Have you ever wondered what filters are and precisely how they can improve the quality of your photographs? In short, filters are pieces of glass that one places in front of a camera to manipulate the incoming light before it even has a chance to reach the optics of the lens. If you search hard enough, it will quickly become apparent that there are a raft of different filters on the market, each with their own unique light altering properties. However, by far the most common versions of filters include ultraviolet (UV) filters, neutral density (ND) filters, graduated neutral density filters (ND Grad) and polarising filters.
Before we even probe the various incarnations of the humble filter, let us clear something up straight away. Lenses are expensive bits of kit, and even the cheapest of them need to be respected. It does not make sense to have finely honed glass in your lens and to place a filter in front of it unless the filter is making a meaningful impact on the final image. Every single piece of glass between your scene and your sensor is taking a little bit away from the final image quality. The use of a filter, no matter which one, must be justified and it is the complete lack of any such justification that represents my biggest problem with the first filter that I’ll discuss – the UV filter.
When talking about filters, it is useful to keep in mind that all filters are in the business of removing the light from a scene – they cannot add to it. The UV filter, for example, is designed to eliminate the ultraviolet light from a scene and here in lies the catch. As human beings, we cannot see ultraviolet light, which means that, effectively, the UV filter is designed to remove something that isn’t 'there' to begin with.
There is one argument to support the use of a UV filter; many camera and lens suppliers mandate them to complete the weather sealing of the camera. What salespeople often neglect to point out is that most entry-level and enthusiast camera bodies are not weather-sealed themselves, making the addition of a UV filter redundant. In fact, UV filters only detract from image quality and in the vast majority of cases provide no additional weather sealing. It goes without saying then that with no material benefit, the UV filter has no place in front of a lens.
Of the filters that do serve a purpose, the simplest to understand is the neutral density filter. The ND filter is designed to reduce all the light entering a camera uniformly. Put another way, the intensity of all the light passing through the filter is reduced by the same amount regardless of its colour. The reds in a scene will have their intensity reduced by the same amount as the greens and the blues. The effect of the reduction in light intensity is often described as placing a set of sunglasses on the lens. Light is cut out of the scene, allowing the photographer a certain degree of creative freedom when deciding upon the exposure settings.
One of the most common – and apparent – outlets for this creative freedom is holding the shutter open for longer. With the ND filter reducing the amount of inbound light, the shutter time can be increased – or even dramatically increased in the case of ‘big stoppers’ – for a given aperture and ISO setting. The liberty to do this permits photographers the opportunity to capture motion in images, for example, silky flowing water or the streaking of clouds across the sky.
A less familiar but equally fitting artistic freedom occurs when the ND filter provides photographers with the option of shooting with a shallower depth of field. For a given shutter speed and ISO, working with the reduced light from an ND filter means that one can use a larger aperture setting than one otherwise might. The larger aperture produces a shallower depth of field, a phenomenon that is most desirable when trying to isolate a subject from its background.
An excellent example of isolating the subject can often be found in outdoor portraits, where photographers are seeking a beautifully blurred background – the consequence of a shallow depth of field – to contrast their pin-sharp subject. In some cases, the use of a wide aperture outdoors may be curtailed as there is simply too much ambient light for the camera to cope with, even when set to its fastest shutter speed. A neutral density filter can help in these situations by reducing the amount of light reaching the camera, making the use of large apertures a viable alternative.
To accommodate the various lighting scenarios that people encounter daily, ND filters are supplied in a variety of different stopping powers. In fairness, when it comes to ND filters, the naming convention chosen by the photographic industry is less than self-explanatory. Garden-variety ND filters typically have their stopping power expressed in ND Number Notation, for example, 0.3ND, 0.6ND, 0.9ND and so on. This system has its origins in the logarithmic nature of light intensity, and for all but the most informed of users, is simply too complicated to elaborate on. The only thing that is important to know is that 0.3ND equates to a 1-stop reduction in light intensity, 0.6ND to a 2-stop reduction and 0.9ND to a 3-stop reduction.
As a reminder, every ‘stop’ is equivalent to a halving of the available light; thus, a 1-stop reduction represents half the available light, and a 2-stop equates to a quarter of the available light. Put another way, a 2-stop is a half of a half and a 3-stop is a half of a half of a half, otherwise known as an eighth.
Typically, when one exceeds three stops of stopping power, the naming system becomes easier to understand. Filters in this realm are often referred to as ‘big stoppers’, ND filters that attenuate a tremendous amount of light. Five, ten and even fifteen stops of stopping power are available, and thankfully the filters are named accordingly. To place the decrease in power into perspective, 10-stops mean that only 1/1024 of the available light is reaching the camera sensor. As a consequence, an exposure with a normal shutter speed of 1/60 of a second would now need to be exposed for a full 15 seconds were a 10-stop filter attached. Needless to say, that with the shutter open for that length of time, motion blur is almost guaranteed, and while the effect is not to everybody’s taste, some of the results are unquestionably spectacular.
Finally, when considering ND filters, it is important to realise that not all filters are created equal. The biggest difficulty with ND filters is that the attenuation of the light is not as uniform as it ideally should be. Invariably, certain of the colours in the spectrum will be attenuated more than others, and this inconsistency manifests as a colour cast in the final image. Each brand and indeed each particular variation of stopping power will have its own optical characteristics and hence its own colour cast. More expensive filters will have better control over their colour output while cheaper ones won’t. Fortunately, out of all the filters, the colour cast of the ND filter is the easiest to correct during post-processing as it occurs throughout the frame. This is not to say that accurately returning the colour in an image to that which was observed is easy, just that it is easier than with other filters which produce localised colour variations.
I should also point out that the presence of a colour cast at all is perhaps the biggest reason why some people prefer not to use filters. The truth is that the effects created by ND filters can be reproduced digitally during post-processing, so why struggle with correcting the colour in photos at all? The answer is that, especially when speaking of big stoppers, the techniques required to achieve an accurate motion-blur-look are cumbersome and time-consuming and certainly well beyond the interests of most photography enthusiasts.
The Introduction to Filters Series
This series of articles takes a detailed look at the four most common filters available on the market today: the Ultraviolet, Neutral Density, Graduated Neutral Density and Polarising filters.