Saturday, 5 July 2008

Circular Polariser

My circular polariser arrived today. It's a Hoya Pro1 Digital filter, meaning it has multiple coatings to reduce flares and ghosting, a black frame, black rimmed glass to reduce internal reflections, a low profile frame, and other useful stuff. It's also pretty damn awesome!

I can't be bothered to explain the physics of polarisation, be it linear or circular. I also can't be bothered to explain why cameras with autofocus or through-the-lens metering need circular polarisers, but I'm sure you can all use Google to find that out for yourselves.

What I will do is show you the extremes of the differences in polarisation, and their effects on the image.

The first image is the greenhouse in my back garden. Notice that there is a lot of reflection from the sky. Reflected light is (partially) polarised, with a maximum of 100% polarisation between 50 and 60 degrees from the normal to the reflective plane. Because of this, I can use the polariser to select the light polarised from the reflection, maximising the reflection effect captured in the image:

Alternatively, I can rotate the filter 90 degrees to almost eliminate the reflected light, leaving only the light transmitted through the glass from the tree behind:

Notice also that the greens became more vibrant; that's due to a similar effect with the scattered light from the sky, and reflection from the leaves themselves.

Since I mentioned the sky, here's a photo which is mostly unpolarised, showing very little contrast between the sky and nearby clouds. It looks rather washed out and weak:

And here's almost the same image with full polarising effect. The sky is a much deeper blue, the clouds stand out more, and the vibrant greens are back. The whole image looks much more colour-correct, and retains the same vibrance throughout:

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