So there can sometimes be a little confusion about neutral density (ND) filters – what are they, why are they useful, what are their differences and what do all the numbers mean? Fear not – I will explain all the above and more, so you’ll know why you may want one (or a few) and which type is best for you.
When you take a photograph in bright daylight you will notice that you have to use a fast shutter speed. This is simply to limit the amount of light that can enter the camera to prevent overexposure. Similarly where you want to use a very wide aperture for shallow depth of field, then problems can arise where your image can quickly become overexposed.
So how do you tackle the problems that occur where you want to use a slow shutter speed or very wide aperture in bright light? Well this is where ND filters come into their own!
What are Neutral Density Filters?
An ND filter is simply a darkened piece of glass that you can place in front of the lens to limit the amount of light that can pass through. Similar to a pair of sunglasses that we would wear on a bright sunny day.
The filters come in different strengths for the amount of light reduction they effect. The reduction is usually measured in ‘stops’ of light. So a 6 stop filter will reduce the amount of light that can pass through by 6 stops of light. In practical terms this means that using a 6 stop filter will enable you to extend the shutter speed by 6 stops to obtain the correct exposure.
We have just seen that the ND filters block out stops of light. Just briefly, different manufacturers use different marking systems to describe their filters.
The ‘F-stop reduction’ amount is just a simple definition of how many stops of light the filter will block out – e.g 2 stops of light etc.
The ‘filter factor’ you often see, such as ND4 (for a 2 stop filter) is a representation of the factor which the filter reduces the light entering the lens.
The ‘optical density’, such as 0.6 (for a 2 stop filter) is a decimal number based on a mathematical equation of the light-cutting abilities.
Neutral Density Filter Types
There are a number of different types of ND filters available:
These simply apply the darkening effect to the whole filter, which affects everything that is photographed.
As the name here suggests – the filter graduates from ND to transparent, so the filter can be positioned to darken the brightest parts of the scene – e.g where you want to balance a bright sky with the darker foreground.
The graduated filter comes either with a hard edge where the transition between the clear and ND is a straight line (useful for shooting where you can see a clear horizon line), or soft edge where the transition is softer and more graduated.
Round filters are the more common type of filter, where they are screwed directly onto the front of the lens. This enables you to apply the filter to the lens quickly and they are quite robust and durable. The only downside with this type of filter is that you need a specific filter for each size of lens that you have – as many lenses have different diameters for the front. It is possible to use step-up/step-down rings which act as adapters so you can use the same filter on different lenses.
Some ultra-wide angle lenses may vignette (have darkening in the corners) where using an ND filter, so this is something to bear in mind. Although manufacturers are addressing this problem with thinner filters.
The system filter has a filter holder that clips onto the lens and then the larger square or rectangle filters are placed into the holder. The holder can be rotated to help position the filter and can hold a number of filters at the same time. If you will be using graduated filters often then this system is more useful as it allows you to be able to position them accurately.
The downside with this system is that it is bulky and heavy and more expensive than round screw-in filters. As the filters themselves are larger and usually glass (although they are available as resin and plastic), they are more prone to breaking and scratching.
Why Use a Neutral Density Filter
So as briefly mentioned above, a neutral density filter will help you to control exposure where you are using a slower shutter speed. You may want to use a slow shutter speed in daytime to:
add blur to an image
shoot with a wide aperture
capture a long period of time
enable some other creative effects to your images
Creative Effects with a Neutral Density Filter
An ND filter will enable the use of a slow shutter which can help capture moving water and waves as a flat ethereal element in an image. The speed of the waves or flow will affect how it is recorded and the strength of filter that you will want to use.
Again using an ND filter will enable the movement of a waterfall to be captured in a dreamy soft flow. You often don’t need a very strong ND filter here as the movement of the fast flowing water will be captured quite quickly.
The movement of clouds can be subtle or strong depending on the wind. A strong ND filter will enable you to capture the movement of slow moving clouds that can be barely noticeable to the naked eye. The image below is a 5 minute exposure where the clouds were moving relatively slowly.
Removing People and Moving Objects
As people or traffic moves through an area, using a neutral density filter to extend shutter times can help remove them from the image. Depending on the speed of their movement, they will often not remain in the scene long enough to be captured in the final image. This is a useful technique to help remove tourists from a popular landmark or place, such as this busy train station below where the pedestrians all walked through the scene and were not recorded in the final image.
I hope that you find this article useful. For more detailed information on Neutral Density filters and all other aspects of long exposure photography, please do check out my book – Mastering Long Exposure Photography – A Definitive Guide, published by Ammonite Press and available at all good bookstores and online.
Naturally please also feel free to contact me if you have any questions or comments relating to this topic or any other photography question. Please do pass this post onto any photographer friends that you think may find this useful.