A what? Some readers will probably ask themselves after reading the title of this post. For those that do not know what a neutral-density filter is, it is a filter that all travel photographers should carry with them as it is the only filter that you should apply before taking pictures, while most other filters can also be applied post production.

What is a neutral-density filter?

A neutral-density filter (abbreviated ND filter and sometimes referred to as grey filter) is basically a sunglass for your camera. It is a filter to reduce the amount of light entering the lens without changing color or hue. Using a neutral-density filter allows the photographer to use aperture, exposure time or for example ISO-settings that would otherwise result in overexposed photos.

Why should you use a neutral-density filter?

Most people know those amazing night time shots of light trails. These images are taken with long exposure settings. Usually a shutter speed between 10 and 20 seconds. When you want a similar effect during day time, there is way too much light to use long shutter speeds or an open aperture. Your picture will quickly become overexposed. To make sure that your picture won’t be overexposed, you can use a neutral-density filter.
You can use a neutral-density filter for time-lapse photography: a neutral-density filter allows you to take pictures such as the light trail that are needed for time-lapse photography. For landscape photography: the neutral-density filter allows you, for example, to blur water, creating a misty effect. And for architecture photography: by using an extreme long shutter speed, you can filter out people.

Different neutral-density filters

There is a variety of neutral-density filters available. It can therefore be difficult to figure out which type of neutral-density filter you need for your setting. To make it even more complicate, every manufacturer uses their own terms to indicate the strength of the filter. For example, an ND2 has an optical density of 0.3, an f-stop reduction of 1 and a fractional transmittance of 50%. This means that when you use an ND2 filter, you need go back one step in your settings. For example from a shutter speed of 1/250s to 1/125s, or aperture from f4.0 to f2.8, or ISO from 100 to 200. The stronger the filter, the more steps you have to go back in settings to take your picture. See below.

Variable neutral-density filters

Seeing the table above, one might conclude that you need a whole range of neutral-density filters to be entirely flexible in your shooting. That is true. However, there are also variable neutral-density filters. Variable neutral-density filters work by placing two polarizing filters together. By rotating the front polarizing filter, you stop a certain amount of light. However, variable neutral-density filters affect the image quality and can even results in undesired effects (such as a large cross in the middle of the frame when using a wide angle lens). For photography, separate neutral-density filters are therefore the best way to go.

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