Polarising Filters

If you've not used a Polarising filter on a camera before you've almost certainly seen the effects by wearing Polaroid sunglasses ( nothing to do with Polaroid cameras ). To the layman these sunglasses make the sky darker, show up strange coloured patterns in car windscreens and even help fishermen spot a fish underwater, because each of these is illustrating a particular characteristic of the filter.

A quick look at the nature of polarised light may help explain how to best use the filters.

Two people hold each end of a rope loosely between them; one person shakes the rope up and down to produce a series of waves. It is also possible to shake the rope side to side and makes waves in that orientation instead. It is not possible to make a rope shake in all directions at once but light can and most light we see around us is scattered in this way. So what? Well, I said most light and the exceptions are the very things a polarising filter takes advantage of, because some natural light is only shaking up and down, or side to side, roughly speaking.

Naturally occurring polarised light

Blue sky on a really clear day

A flat, non-metallic surface such as glass or water

 

Examined microscopically, the working part of a polarising filter acts like the slats of a venetian blind and unless the light from an object is shaking in the same direction as the slats, it simply hits them and does not pass through. By aligning the slats to cut across the direction of light from a blue sky it will be stopped, making it appear darker than an unfiltered scene.
This simplifies what is going on in and in practise the light in the sky is never 100% polarised or else the sky in a polarised photograph would be completely black, but the darkening effect can nevertheless be quite pronounced and thus the sky appears a darker and richer blue.

 

 
no polariser with polariser

No Polariser

With Polariser

 

 
no polariser with polariser

No Polariser

With Polariser

Normal, scattered daylight becomes polarised as it bounces off the surface of water and so a filter allows us to see through the bright reflections and into the more subdued light coming from underneath the surface: the fish is revealed below the sparkling surface.

 

 
no polariser
with polariser

No Polariser

With Polariser

The Cheltenham park above was photographed in the morning and the light not particularly clear so the darkening effect in the sky is not dramatic, however the colour of the trees is more saturated. The glass Hoya filter also introduced a warmer overall tone so bear in mind that a filter may effect colour and require an increase in exposure by 1 - 2 stops.

If you are thinking of buying a polarising filter it’s worth knowing there are two types available: linear and circular, which act on the light is different ways and suit particular camera types. A simple manual camera, one without autofocus or spot-metering, can use either filter type, but it is recommended that more sophisticated SLRs and dSLRs use circular polarisers. The design of many autofocus systems makes use of polarisation, so a linear filter might interfere with its operation.
As filters are expensive it is certainly worth checking your cameras’ documentation to see which is suitable.

 
The Darkroom UK Ltd
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