Polarising filters (or Polarisers as they are also known as) are a very important light control tool in the photographer’s kit bag, they can be used for 3 different reasons.
Glare and light reflected from non metallic objects (e.g. glass, water) can be altered by the rotation of the polarising filter. This effect can be seen through the lens of reflex cameras as the filter is rotated to adjust the polarisation state.
When used outside on sunny days, polarising filters have the effect of saturating colours and deepening blue skies. They can also be used in conjunction with other filters (although the polariser should always be placed in front of polyester filters to ensure it functions correctly) for a wide range of effects.
A not commonly known fact, is that a polariser can also be used as an ND filter when the polarisation state has been adjusted to minimise any saturation or sky deepening if not required.
Polarising filters are available in two types, Linear and Circular. Initially confusing, these terms do not refer to the physical shape of the filter, but to the way the filter changes the light waves to achieve the polarising effect. The type of polariser required (Linear or Circular) depends entirely on the type of camera the photographer is using.
Modern reflex cameras with autofocus or complex metering systems, will only function correctly when a Circular polariser is used, as a Linear Polariser will affect the metering and autofocus operation. Manual focus cameras of all types (35mm, Medium Format and Large Format) can normally be used with either a linear or a circular type.
Both linear and circular polarisers are available from Lee Filters in both a rotating 105mm diameter ‘screw in’ version and a 100x100mm square version. The 100mm square polarisers are to be used in the filter holder in a standard 2mm resin filter slot; and the entire holder is then rotated to achieve the correct polarisation state.This is recommended for studio use or for when only the polariser and, possibly, standard filters are to be used.
The 105mm rotating polarisers filters are attached to the filter system via a special adaptor ring that is fitted to the front of the holder. Critically, this allows the polariser to be rotated independently of any grads used; and is ideal for landscape photography where extra flexibility is required.
Typically, polarising filters require a 1 2/3 stop exposure increase, but this should be checked with individual cameras and metering systems.