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Cross polarisation

Cross polarisation - By using two polarising filters - one at the light source and one on the camera - spectacular images can be created from clear plastics.

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While the weather and short daylight hours restrict our outdoor photography, here's something you can try at home to create vividly colourful photos with loads of impact.
Words and photos Peter Bargh ePHOTOzine

Plastic - it's one of our most commonly used materials, which we take for granted. Here's a chance to appreciate it for its photographic qualities. I remember visiting the Bradford National History of Photography Museum when it first opened and one of the most impressive displays was of a model railway, with a bridge showing stress patterns as the engine passed over it. It wasn't till a few years later that I realised the effect was created using polarised light.

By using two polarising filters - one at the light source and one on the camera - spectacular images can be created from clear plastics. The effect is achieved when rotating the camera polariser, which causes 'cross polarisation', and the darkening of the light source polarising filter. The colours seen in the plastic are the result of diffraction of white light into various parts of the spectrum. The closer the cross polarisation the stronger and more saturated the colours.

To attempt this type of photography you need a light source which could be an electronic flash, either portable or studio, or tungsten light. The advantage of using studio flash over a normal flashgun is that you have a modelling light to see the effect you will create. Tungsten light or a slide projector will produce the same visual effect but if daylight film is used a colour correction filter is required to prevent a yellow cast.

You also need polarising material. This must cover the light source, so if a projector is being used or a small flash head, you can usually get away with using a normal round or square filter that you'd use on a lens. If you decide on studio flash or tungsten lighting a larger filter is necessary. Sheets of polarising gel are available in various sizes from manufacturers such as Lee Filters or from a Jessops shop. You also need a polarising filter for your lens.

The subject matter should be placed between the camera and light source. The type of plastic that cracks when flexed seems to produce the most vibrant colours. Items such as rulers, cassette and CD cases, plastic catering cups, measures, Cokin and Cromatek filter cases and even some transparency mailing boxes produce marvellous effects. Attach a lens to the camera that will fill the frame with the subject - a macro lens is the best solution, but a close up lens added to a non-macro lens will reduce the cost. When selecting film pick a slow speed for sharp, fine grain results or fast film for grainy effects and preferably slide film to give the best saturation of colours. Print film may confuse the automatic processor resulting in disappointing images.

If you use the camera's built-in meter make sure you use the correct polarising filter. There are two types, circular and linear, and because of the latest technology some cameras give incorrect readings when a linear filter is used - check the instruction book or ask your dealer. As a guide auto focus or multi spot reading cameras need circular polarising filters and older manual cameras need linear. You can use the wrong one providing you compensate for the metering error. Take a test film and bracket exposures making a note of the settings so they can be repeated.

Equipment used
I began my trials using a Jessops Powerflash MD400 studio flash head for the light source, fitted with a soft box to diffuse the light. The more recent Jessops Portaflash will work just as well. A sheet of Powerflash 8.5in polarising filter was placed on top of the soft box material. The subject was then placed on top of the filter. When viewing the image through the camera I noticed that any marks on the polariser (gel filters easily scratch!) became quite prominent. To avoid this I lifted the subject off the polariser (using a sheet of glass supported at both edges) use either cardboard boxes or, when working with a smaller flash, an old chair with a removable seat provides a great frame support. Making a gap of about 12in from the filter throws the scratches out of focus but creates a new problem - the polarising sheet is now too small so the edges can be seen in the photograph. Bringing the glass closer to the filter reduces the gap from the subject to around four or five inches and the filter is then a satisfactory size. Another option is to buy a bigger sheet of polarising gel, but this can be expensive.

When using flash you need to take a flash meter reading. I use a trusty Sekonic L308B. Point the meter at the subject and take a reflected light reading, then opened up the aperture 1 1/2 stops to compensate for the polarising filter on the camera. In my tests the aperture needed was f/16.

Set the camera's shutter speed to the flash sync speed - 1/250sec on my Nikon FE2. Switch on the light source and position the subject on the glass above the polarised light. Now frame the picture, adjust the camera's polarising filter and welcome to the glorious world of Technicolor!



This measuring cylinder makes a colourful and detailed choice. I laid the object down on the glass surface with the graduation marks nearest the flash. I focused through the plastic on the markings that appear in reverse. The result has to be viewed from the wrong side to reverse the numbers. This shot has showed me that the plastic must be spotlessly clean. I use this graduate daily and a chemical stain has become a prominent feature between 15 and 16oz!


Here is a simple but effective arrangement of round filter cases from the likes of Hoya and Jessops. With a Cokin P series case producing the straight line in the background.


Three CD jewel cases sandwiched and closely cropped creates this image with strong but simple shapes and colours.

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Comments


24 Aug 2007 1:25PM
Another way to produce polarised light is to use a TFT monitor (won't work with CRT) - a plain, white image displayed full screen (e.g. in Photoshop) should provide a suitable light source.

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