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Lenses: Filters

While only a few photographers choose to get really creative with filters for starbursts and other special effects, most will have at least a polarising filter lurking around in their camera bag. But whether you are scanning film or shooting digital, the proliferation of easy-to-use image manipulation programs for computers raises new questions about the need for photographic filters.

Traditional glass or resin filters attach to the front of the lens, altering the light before it reaches the film or digital sensor, and often changing the exposure. However, most photo editing programs either have built in ‘photo filters’ or give you so much control over the image that it is simply a matter of a little manipulation to achieve an effect similar to using a front-of-lens filter.

So do you really need the hassle of carrying, fitting and using filters, or should you shoot with a naked lens, safe in the knowledge that you can dress up the image later?

The answer is that you no longer need all of the filters − provided you are willing to spend a little time in front of a computer afterwards. Of course, you might find that attaching a filter to your lens a lot easier and considerably less hassle than achieving the same effect on your computer. It all depends on your computer skills and the results you are after.

Reproducing filter effects

Warm-up, neutral density and polarising filters are the most commonly used. Whether you are shooting film or digital, a subtle rather than overpowering effect is usually the best approach. But how do you replicate the effect of traditional filters on your computer?

Warm-up filters

Shifting the colour balance is an easy filter effect to reproduce digitally. It can be done very simply in-camera or in RAW processing after the exposure.

The 81 and 85 series of photo filters are widely used by landscape and wildlife photographers to add warmth and atmosphere to the scene. The filters hold back strong blue tones, allowing the yellow and orange light to have a more pronounced effect. This gives the impression of a scene bathed in evening sunlight. Warm-up filters are available in different strengths so you can choose which level of filtration you prefer.

The easiest way to replicate a warm-up filter with a digital camera is to adjust the white balance setting. If you move this to a higher value than the actual colour temperature of the light, you will cause a colour shift in the image towards yellow/brown. If you are shooting in average daylight, which has a colour temperature of about 5500K, try a setting of about 7,000K. Electronic flash has a colour temperature to match daylight, so a setting of 7,000K is also worth using if you want to add a little warmth to skin tones in portraits.

The image from a RAW file processed with the white balance set to daylight give natural colours, but lacks warmth. The same image processed with the white balance set to Shade brings out the autumn hues of the woodland scene.

The colour temperature range of EOS digital cameras varies a little from model to model, but is usually from around 2000K or 2800K to 10,000K. Press the AF-WB button on the camera and select the K (colour temperature) option from the LCD panel, then turn the main dial to choose the value.

Altering the colour temperature in-camera is only necessary when you are shooting in JPEG mode. Here, the camera will use the setting when it processes the file.

If you are shooting RAW files, the camera will store the settings you used to take the shot as metadata, but you have the option to apply a different colour temperature, sharpness and contrast when you come to process the file outside the camera. Not only that, but RAW processing is always done on a copy of the file, so you can try one set of values and save the image, and then work on another copy of the original RAW file with a different set of values. In short, you can keep playing with the file until you obtain an image that pleases you.

JPEG shooting, on the other hand, allows you to experiment with the image in-camera and spend less time in front of the computer. It is possible to adjust some of the settings of a JPEG image on a computer, but this is neither as controllable nor as effective as working with a RAW file.

Polarising filters

A polarizing filter has two distinct uses in photography − it can darken blue skies and can reduce reflections from non-metallic surfaces.

Blue sky can be darkened using a polarising filter.

It is almost impossible to create the effect of a polarising filter digitally, so this is one filter that still has a place in a good photo outfit.

The amount of reflected light absorbed by a polarizing filter depends on the degree of rotation of the filter on the front of the lens. However, the effect produced can be seen through the viewfinder, so all you have to do is turn the filter until the maximum effect is seen. One point to keep in mind, however, is that a polarizing filter can only darken skies that are already blue. It has no effect on grey skies.

A polarising filter helps to remove reflections from non-metallic surfaces, such as glass. The effect is strongest when the camera is at an angle of about 45° to the surface.

Polarising filters have the maximum effect on blue skies when the lens axis is at right angles to the sun. When the sun is overhead, the effect is fairly consistent around the horizon, but if the sun is low only bands of sky to the north of south will show a strong polarising effect.

Always use a circular polarizing filter with EOS cameras, rather than a linear polarizing filter. Both look the same and give similar results, but linear filters can produce incorrect exposure meter readings.

Polarising filters are not recommended for use with extreme wide-angle lenses (around 24mm or wider) as the front of the filter mount can intrude at the edges of the frame, giving dark corners (an effect called vignetting). Only professional EOS cameras − the EOS 1 series − have viewfinders with 100% coverage. Other models give coverage between 92% and 95%, which means that the vignetting may not be visible through the camera viewfinder.

Also, wide-angle lenses take in so much of the sky that you can often see a change in the polarising effect from one side of the scene to the other.