What are Neutral Density Filters in Photography and Why You Need One

What does an ND Filter Do?

An ND filter reduces the amount of light that passes through it, therefore reduces the amount of light that is recorded on the camera's sensor. It is basically a darkened piece of glass that is designed to limit the amount of light passing through, not to add colour or polarization.

Adding an ND filter allows for creative effects such as using a wider aperture or a using a longer shutter speed that would not be possible otherwise. They are most useful in bright conditions where there is a lot of available light.

In this example, the first photo looks rather static with everything frozen in place. The longer exposure of the second shot has resulted in the water movement in the sea smoothing out, and the clouds blurring across the sky. This example also highlights the effects of using cheap ND filters - they are not 100% neutral and introduce a colour cast. These were cheap plastic filters, I would definitely recommend spending the extra for a quality set of glass filters.

Type of ND Filters

There are different types of neutral density filters, and they offer different strengths, depending on how much light you want to block. This is typically measured using the number of "stops" of light the filter blocks.

Commonly used filters are 3 stop filters, 6 stop filters, and 10 stop filters. In this example we can see three ND filters (solid) and graduated filters where the amount of light blocked varies down the filter. Graduated filters are commonly used to preserve sky detail whilst exposing for the foreground.

A "stop" is term used to measure the of the amount of light. If you increment by 1 stop, you are doubling, or halving, the amount of light. So for example, if you go from a shutter speed of 1/250th of a second to 1/125th of a second (twice as long), you are doubling the amount of light.

• A three-stop filter would go from a shutter speed of 1/250th of a second to 1/30th of a second.
• A six-stop filter would go from a shutter speed of 1/250th of a second to 1/4 of a second.
• A ten stop filter would go from a shutter speed of 1/250th of a second to 4 seconds.

Solid or Graduated Filters

Solid neutral density filters are just dark bits of glass designed to slow exposures. A 0.9 (3-stop) ND is a good general-purpose filter handy for slowing the world down. You can see examples of each in the photo above. As I said above, the thing to remember with all filters is the optical quality. A poor quality filter can cause a lens flare, rob a lens of its precious performance and introduce colour cast or fringing.

A graduated filter is dark at the top and clear at the bottom. These are typically used to balance the exposure between the land and the sky. There are also two types of graduated filter - hard and soft - and this refers to how sharp the gradient transitions. Soft has a smooth gradual transition, while hard is more abrupt.

When Should I Use An ND Filter?

An ND filter is ideal for use in landscape photography, particularly when you want long-exposure effects such as a milky effect in water or to show the movement of clouds in the sky.

Water, and particularly waterfalls, are perfect examples of when you would want to use an ND Filter. Moving water creates a really nice effect when you use low shutter speeds (1/15th of a second and longer).

Achieving low shutter speeds during the day is practically impossible. Even if you close the aperture as small as possible, say f/32, and use an ISO 100 setting, in most cases you'll struggle to shoot at shutter speeds slower than 1/30th of a second.

This is when the ND filter comes in. We can use the ND filter to cut down on the light and shoot for longer shutter speeds without having to resort to crazy-high apertures. Using apertures above f/16 is generally not recommended as diffraction can reduce the quality and sharpness in the photos.

What Do I Need to Look for When Buying An ND Filter?

There are a number of different features to look for when buying an ND Filter, but the most important is quality. Poor quality filters will introduce distortion, colour cast and other negative effects. Spend a little extra to get quality filters from a good manufacturer.

There are two main options for attaching filters to lenses. Filters can either screw directly onto the lens or they'll slot into a filter holder on your lens. Each filter type has its own pros and cons.

Screw-in filters are easier to put on and remove, however different lenses have different thread sizes, and if you have a number of lenses, you'll need different size filters for all of them. If you're buying a screw-in filter, you need to make sure it is the same size as the screw thread on your lens. This will be a number measured in mm, and is usually written on the lens, for example ⌀ 67mm.

Slot-in filters (as shown above) allow you to more easily stack filters, and as they attach to the lens body, once you have bought the filter holder, you can use the filters across lenses. Individual filters are generally less expensive than screw-in filters, after the initial investment in the mounting system.

The final thing to consider when buying an ND filter is that some cheaper filters can cause a colour cast. Whilst this can usually be corrected in post-processing, it is something to be aware of.

Do I Need Anything Else With My ND Filter?

A tripod is essential for using an ND filter. This is because we are reducing the shutter speed to one that is below the threshold of stability that image stabilization is not able to cope with.