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Friday, 6 July 2012

Polarising Filters - How To Use Them? Why?

Digital photography enables us to recreate most filters effects that we used to produce using optical attachments on our cameras. But there's one filter that should still be in every photographer's gadget bag. Let's forget the crazy world of multi-images, the colourful starbursts and the dreamy diffusers - the essential item you should own is a polarizing filter. A genuine polarizing filter can have a dramatic effect on your photographs, but it won't ruin them, which is one common criticism of special effects filters, such as spot colours and multi images.

polarising filters
The classic effect a polarising filter has on the sky. Left shows a non polarised photograph and right with a polariser attached. Notice the saturation in the sky, making it vivid blue, but also the improved detail in the foliage and blooms, that give the picture more clarity.
How they work
Polarising filters work by suppressing surface reflections from non-metallic objects, by blocking the rays. The amount of suppression depends on the angle of the reflected light, the rotation of the filter and the amount of polarisation. You also see an increase in colour saturation, as the glare caused by the surface reflections often lightens the subject.
polarising filters
Polarsing filters are perfect for landscape photography. Here a warm tone version was used which has made the picture look more like a summer's day.
Some image editing software suggests it can offer the same results as a polarizing filter, but it misses the mark. A digital polarizing filter can enhance contrast and colour saturation, but it can't remove the reflections caused by light - well not yet anyway!

polarising filter reflections
Here sunlight has caused glare on the varnished wooden sign which can be reduced when a polarising filter is used.

Photos of non-metallic surfaces aren't the only subjects that benefit from the use of a polariser. Light reflected from water and glass is also polarised and using a polariser enables the photographer to see through the glass or water.
polarising filter results
A polarising filter used on water kills reflections, allowing you to see below the surface.

Another essential use of a polariser is for creating those rich blue skies that are normally seen in travel brochures and on postcards. Light scattered from a blue sky is polarised, making many blue skies look drab, add a polariser to the equation and you will produce a lovely dramatic blue sky.

How they attach 

There are three types of polarizing filter. The most basic is a sheet of polarizing gel, which you would need to cut to size and hold over the lens or mount in a gelatin holder. Then we have precut and mounted versions, made by the likes of Jessops and Cokin, that slot into a filter holder and mount onto the front of the lens. The third option is the round type that has a sheet of polarising material sandwiched between two pieces of optical glass. The screw-in variety has a rotating front ring so you can adjust the filter while looking through the viewfinder until the reflections are reduced. The holder variety is usually round and can be rotated in the holder.
Ten top tips on using polarising filters
  • When shooting to prevent reflections, it's best if you are at an angle of around 35 degrees to the reflective surface.
  • A polariser can be attached on top of an existing UV protective filter, but if you do so, be aware that the depth of the filter rims may cause a small amount of vignetting (darkening of the edges) on wider-angle lenses. And as there are more glass to air surfaces the image quality could be degraded. So where possible it's better to remove any other filter and just attach the polariser.
  • When shooting skies ensure you shoot at the best angle - for rich blue skies move around so that the sun is at 90 degrees to the subject - anything more or less and the saturation is reduced.
  • Avoid using a polarising filter on a lens wider than 28mm as the effect can look false because only a proportion of the sky will be deeply polarised.
  • Watch the exposure. A polariser has a neutral grey look, which won't affect colour, but does reduce the amount of light reaching the film/CCD. If your camera has through-the-lens metering it will calculate the exposure difference automatically, which is roughly two stops.
  • Use two polarisers together as variable neutral-density filter with between two and nine stops light reduction.
  • Use two polarisers one over the light source behind a plastic subject and one on the camera for a cross polariser effect.  There's a technique how to do that here: Cross Polarisation
cross polarisation effect
Cross polarisation is a colourful technique that makes use of one of the polarising filters more creative features.
  • When using a polarising filter on an older camera that doesn't have TTL viewing hold it up to your eye and rotate until the filter is showing the best effect. Then mark the uppermost point of the rim with a china graph pencil or tape, screw it onto the lens and rotate so that the tape or mark is at the uppermost position. Some filters have a white line mark already printed on the rim which can be used as a gauge.
  • Buy a slim version for use on a wide-angle lens to prevent the mount causing vignetteing
  • Don't use a polariser when shooting through an aircraft window, you will record distracting patterns in the window.
polarising filters
There's one incident when a polariser should not be used. Here I increased the colours in the sky and made the ground look more saturated by using the polariser (left), but look at the rainbow pattern. This is caused by the material used in the windows and has produced the cross polarised effect.

A Guide to buying a polarising filter

Buying a polarising filter - You may be aware that a polarising filter is one of the most useful additions to your camera kit, but there are a few things you need to decide before buying one. First thing you should know is that there are two varieties - linear and circular. Although they're both physically round, a linear variety can have an effect on the autofocus or metering accuracy of any camera that uses a semi-silvered mirror or prism to split the light entering the viewfinder.

Guide to buying a polarising filter

You may be aware that a polarising filter is one of the most useful additions to your camera kit, but there are a few things you need to decide before buying one.

Linear or Circular?

First thing you should know is that there are two varieties - linear and circular. Although they're both physically round, a linear variety can have an effect on the autofocus or metering accuracy of any camera that uses a semi-silvered mirror or prism to split the light entering the viewfinder. This is known as a beam splitter and is used by most modern SLRs to calculate exposure and focusing distance.

As a rule use a circular filter if you have an autofocus camera or a manual focus model with a spot meter such as the Canon T90 or any modern digital SLRs.

Screw-in or System filters?

Next thing to decide is whether to buy a round, screw mount filter or a system version that slots into a filter holder. Round ones are often easier to adjust and feel better built. They're also more compact to carry around. The disadvantage is larger sizes are more expensive and, if you have two lenses with different filter thread sizes, you may need to buy two filters, whereas a system type would just need another adaptor ring for the filter holder.

The filter holder type can also cause vignetting when used on some larger thread wide-angle lenses. Equally, a larger filter holder may prevent vignetting when used on a smaller thread wide-angle.

Guide to buying a polarising filterFeatures to look for
  • If you have a camera without through-the-lens viewing look for a filter that has an index mark printed on it. This will help you align the filter correctly.
  • Some filters have a small screw-in arm to help rotate the filter, which is useful if you intend using the filter when wearing gloves in cold weather.
  • Filters with thick rims can cause slight vignetting when used on wide-angles. The latest Pro1 D versions from Hoya have slim rings.
  • Check whether the filter is circular or linear. It will often say PL CIR if it's circular, while linear often just has polariser or PL marked on the rim.
  • Polarising filters for square system holders are still round so they should rotate easily in the holder.
Who makes them?

Screw-in filters

Screw in filters attach to your lens' filter thread and are the least bulky option. If you have a number of lenses with different filter threads it can be an expensive option.

A brass mount with solid ring and deep filter thread make this a heavy filter that feels substantial compared with its Japanese competition. Extremely neutral throughout the range with plane parallel polariser material that the German makers, Schneider, say will guarantee optimal image results. The one to choose if your budget can stretch to the hefty price tag in sizes from 46 to 95mm.

Suppliers of one of the largest accessory ranges made, with thousands of items for video, stills and audio markets. Recently they have introduced a range of silver finish filters that match the modern styles seen with newer compact digital cameras and AF SLRs, This range includes the HTMC Circular polariser in sizes from 25.5mm to 77mm. In conventional black rimmed options there's a Linear in sizes from 49mm to 82mm, Circular in sizes from 27mm to 82mm and a HTMC circular in sizes from 37mm to 86mm. Hama also produce an slim mounted circular version for wide-angle lenses in sizes from 49mm to 77mm.

Ksemannn polarising are very expensive, but different in that the after the sandwiching of the polarising layer the filters are polished plane parallel and the edges are thoroughly sealed and mounted in a precision rotating mount with numerical scale. The special seal helps if you're using it in difficult climates such as sub tropical. They are available in linear or circular versions along with a linear warm polariser all in sizes from 39mm to 105mm along with Hasselblad and Rollei bayonet mounts.
Heliopan linear and circular polarisers are made from top quality Schott glass (made by the Zeiss group) and come in black anodised brass mounts in sizes from 39mm to 105mm.

Super HMC Pro Multi-coated circular polarisers are difficult to make because the heat needed to multi coat the outer surfaces can damage the polarising sheet that's sandwiched between the glass. Hoya have managed that with this version available in sizes from 49mm to 82mm and it's ultra thin and lightweight. Hoya also make a normal circular polariser in sizes from 27mm to 86mm and linear in sizes from 39mm 95mm.

Produce a wide range of low cost screw-in filters including linear options in sizes from 46mm to 72mm and circular varieties in sizes from 27mm to 77mm.

Have a range of screw fitting polarisers in sizes from 39mm to 86mm for the linear variety and 27mm to 86mm for the circular versions. They also produce 84mm linear and circular versions for use in Cokin P series holders.

The company that makes lenses also has a range of circular polarisers with slim mounts for their lenses. They range in sizes from 46mm to 82mm for normal coated versions and there's also a multi-coated version in sizes 86mm, 95mm and 105mm.

System filters

Square filters fit onto your lens with removable adaptor rings. You only need one holder and one filter which can be attached to a variety of lenses using different size adaptor rings.

Guide to buying a polarising filterCokin
French manufacturer Cokin was the originator of the special effects system with A (amateur) and P (professional) sizes. The A series are 67mm square and designed for use on 35mm cameras while the 84mm square P series are more suitable for larger medium-format systems and also come in handy when you use wide-angles to helps prevent cut-off.

More recently a larger Z-Pro and X-pro range were added to accommodate those lenses with large filter threads, especially suitable for the modern ultra wide-angle lenses.

Both linear and circular polarisers are available, which are very neutral in colour. Unlike other resin Cokin filters these are mounted in glass making them much heavier. They fit into the filter holder's back slot so you can rotate them easily.

A British brand with a unique box shaped filter holder that holds the 76mm filter in place while acting as a lens hood. The filter fits into the back slot of the box holder and you have to flip down the cover of the box to rotate the filter. This can slow you down, but you do gain the benefits of a superb light tight hood. A 100mm version is also available and both are glass mounted with circular or linear options.

An 85mm system with an option of attaching a 105mm screw thread Kaesemann polariser to the front of the 100mm holder.

Jessops make a range of budget priced filters for their effects filter system which are, like Cokin, available in two sizes Standard (67mm slot) and Professional (83.5mm slot). Linear and circular polarisers are available in each system.

A system originally developed for the professional photographer who needs the very best quality. The filter holder is larger and accepts 100mm square filters. Their linear polariser is made from Butyrate and is less than 1mm thick so needs to be mounted in a gelatine holder for the best support. It's available in 75mm, 100mm and 150mm square versions.

Square system from the popular maker Hoya. A wide range were made and the system had a useful rubber hood that attached to the outside of the filter holder. Several of the filters in this range were glass. The system was 75mm square and excellent quality. You can pick these up second-hand Used Hoyarex Filters and they're well worth the investment.


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