A neutral density (“ND filter) is a piece of photographic equipment that is used to decrease shutter speeds in creative photography to convey a sense of movement in a photograph.
The best ND filters are constructed out of glass or high-grade resin and are placed over the camera lens to restrict the amount of light hitting the sensor. ND filters are neutral gray in colour and do not affect the colour saturation or hue of the photograph. ND filters are typically available in one stop, two stop and three stop versions, although some manufacturers make a five stop and ten stop variety for times when an exceptionally long exposure is required for the desired effect. The stronger the ND filter the darker it is. For example, the ten stop ND filter will be much darker in colour than a one stop ND filter. It is possible to use more than one ND filter at a time, a term known as stacking, although it is not recommended to use more than three in a stack as the image quality is likely to suffer somewhat.
Typically, there are two different types of ND filters. The first is the screw in type, which as its name suggests simply screws in to the end of the lens. These filters are circular in shape. Many photographers keep a UV filter on the lens at all times in order to keep the lens glass clean and protect it from scratches, and if you are one of these, you must remember to remove the filter before attaching the ND filter. The screw in filters are advantageous in they are easy to use. However, if you have a range of lenses all accepting different filter sizes, and this is very common, you are going to need a separate ND filter for each different size lens, which is likely to become expensive.
The second type is a variation of the screw in type and consists of a square or rectangular shaped holder, which attaches to the lens by screwing it in to the lens’ filter holder. The ND filter, which is also square or rectangular, sits in the holder. The main disadvantage of this square type ND filter arrangement is there is more equipment to carry around. That said, the square type ND filter arrangement is the most popular and it is easy to see why. Having a holder means it is possible to change the ND filters quickly and easily without having to constantly screw filters in and out of the lens, hence reducing wear and tear and potential damage to the lens. The ND filters can be used with all your lenses. You will need to buy different sized rings for the holder but these are inexpensive compared to shelling out for a whole filter, so there is a definite cost saving to be made with the square type ND filter set up.
Look through any photographic magazine or search on any photography equipment supplier’s website and you will find many companies that make both screw in type ND filters and the square type ND filters. As with everything, Ebay is flooded with many cheap and cheerful ND filters from China, so if you only want to experiment with ND filters and want to spend as little as possible this may be your first port of call. If you want a decent set of ND filters then avoid Ebay altogether and buy products from a reputable company such as Lee, Cokin or Hoya, all of which produces good quality and well made ND filter sets.
So, with the above in mind how do we go about using ND filters? Using ND filters is not difficult and it is easy to pick up. Anyone can do it. There is no additional specialist knowledge or theory to learn so anyone who can take a photograph can use a ND filter. It is no more than following a series of methodical steps.
The first thing to do is to set up and compose the shot. Leaving the ND filter off the lens for the time being put the camera on a tripod and frame the shot. With the camera set in aperture priority mode set the desired aperture and gently press the shutter button to focus and take a meter reading. Remember, the objective is to reduce the shutter speed for a longer exposure to create some motion blur.
Now you have the meter reading it is simply a case of deciding how much longer you want to want the shutter to stay open for, i.e. reduce the shutter speed. If you want to reduce the shutter speed by one stop the one stop ND filter is required, if you want to reduce the shutter speed by two stops the two stop ND filter is used and so on.
So, let’s say the metering reading is ISO 200, f16 at 1/4 seconds and we want to reduce the exposure by three stops. The new settings would be ISO 200, f16 at two seconds. Reminder: to reduce the exposure by one stop double the shutter speed, to adjust by two stops double the shutter speed again and to adjust by three stops double the shutter speed again. The sequence would be 1/4, 1/2, 1, 2.
The next step is to place the ND filter over the lens. If using a screw in type ND filter screw it in to the filter part at the end of the lens, remembering to remove any other filters that may already be on the lens first. If using the square type ND filters attach the holder to the end of the lens, once again remembering to remove any existing filter, and drop the relevant ND filter in to the holder. With the ND filter in situ, it is a matter of pressing the shutter release and taking the shot.
Once the exposure is complete you should always check the LCD screen and the histogram to ensure the image is how you want it. If the movement blur is not strong enough take the shot again using a stronger ND filter or stacking ND filters as necessary. For example, if you want to reduce the exposure by five stops and do not have a five stop ND filter it is possible to achieve the same effect by stacking the two and three stop ND filters for the exposure. Alternatively, if the movement blur is too strong we need to take the image again using a weaker ND filter.
Now we know what a ND filter is, a good company to buy them off and how to use them we need to explore the best situations in which to use them. Many photographers believe that an ND filter is used in landscape photography to balance the sky and the foreground to prevent a washed out sky and correctly exposed foreground or a correctly exposed sky and an underexposed foreground. An ND filter affects the entire frame therefore a ND filter is not best suited to this exercise.
Arguably, the best situation in which to use a ND filter is when taking images of fast moving water or clouds. Increasing the exposure time will make water smooth and wispy, which creates mood and atmosphere. Using a ND filter it is possible to make waves milky rather than freezing them. ND filters are suitable for taking photographs of seascapes, rivers and waterfalls. Similarly, increasing the exposure when taking photographs including moving clouds will render them wispy and smooth, once again this adds mood and atmosphere, which often makes for a more pleasing image.
ND filters are a great piece of equipment for creative photographers that want to add another dimension to their images by adding some movement and intentional blur. A set of filters is a must have accessory in all photography equipment bags and the square type holder system is highly recommended.