Polarising Filters

There have been a few requests of late asking for information on Polarising Filters. These request have gone along the lines of....
What does a Polarising filter do? Do I need a Polarising filter? Is it worth me buying a Polariser?

So I've decided to write a quick guide to the Polarising Filter, but before I start on the guide to the polariser I would like to give you a piece of important advice.
Every lens you use should have a Skylight 1B or a UV Haze filter on it!
Why? Protection, that's why! Most modern lenses are designed by computer, are well constructed, are ground from the finest optical glass and are expensive. A slip of the hand could send it tumbling, a child could stick a jam sandwich to it, or while cleaning it a grain of dirt could scratch it. Put a 1B or UV filter on the end and you might just need to replace a few quids worth of filter rather than an expensive repair or lens replacement. 1B and UV filters are neutral in colour and don't stop any visible light so they can be left on permanently. I have a UV filter permanently on every lens I own.
......And don't buy a cheapy, go for a quality piece of glass to match your quality lens, there's no point in having a quality lens and putting a poor quality filter on the end!!!

What does a Polarising Filter do?
Well, without getting technical it cuts down reflected light. Light in the sky is reflected off such things as water vapour and dust particles that are always present in our atmosphere. the polariser will cut down the amount of this "reflected" light that reaches our camera sensor or film. This will enhance the contrast and colour saturation of our picture. It will also cut down reflections in non-metallic surfaces such as water, glass or gloss paint, again, improving contrast and saturation.

Do I need a Linear or Circular Polarising Filter?
There are two types of polarising filter, Linear and Circular (sometimes known as a CPL). The Linear filter is now a bit "old school" and is rarely used in digital photography. The Linear Polariser is not compatible with auto-focus and modern metering systems so should in general be avoided.
The Circular Polariser or CPL is the one you need to look out for, it does the same job as the Linear Polariser but is compatible with modern auto-focus and metering systems.

So, what do you know so far?
You should fit a UV or 1B filter to every lens, and you need to get a Circular Polariser (and not a Linear Polariser) to improve the contrast and saturation of your images.


How do you get the darker skies effect?
A Polarising Filter needs to be rotated to vary the Polarising effect, when you screw a Polariser to the front of your lens it is designed in a way that still allows you to rotate the Polarised glass within the filter mount.
The effect will vary as you rotate the filter, from no effect turn the filter 90º and you get maximum effect, another 90º and it goes back to minimal effect, a further 90º to get the maximum effect again and another 90º to go back to minimal (and you've now rotated 360º back to where you started).

This shot was taken with a Cokin Circular Polariser turned to achieve no Polarising effect...


This next shot was taken a second later of the same scene but with the Circular Polariser turned 90º


Both the above shots were at ISO200, manual white balance, on aperture priority @ f8.
As you can see, the Polarised version has darkened the blue skies, improved contrast slightly and improved the colour saturation.
The first (lighter) shot was at a shutter speed of 1/320th and the second (more saturated) shot at 1/250th.
The reason the shutter speeds are different between the shot is because as you rotate the Polarising Filter to increase the Polarising effect you are cutting out the reflected (non polarised) light, so you need to increase your exposure slightly.
Increasing exposure to compensate for Photo Filters is known as a Filter Factor. A Circular Polariser (of any make) has a Filter Factor of 3-4, which means an increase in exposure of 1½ to 2 stops. But don't worry, you don't need to do anything because your camera's meter will automatically compensate. Basically, the Polariser acts as a neutral density filter stopping some light reaching the camera.

Now the tricky bit. A Polarising filter works best when you are taking a picture 90ºto the sun, e.g.. taking a shot of a landscape with the sun directly to your left or right. Point your camera directly towards the sun or have the sun directly behind you and you will minimise the effect.

Take a picture of a car and you can alter (or eliminate) reflections on the paintwork or in the window/windscreen to allow you to see inside, as well as improving the contrast and colour saturation. The same goes for shooting throgh a normal house window for example.

Should I just leave my Polariser on permanently?
No, there will be many situations where due to the lighting conditions or the angle of the shot the Polariser will have no effect, and then you will just have a permanent neutral density filter on the end of your lens.

Options:
If you have 3 or 4 (or more) lenses, then a Polarising filter to fit each will work out darned expensive (a Polarising Filter is one of the most expensive filters you can buy). A good option is to buy into one of the "filter systems" on the market.
You have 1 filter, with a filter holder and an adapter ring. You buy a cheap (just a few pounds) filter ring for each lens, then the filter holder can be swapped between all your lenses, the filter goes in the filter holder so 1 filter fits all your lenses. Buy another lens, just get another adapter ring and all your filter will fit it! The filter holder normally holds 3 or 4 filters so you can mix and match effects. It's no problem to have a Polariser and a Neutral Density Graduated, and perhaps an 81 warming filter all on at the same time!
Filter systems are made by Lee Filters, Kood, and the most popular Cokin.

As with any filter, buy a quality item from a reputable manufacturer like Hoya or Cokin. As I mentioned earlier, there's no point in having a good quality lens and then screwing a poor quality filter on the end, you will get soft pictures.

...A Very rough guide to price

Hoya UV Filter
52mm Thread £9.50
58mm Thread £12
67mm Thread £20
72mm Thread £25

Hoya Circular Polariser
52mm Thread £25
58mm Thread £31
67mm Thread £47
72mm Thread £48

Cokin "P" System
Circular Polarising Filter £58.50
Filter Holder £8
Adapter Rings £7.50 each

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