Choosing and using
the right filters for
With the advent of video capture on DSLR cameras the craft of cinematography has moved within the reach of many of those who previously just thought in terms of stills. Carey Duffy (Technical Director of filter company Tiffen in the UK) writes for CPN on what to look for when selecting the right filters for photography and filmmaking...
Prior to the HD video revolution, shooting professional quality motion images was only possible with the use of either a broadcast video camera or a 35mm or 16mm motion picture film camera. For the still photographer the investment to enter this area would have been huge but now technology has opened up the field to a whole new audience, who are having to learn the practical skills required to enter this emerging sector.
The first thing to realise is that, despite apparent similarities in what you are trying to achieve, the techniques you need to master to tackle cinematography and still photography are quite different. Understanding why these differences exist and what is required at a basic level to enable competent recording of an image will always be fundamental to each of these crafts.
For example, a cinematographer may have to consider the changing light conditions that will occur during a camera movement. Add that to the need for continuity as a basic requirement of storytelling and it starts to become clear why cinematographers find themselves having to work to a discipline which is very different to that faced by the still photographer. Exactly how a story is produced and told with a moving camera is defined by the cinematographer’s choice of tools, and there are many specialised products that are dedicated to the video-enabled DSLR to help you get the best results.
Filters are one of the many tools that both a photographer and a cinematographer employ to control the light entering the camera lens, either as a physical necessity, for aesthetic effect or even to achieve both these ends. A good tripod, lenses and a few other tools are likewise common in both disciplines. However, when shooting movement there is a physical difference from stills, in that a constant shutter speed has to be used. Then the question of how to control the intensity of light, especially for exterior shots, means that the use of filters as a primary tool becomes paramount.
Physics v aesthetics
Many people ask me why cinematographers, directors of photography (DoPs) or lighting cameramen use so many filters. The simple answer is ‘shutter speed’ – frames per second (fps). The constant shutter speed of a moving image camera means you must be able to keep a continuity of light reaching the image sensor, regardless of lens or camera angle. This is so that your footage will edit together and will not look as though it was exposed under different lighting conditions.
Neutral density filters are must-have tools to control excess light, and they are manufactured in a range of single stop densities. Of course the ability to increase, or open up, exposure via the lens iris is still a tool for controlling light, but cinematographers always try to shoot a scene at a constant stop, or within a certain stop range. Again this is to help control the look, including the depth-of-field (DOF), for continuity. Very rarely do you see a scene in any production where there are great differences in the DOF, from extremely shallow to very long DOF.
A photographic filter tool that has recently entered the motion-capture world is the Fader or Vari ND. This filter uses a combination of two polarisers that, when rotated against one another, reduce or increase the light. This is a very useful ‘run and gun’ filter that can do a good job, but for precise light control solid ‘single stop’ NDs remain the preferred choice of the professional.
Polarisers are used either to increase the contrast and colour saturation or to remove unwanted reflections, and they are one of the most frequently used camera filters for both still and moving image photography. However, you will have different considerations when using them in both mediums. If you’re shooting a moving image with a polariser, for example, you need to think about the change in the direction of light, and so of its polarisation, as the camera moves.
The simple act of moving a camera when a polariser is used will effectively lead to changes in the way light reaches the camera’s sensor. Since the direction of the polarised light has changed there will be inevitable changes in the image’s contrast and colour saturation or of the unwanted reflections. It is for this reason that polarisers are primarily recommended for locked-off video shots rather than shots where the camera is actually moving.
Graduated filters are also commonly used in cinematography, none more so than neutral density ‘grads’. Just as in still photography, these filters are designed to even out the exposure between the sky and foreground. They are manufactured in single-stop density steps, with both ‘soft edge’ for wide-angle and standard lenses and also ‘hard edge’ for use on longer lenses, where they produce the look of a soft edge. They can also be used architecturally, and here they would be lined up against hard-lined objects or buildings to allow you to even out the exposure or perhaps to add a dramatic effect.
Another extremely useful member of the ND grad family is the Neutral Blended Ratio Attenuator (NBRA). This is simply a full cover graduate that starts at its full ND stop density at the top of the filter and very softly graduates down to nothing at the bottom of the filter. The NBRA is an extremely useful filter, not only to control the light of uneven landscapes or urban horizons in exterior locations but also for interiors too.
The effectiveness of an NBRA depends on its positioning and chosen density in relation to the focal length of the lens in use. Unlike a soft- or hard-edge grad, an NBRA can be used when the camera is moving, as the line of the graduation is continuous. This has the effect on the shot of either light falling away as the camera moves into the darkest part of the frame or, conversely, to reveal as the camera moves away from the darkest area of the frame and then moves towards the light.
Colour graduated filters, meanwhile, help to enhance establishing shoots and moods for parts of the day, such as sunset to sunrise, blue skies to enrich a sunny blue sky or even green grads reversed to enhance grass in the foreground of the frame.
An important consideration to bear in mind is that since filters will need to be continually changed during a scene, conventional screw-in filters could slow up your schedule. This is why professional cinematographers and camera assistants tend to use matte boxes. Not only does a matte box serve as a lens shade across the wide variety of front-element sizes of your lenses but, by also keeping all of your filters to one size, all lenses can be filtered quickly and simply. Matte boxes also have rotating stages, whereas a sunshade has no rotation. This movement allows creative control of a filter’s placement for maximum effectiveness.
Contrast control filters
All of the filters mentioned so far generally deal with controlling light at a constant shutter speed, or fps, for exterior locations. Your effectiveness in shooting a clean exposure in video mode on DSLR cameras is very important, as the latitude of the stop range is commonly agreed to be around five stops. This means that setting your exposure may lead to clipping in over-exposed areas of the frame, leading to data loss, or underexposed dark shadows where you may have a requirement to see some of the action.
As shooting at a constant shutter speed presents exposure difficulties to cinematographers, contrast control filters also have a very important part to play in the ability to control uneven exposure within a frame. For this reason contrast control filters come in a number of different varieties and densities. They are not designed by stop densities but by increment, to take into account the different focal lengths of lenses used and to create continuity across a scene.
For example, a stronger density is generally used on a long focal length establishing shot, since the light hitting the image sensor is only passing through a small part of the filter’s area. Conversely, a lighter density of filter is used for a wider close-up shot as more of the filter’s area is being used. This switching between densities depends on focal length and the image in frame. It’s just one part of the effort a cinematographer needs to put in to ensure the continuity of the look across all of the film that is being produced for a particular project.
Low contrast filters create a small amount of ‘localised’ flare near highlight areas within the image, and this reduces contrast by lightening nearby areas of shadow, leaving highlights almost unchanged. Soft contrast filters include a light-absorbing element that, without exposure compensation, will reduce contrast by also darkening highlights, and cinematographers use these when lighter shadows are not desired. In both cases, the mild flare produced from bright highlights is sometimes used as a lighting effect.
The Tiffen Ultra Contrast filter series uses the surrounding ambient light, not just light in the image area, to evenly lighten shadows throughout. You can use it where contrast control is needed without any other effect on sharpness or highlight flare being apparent. You can also manipulate contrast via internal camera settings and combine with filtration for a tailored look in your film.
This feature is merely intended to be an introduction to the world of filters, so you can consider what filter tools you should be employing to burn-in your look on your moving images. Each production, scene and location requires individual consideration if you are going to achieve your desired look.
Knowing what tools will enable you to achieve the look you are after is part of the job of being a cinematographer. If you have filters readily to hand they can also provide a crucial last-minute fix to unexpected problems that digital post-production software can’t touch.
Biography: Carey Duffy
© Carey Duffy
Carey Duffy, son of legendary fashion photographer Brian Duffy, is Tiffen’s Technical Director and Operations Director of the London Filter Company in the UK. A passionate photographer himself, he is a regular speaker at photo events and shows on the importance of filters and camera technique.