There are two approaches to the use of optical filters with digital cameras. The first asserts that any extra glass inserted in the light path between the subject and the image sensor can degrade the image in various ways. The second maintains that filters are as useful to digital photographers as they were to those who captured pictures on film. The truth lies somewhere between these extremes.

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There are two approaches to the use of optical filters with digital cameras. The first asserts that any extra glass inserted in the light path between the subject and the image sensor can degrade the image in various ways. The second maintains that filters are as useful to digital photographers as they were to those who captured pictures on film. The truth lies somewhere between these extremes.

In practice, many of the effects that could only be achieved with filters when photographers captured pictures on film can now be achieved more easily – and with greater control – with decent image editing software. And, if the adjustments aren’t successful, it’s easy to start afresh. However, some filters have quantifiable advantages for photographers and some shots are feasible only with one or more filters in place. In this feature we will look at some of the main reasons for and against using optical filters to help you assess the quality and functionality issues filters raise.

Useful Filters

Polarisers: A good polariser can be very useful for digital photographers, not only for controlling specular reflections off shiny surfaces but also for controlling image contrast and colour saturation. Polarisers can also be used for cutting through haze. Used properly, a polariser will darken the tone of a blue sky, highlight clouds, suppress specular highlights and increase colour saturation, producing pictures that look more intensely ‘real’ than shots taken without a polariser. They can also increase the dynamic range in some shots.

Two types of polariser are available: linear and circular. Linear polarisers will work with almost all types of cameras (digital and film) and are usually cheaper than circular polarisers. However, circular polarisers will do a better job with cameras that use split-beam optics for focusing and metering, such as digital SLRs.

A couple of cautions should be noted when using polarising filters. Firstly, most polarisers will reduce the light entering the camera by one or two stops. In strongly polarised ambient lighting, a reduction of three stops is common. Secondly, the changes made by adjusting a polariser may not be visible on a camera’s LCD screen, particularly when the screen is small with low resolution and you’re working in bright outdoor lighting. The viewfinder will give you a better idea of the effect – but only if you’re looking through the polariser itself, so make sure the filter covers the viewfinder’s porthole. It’s easier to use a polariser with SLR cameras because they give a true ‘through the lens’ view of the subject.

Neutral Density Filters: Available in plain or graduated format with varying degrees of filtration, ND filters have no effect on subject colours; they simply suppress the amount of light entering the camera. In bright ambient light, an ND filter can be used to achieve:

  • a slower shutter speed to enhance motion blurring, or
  • a larger aperture to facilitate selective focusing (subject-background separation).

You may be able to accomplish the same thing with a digital camera by selecting a lower ISO setting, with the added bonus of reduced image noise. But when you can’t reduce the ISO enough, one or more ND filters can help you achieve either objective. ND filters are particularly useful with compact digicams, which seldom offer apertures smaller than f8. Under bright conditions, that aperture floor can be a problem, but ND filters can fill the gap. They can also allow photographers to shoot close to their cameras’ resolving power ‘sweet spot’, which is usually around f5.6.

Graduated ND filters are dark grey at one end and clear at the other and, like the tinted windscreen on a car, allow you to balance out the effect of bright skies so foreground details and colours are reproduced with a more natural looking balance. They also increase the dynamic range in the captured shot by two or more stops. Most GND filters are square in shape and slightly wider across than the diameter of the camera’s lens. You should be able to adjust the position of the dark zone to provide the filtration exactly where you want it.

The main problem with using GNDs on compact digicams is the difficulty in seeing and adjusting the transition via a difficult-to-see LCD screen. They are much easier to use with digital SLRs.

Infrared Filters: IR filters became popular with film photographers for the almost surreal effects they produced with living vegetation and skin tones. Even if you could simulate these effects with post-processing, it’s doubtful you could do so convincingly – or easily. For this reason, IR filters have a role to play in digital photography, especially for photographers who enjoy working in monochrome. They can also be used for taking pictures in colour. However, we have found different cameras can produce radically different results with IR filters so some experimentation will be required. Sample shots from two different cameras are shown on these pages.

Because IR filters work by filtering out all wavelengths except the red and infrared, they require a sizeable adjustment to exposure. Few metering systems will deliver correctly exposed shots on their own, regardless of the selected metering mode. Over-exposure by up to three stops is commonly required and the dark red images produced with an IR filter are almost impossible to evaluate on a camera’s LCD screen, especially in the bright conditions where you are most likely to use this type of filter.

Less Useful Filters

UV Blockers: In the past, most photographers attached a UV, haze or skylight filter to each lens to protect the front element from scratches and environmental grime. Such filters also added clarity by reducing the effect of UV scattering in high altitudes or for telephoto shots over water. However, most digital cameras are too insensitive to UV for such filters to be beneficial. Furthermore, the addition of a filter can increase the number of internal reflections in the optical system, thereby increasing flare. Filters can also collect the very dirt and grime you are trying to avoid – and you must be very fastidious about their cleanliness if image degradation is to be avoided.

Haze and skylight filters are usually lightly tinted; and some can impart a noticeable yellow or orange cast to shots. Haze filters cut out more UV-A than regular UV filters but also suppress some visible blue. Skylight filters also cut some blue but are no more effective in the UV-A band than regular UV filters.

Colour Filters for B&W Photography: Most of the effects formerly created by red, orange, yellow and green filters with B&W film can be replicated with good image editing software, which makes such filters somewhat redundant for digital photographers. However, it’s worth noting that the transmission spectra of the Bayer pattern filters on digital cameras’ sensor arrays may be quite different and the results obtained by using a coloured filter may not match the effects obtained when shooting on film. The image processing system used by individual cameras may also produce different results. Some experimentation will be required if you wish to use these filters for B&W digital photography.

In the next issue we’ll look at some of the many effects filters you can use to add variety and excitement to digital photographs.