Neutral density filters are primarily used for balancing exposures. By balancing exposure, I mean adjusting the amount of light that enters the lens so that the scene appears uniformly lit across the frame and every inch of it is exposed properly. A properly balanced image will show a histogram that is clustered more towards the middle of the graph. Too much on the right and you have an over-exposed image and too much on the left means you have under-exposed.
Technically speaking, an under-exposed image is better than an over-exposed one, because you would still be able to salvage some details from an under-exposed image. You would need to shoot in RAW for that. However, getting the exposure correct in-camera is the best option by far.
The reason these filters are called as neutral is because they stop light across the spectrum equally, without any bias. Consider them as shades for your camera lens.
Neutral density filters are pieces of glass that are placed in front of the lens. They come in a wide variety of sizes and light stopping power. The two main shapes are circular and rectangular. They come in two varieties, normal and graduated. Graduated neutral density filters are also known as Grad NDs. Grad NDs again are divided into two main types, hard-stop and soft. It is to do with the transition between the clear and the light stopping sides of the lens. There is yet another variety known as Variable neutral Density filters. Many other types are also available such as the reverse ND filter. These have specific uses.
The light stopping power of neutral density filters are usually expressed in numbers such as 2, 4, 8 and 16. An ND2 filter will halve the amount of light entering the lens for the given aperture and shutter speed. The same way a ND4 filter will reduce the light to only 25%, at the same aperture and shutter speed. Sometimes, the associated number is 0.3, 0.6, 0.9 and so on. 0.3 means one-stop of light, 0.6 means 2-stops of light and so on. As such a 1.8 ND filter has a whopping 6-stop light stopping power. It allows you the freedom to increase the exposure time and or use a significantly larger aperture than what can be used otherwise.
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Let’s say the camera meters the scene at f/11 and 1/200th of a second. ISO is 100. If you wish to use a shutter speed of 1/100th of a second, you will need to be able to stop half the light with the new exposure value to eliminate the possibility of a washed out image. What you need is a ND2 filter.
Circular neutral density filters
The circular ones are the more common variety. They are also preferred by amateurs because they are easy to carry and easier to use too. However, they have some serious drawbacks. These filters do not slide up and down across the front of the lens, making it impossible for photographers to lineup the horizon line with the demarcation between the clear and the dark sides of the filter without changing the composition. Secondly, if your lenses are of different diameters you will need to buy separate filters for each lens.
Rectangular neutral density filters
The rectangular neutral density filters (sometimes also referred to as square filters) are the professional’s choice. Having said, that these filters are a bit more cumbersome to set up. These require a filter holder to slide the filter and an adapter ring to attach the filter holder to the lens. One of the most popular filter systems is Cokin’s P series. These system allows use of filters with lenses having a diameter of up to 82mm. The filter width can be a maximum of 84mm. A wide variety of adapters are available. You will need to buy an adapter ring matching the filter thread specification of your lens. If you are unsure about the filter size of your lens simply look at the reverse of the lens cap. There should be a number mentioned followed by a symbol which looks like this ø.
There are many advantages to using a rectangular filter system. These filters slide up and down the filter holder allowing you to precisely line up the transition line against the horizon, without having to move the camera itself. Secondly, if all your lenses are within 82mm you could use the same filter and filter holder for all your lenses. All you need to purchase is a filter holder for each of the lenses.
How to use a ND filter
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A circular filter needs to be screwed on to the front of the lens in order to be used. The only creative freedom you have is choosing the right filter. From here on you simply need to get the composition and framing right. Please note that you will need to switch to manual exposure when using a ND filter because otherwise the camera’s exposure metering system will try to compensate for the lack of light.
With a rectangular ND filter you have a little more creative option. Sliding the ND filter up and down creates an effect known as feathering. This blurs out the transition line (ND Grads) allowing for better exposure balancing across the frame.
As already discussed briefly above, graduated neutral density filters are a variation over the vanilla neutral density filters. These allow a user to block light off of a portion of the composition while allowing light from other areas of the composition to reach the sensor. E.g., if you are photographing a waterfall in a thickly wooded area, the foreground would be comparatively darker than the background. Ambient light of the sky needs to be balanced else the composition will be washed out. A graduated neutral density filter will allow you to achieve that perfect balance.
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Again, if you are photographing a sunrise or a sunset, that perfect balance between the foreground and the background is difficult to be achieved without the use of a graduated neutral density filter. In this case, however, you will need a reverse graduated neutral density filter or also known as the reverse ND grad. ND Grads come in both circular and rectangular varieties.
Variable ND Grads
The last, but certainly not the least important, by any stretch of imagination, the variable ND grads are a piece of ingenuity. These are usually available in circular sizes. These filters have a several pieces of glass compressed together. To use it just screw it on to the front element of your lens. Now, when you turn the filter slowly, its light stopping power increases gradually and then decreases. The effect is similar to what you would expect in a circular polarizer but in this case instead of the light getting polarized it gets stopped. Works great if you are going to need a ND filter but not sure how much light stopping power you need.
Which one should I buy?
If you are merely dabbling into the idea of ND filter systems and the art of creative photography, then a circular filter would be ok for you to start with. However, if you are more into precision high quality photos and need to be able to use several combinations of ND grads, NDs and polarizers, then you should be investing in a rectangular system.