like using the exposure compensation button, except, in this case, you don’t have to change the exposure value. You are basically putting on the sunglasses on your lens to allow you to shoot at slower shutter speeds..
How Slow a Shutter Speed You Can Use?
Well, that depends on the ambient light reading as well as the ND filter that you are using. If it’s too bright, let’s say LV 15ish, you wouldn’t be able to use too slow a shutter speed unless you are able to stop something like 3 to 4 stops of light at least. Every time you stop down, you reduce the amount of light entering the camera. So if the camera meters the scene at f/8 and 1/250 of a second, with a 3-stop ND filter you would be able to shoot at f/8 and 1/30 of a second. Even at that speed the effect may not be what you are after. So most photographers stack two or more filters together. A 3-stop plus a 1-stop filter combination (essentially making the combination a 4-stop filter) will allow you to use an even slower shutter speed of 1/15 of a second. Manufacturers know this and often sell Neutral density filters in kits with varying shade levels.
Variable Neutral Density Filters
If you are not interested in carrying many filters with you and stacking them together to get the right shade, you can opt for a Variable Neutral Density filter. This filter is capable of variable light reduction. They have a scale that you can refer to, and by simply rotating the filter’s ring you can change its light reduction by several stops. While this sounds like a very handy thing to have, keep in mind that many of these “ultimate” natural density filters can create “dark shadows” in some corners of an image. Some of them can also affect image colors. But the one major problem with Variable ND filters is that they can sometimes create a dreaded X over the entire image. This happens due to the structure of the variable filter and how its element interpolate.