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IMG6270Polarising filters

Most filter effects are either achievable through camera features or in post-production in this digital age. But the one effect you really can't recreate is polarising – it's the essential tool for saturated colours, reduced reflections and more.

Have you ever wondered how some photographers manage to get such saturated colours in their images? It’s all down to the effective use of a polarising filter. This cuts through some of the reflected light to show you the true colours of the subject.

We tend to think of polarising filters doing two different jobs – darkening blue skies and reducing reflections from water and glass. In fact, these are two sides of the same effect.

Blue sky thinking

A polarising filter can help to darken blue skies. However, the strength of the effect depends very much on the blueness of the sky to begin with, the angle of the sun to the camera lens axis, and the rotation of the filter.

CJB180627Oxford076CJB180627Oxford075University Church in Oxford, taken with the EOS 760D, EF 28-105mm f3.5-4.5 USM lens at 28mm.

Far left: Without polarising filter, 1/128 second at f18, ISO 800

Left: With circular polarising filter, 1/195 second at f18, ISO 800

thumbnailPolarising filters produce the most dramatic effect when you are photographing at right angles to the sun. Hold your hand with your thumb at right angles to your extended index finger. Point your finger at the sun. Now rotate your wrist. The arc drawn by your thumb is the area of the sky which will be most affected by a correctly positioned polarising filter.

For example, if the sun is overhead your thumb will draw an arc around the horizon – every point on the horizon will be equally affected by the filter. If you shoot shortly after sunrise or sunset, only the horizon at north and south will be affected (your thumb draws an arc over your head from north to south).

WYSIWYG

With the polarising filter on your lens, what you see is what you get (WYSIWYG). This is the advantage of using a polarising filter with your EOS camera – looking through the viewfinder you will see the effect change as you rotate the front filter, allowing you to judge exactly how much polarisation you want, if any. You are in complete control.

The best effects occur when the sun is shining and there is already some blue in the sky. However, polarisers can sometimes improve your image even in overcast conditions. If in doubt, try taking shots with and without the filter.

Reduced reflections

Polarisers remove, or reduce, the amount of glare reflecting off non-metallic surfaces. These include glossy leaves, grass and rocks, as well as glass and paint, most of which reflect a considerable amount of white light on a bright day (and even more on a ‘white sky’ day). Using a polariser here has the effect of saturating these colours, because you are seeing the true surface of the subject.

IMG0141KewAutumnTintsNovember2012650DIMG0142KewAutumnTintsNovember2012650DTaken at Kew Gardens with an EOS 650D with EF 70-300mm f4-5.6L IS USM lens.

Left: Without polarising filter, 1/640 second at f8, ISO 800.

Right: With polarising filter, 1/500 second at f6.3, ISO 800.

The effect can be used for foliage in landscapes as well as close-ups of flowers. The increased saturation in the foliage (above) is partly due to the sky being darker, but mostly because the polariser has removed the reflections from the surface of the leaves so that you can see the underlying colours.

Which angle?

Although you can use a polarising filter to reduce the reflections from a non-metallic surface, the effect is minimal with the camera square on to the subject.

CJB180627Oxford038CJB180627Oxford040View onto the river Cherwell from Magdalen Bridge in Oxford, taken with the EOS 760D, EF 28-105mm f3.5-4.5 USM lens at 28mm.

Far left: Without polarising filter, 1/80 second at f9, ISO 400

Left: With circular polarising filter, 1/195 second at f9, ISO 400

The optimum angle for the camera is around 37 for water and about 34 for glass, though you do not need to be this precise for most subjects. If you look through the camera viewfinder you can see the reflections start to disappear as you rotate the polarising filter in its mount. If they do not fully disappear, try altering the angle of the camera to the reflective surface.

Keep in mind... if the surface is not flat, you may not be able to eliminate the reflections completely.

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How polarising filters work

When rays of light pass through the lens of your EOS camera, most of them are reflected up to the viewfinder by the reflex mirror. However, the centre of this mirror is semi-transparent. Some of the rays of light pass through and hit another, smaller, mirror (often called the ‘sub-mirror’). Here, they are reflected down to the autofocusing sensor in the base of the camera.

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The light rays are polarised as they are reflected from the sub-mirror. However, if the light is already polarised before it reaches the mirror, it will not be reflected. This means that if you use the simplest type of polarising filter on the lens – the linear polariser – the amount of light reaching the autofocusing sensor will be reduced. If this happens, the autofocusing system may not work, or at best will give unreliable or inconsistent results.

There can be a similar problem with the exposure sensors in the viewfinder area. The light reaching the sensors will be reduced, which means that the exposure readings may not be accurate.

These problems can be resolved by using a circular polarising filter. This is a linear filter with a ‘quarter-wave’ plate bonded to it. In simple terms, this plate ‘stirs up’ the light after it has passed through the linear filter, in effect giving it a circular motion.

This means that you gain the effect of polarised light, without the limitation of all the rays oscillating in the same direction. A few of the rays will still be lost when they are reflected from the mirror, but the majority will get through to do their job. The autofocus and exposure systems should give reliable results.

Most polarising filters you buy today will be the circular type. If you are buying second-hand look for ‘Circular’, ‘CPL’, ‘PL-CIR’ or similar engraved on the filter mount.

 

 


 

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Here to help 01869 331741

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DId you know?

If you try and photograph a rainbow with a polarising filter attached, it will disappear!

 

 


 

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