The neutral density (ND) filter is second in importance to only the circular polarizing filter (C-PL). I will write a separate dedicated article on the importance of the C-PL, which will highlight the many advantages of this wonderful accessory for your lens. In this article, however, I shall be concentrating on the neutral density filter and its many avatars, its uses and advantages.
In a previous article on landscape photography I referred to this very versatile filter, albeit briefly. Well, the ND filter is actually made out of glass. Some cheaper ones are made of resin. Its sits in front of a lens and blocks light. It is coated in a way so that it stops light but doesn’t create any color cast, meaning all wavelengths of light pass through equally. They are primarily used for balancing exposures and stopping light for creative photography.
To make it simpler to understand let’s take an example. Let’s say that you are shooting outdoors on a clear bright day. You want to use a wide aperture so that you can create a shallow depth of field. However, with the bright ambient light you are constantly overexposing the scene, even after using a fast shutter speed. In a situation like this a neutral density filter works as a sunglass for your camera. It allows you to use a small f-number (wide aperture) without risking overexposure.
Let’s take another example. Let’s say you are now shooting at the beach. It’s afternoon and it’s sunny. Let’s also presume that you are trying to use a long shutter speed to capture the tide coming in, in effect trying to capture a misty effect of the water. Unfortunately, the meter of your camera is telling you that you are overexposing the scene way too much. If you take the shot regardless of what the meter says, the image will be completely washed out. To overcome the problem and get away with what you want, you will need to use a ND filter.
Then there are some really innovative uses too. Let’s say you are shooting wildlife in Kenya. The sun is lower to the horizon and decide to take a few shots of an African sunset. Suddenly you notice some wild elephants at a distance. Quickly setting up your camera and taking a few shots you realize that the elephants are not quite 100% silhouetted. A grey tone is still comprehensible. To make the herd pf elephants completely silhouetted you can use the neutral density filter to stop some light.
Things you need to keep in mind when buying these filters
The light stopping power of these filters are expressed in either stops like 1-stop, 2-stop, 3-stop or as 0.3, 0.6, 0.9 and so on. They both mean the same thing. A 1-stop ND filter will half the amount of light reaching the sensor. A 2-stop filter will allow only 25% of light to pass through and so on. Let’s say a scene meters f/8 at 1/125 of a second. You need a slower shutter speed, something around 1/15 of a second. So, basically you need a three stop slower shutter speed. This can be achieved by using a 3-stop ND filter.
ND filters come in two shapes, square and round. Round filters attach to the front of a lens by screwing. In order to use square filters you need to buy an adapter ring and a filter holder, both separately available. Round filters will come with clear specification mentioning their size. In order to buy a filter for your lens refer to the filter diameter specification mentioned at the front section of the lens.
Square filters are available in various sizes. Lee, a very popular brand makes filters that 100mm in width and require filter holders that are compatible. On the other hand a very popular filter holder system, Cokin’s P series can only accommodate 84mm filters. Thus Lee filters are not compatible on the Cokin-P series holders. Singh-Ray, another popular brand makes 84mm filters which are compatible with the Cokin holders. Keep this aspect in mind when buying filters, holders and adapters.
A major advantage of square filters is that a single filter can be used on all your lenses. All you need to do is buy the correct adapter ring for the specific lens. If you prefer round filters, you will need to buy separate filters for all your lenses.
These filters are either completely dark or graduated which means the intensity of light stopping power increases from the clear to the dark edge. There is also a reverse graduated variety which has a clear edge and a graduated one which increases in intensity towards the middle of the filter. This variety is particularly suitable for sunrise and sunset photos because the sun is closer to the horizon which matches with the maximum light stopping power at the middle section of the filter.
Invest in a filter that is made of glass and is multicoated to ensure there are no color casts, defects that reduce the optical performance of the lens. Buy several filters of different strengths because a single filter may not be enough. That will give you greater flexibility to work with.