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| How a Polarising Filter Works A polarising filter has the effect of limiting the light waves to one direction or plane. When fitted to the camera, the filter's adjustable collar should be slowly turned while looking through the viewfinder, blue sky will darken then lighten, choose how dark you want it and take your photo. If you have a lens that rotates the front element when focusing you'll need to make sure the image is focused before using the filter. There are two types of filter, linear and circular. For modern cameras using autofocusing and TTL metering a circular filter is the correct choice. When using a polarising filter the exposure increases by 1 - 3 stops. This exposure adjustment will be made by the camera's metering system so no compensation is required. Polarisers work best in sunny, bright weather when there's increased polarised light around. Maximum effect is gained when the camera is at right angles to the sun. To find out where this is, use the fingers of your hand to to point in the direction of the sun and spread your thumb as far away from your fingers as possible, your thumb will be at the point of greatest polarisation. In cloudy weather or when the sun is directly behind you the filter may make no difference at all. At high altitude or when there's an abundance of polarised light you can get the other extreme with the sky turning to an inky blue-black which may look unnatural. It's not advisable to use a polarising filter with a lens wider than 28mm as the sky is likely to be uneven across the frame giving your shot an abnormal appearance. |
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| Should you purchase a Polarising Filter? Polarising Filters are expensive, usually at least twice the cost of a standard filter. If you don't take many landscape photographs you may feel it's not worthwhile getting one. Polarisers are not suitable to be used as a filter to protect the front element of a lens because of the reduction in light transmission, a UV is a preferable choice. With compact cameras it's commonplace not to be able to fit filters, check your instruction book. In theory you could hold a filter in front of the lens but this isn't a practical solution. SLR lenses and long zoom cameras usually have a filter thread. Filters come in different price ranges, it's not advisable to fit a cheap filter on an expensive lens. Screw-in filters are ideal if you only use one lens or are fortunate enough to have lenses with the same filter diameter, otherwise you'll have to purchase extra filters. Another option if your lenses have different sizes is to buy a square filter system made by Cokin, Lee or Kood. |