Canon EOS Digital SLR Product Range - Eos40D, Eos400D

Filters can add a new dimension to your digital photos and are a great asset for anyone who owns a digital SLR camera. They can help you to compensate for certain problems related to lighting and allow you to add some interesting effects to your digital photos.

Polarising and graduated filters are the most useful filters for most types of photography and they're well worth having in your camera kit. Polarisers have a similar effect to Polaroid sunglasses: they suppress unwanted reflections and cut back glare. Skies will take on a deeper blue when photographed through a polarising filter, making clouds stand out more obviously. And unwanted reflections from glass, water and other shiny surfaces can be, if not totally eliminated, at least brought under control.

Polarising filters are often used to bring out the blue in the sky and emphasise the structure of clouds.

Polarisers can also make still water look more translucent and increase the colour saturation in leaves and grasses, especially in dull weather. The effectiveness of the filter depends on the relative angles between the camera, the subject and the sun. How much you rotate the filter can also be important.

Two types of polarisers are available: linear and circular. Linear polarisers are usually more effective (and often less expensive) than circular polarisers. However, circular polarisers work better with cameras that have through-the-lens metering and autofocusing systems (i.e. all digital SLRs).

Polarisers normally consist of a double ring, one part of which screws onto the front of the camera?s lens, while the other holds a sheet of polarising glass. Looking through the camera?s viewfinder you rotate the polarising element until you achieve the desired effect.

1. Don't wear Polaroid sunglasses when shooting through a polarising filter. The two polarisers can cancel each other out, making your viewfinder look black!
2. Polarisers reduce the amount of light entering the camera by between one and 1.5 stops. You may need to compensate by using a faster ISO setting, wider lens aperture or slower shutter speed.
3. Because they are relatively thick, a polariser may produce vignetting (darkened edges on pictures) with wide-angle lenses. They are best used with 'normal' and telephoto lenses.
4. Don't use a polariser for shooting panoramas that will be stitched together. As you capture the shots, each shot will be differently polarised and it will be impossible to match the skies exactly.
5. You don't need to use the full polarising effect unless you want to. On a clear day it may make a blue sky too intense ? or affect one side of the shot more than the other.

Graduated filters are grey (known as 'Neutral Density') - or coloured - on top and fade gradually until they are totally clear about half way down. Between the colour and the clear areas the filter transitions through either a soft-edged or hard-edged zone that reduces the likelihood that the transition will be visible in the shot.

The main use for these filters is to add impact to shots in which the sky would otherwise be white or uninteresting and overcome exposure problems in shots with large areas of bright sky. Used effectively, a graduated filter can direct viewers' attention to important areas in pictures.

Graduated filters come in both and coloured versions - and in a wide variety of 'strengths'. Densities of one, two and tress stops are commonly available in grey and the more popular colours. It's best to buy filters that fit into a holder that attaches to the lens, rather than screw-in filters that can be difficult to position accurately.

Graduated neutral density filters allow you to add some interest to an otherwise over-bright sky and direct viewers' attention to the key areas in the picture.

The trick to using graduated filters is positioning them correctly. That can depend on the type of subject you're shooting - and it will take practice to learn what works best. Use the depth-of-field preview button on the camera to stop down the lens to the shooting aperture. This makes it easier to see the edge of the graduation and how it will be recorded. If the edge is too visible, try opening the lens aperture by a stop or two.

It?s best to take an exposure reading before sliding the filter into position. Then, with the depth-of-field preview button held down, move the filter up and down until it's positioned correctly. Don?t be afraid to tilt the filter holder slightly to match the horizon line.

1. Exposure problems. Metering can be tricky with graduated filters if you use either multi-pattern or centre-weighted metering patterns. Swap to spot metering and check out different parts of the frame to find out whether you need a filter at all - and how much light it should block out. It's best to meter the foreground and set the exposure manually before fitting the filter.

2. 	A light 'tobacco' graduated filter adds drama to the otherwise uninteresting sky.

   Hard-edged graduates should be kept for shots in which there is a well-defined transition line, such as sunrises or sunsets over water or landscapes with a level horizon. For all other types of subjects, soft-edged filters work better because they allow for positioning errors.
3. Use coloured graduates with care to avoid obvious and unnatural-looking effects.