Photography News | Issue 59 | photographynews.co.uk
Camera School Here we lift the lid on all things camera related, showing how to get better results from your mirrorless or DSLR, and providing all the info you don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how polarising filters work, and when to use them
Words & pictures by Kingsley Singleton
In the pantheon of photographic filters, polarisers are vital. Their effects cannot be replicated digitally; they change the look of a picture in ways that software cannot. Like an ND filter, a polarising filter blocks light, but instead of controlling the overall intensity, it instead blocks only light that’s reflected along certain angles from the subject. The filtering of this light reduces glare, controls reflections and, thanks to the more diffused light, boosts saturation and improves contrast. There’s no doubt that polarisers can improve the look of scenes, particularly those with light reflecting from water or rocks, or where the light is hitting foliage. With polarisation taking place, the amount of glare is reduced and the picture is more calming. With water, for instance, you will often be able to see what’s beneath the surface of the water more clearly because the reflection on the surface is removed. Another benefit is in making blue skies more intense. The removal of reflections also works on shiny subjects like paintwork, plastic and glass, so it’s useful when shooting cars or products. But reflections frombare metal aren’t reduced. Polarising filters come in two types, linear and circular; this denotes how they polarise the light, not the physical shape of the filter. Linear polarisers don’t tend to be used with digital cameras as they disrupt the metering and AF systems, so don’t buy one unless you’re happy to work with manual exposure and manual focus. For that reason, linear filters do tend to be cheaper, though. Although you can buy square polarising filters and use them with a filter holder, polarisers are almost always of the screw-in type as thismakes themeasier to use, especially in conjunction with other filters. You just need to pick the one that matches the filter thread of your lens. Alternatively, if you’re using a filter holder that accommodates a screw-in polarising filter, pick the matching size for that. HOWTO USE A POLARISING FILTER The amount of polarisation from the filter is controlled by rotating it; this is achieved by the filter incorporating a bezel that can be turned while the filter is attached. However, a polarising filter’s effect is dependent on the camera’s view in relation to the sun. The polarising effect is at its greatest when you’re facing 90º to the sun; ie. at a right angle. This should be fairly easy to calculate with your eyes, but if in doubt, you can also gauge WHAT TYPE OF POLARISER DO YOU NEED?
Images Here the use of a polarising filter reduces the reflection on the water and glare from the lily pads, making colours in the scene clearer and richer. The reflection isn’t completely removed due to shooting at 16mmwhere the angle is too wide for the effect to be even.
it by pointing your index finger at the sun and raising your thumb so it’s perpendicular to your index finger; wherever your thumb is pointing while your index finger is pointed at the sun the greatest effect of the filter will be. Next you just need to rotate the filter itself, which determines which angles of light are filtered, so look through the viewfinder as you do it or on screen to gauge the effect. Because a polariser is blocking a portion of the light being recorded by the camera, you’ll notice that there’s a fall off in the brightness of an exposure as the polarisation increases. Therefore if you’re shooting in aperture- priority mode (A or Av), the shutter speed will lengthen. The change in exposure depends on the amount of polarisation, but is around one to two stops. Therefore, if the unfiltered exposure is 1/60sec, with two stops of polarisation the shutter speed will be 1/15sec.
using wide-angle lenses can give polarised shots a patchy look. For example, if you’re shooting horizontally with a lens that has a 100º angle of view, and aiming it at 90º to the light, the right and left edges of the frame will only be 40º to the light, and therefore the effect will be less obvious there than it is in the centre of the frame; the polarisation is fading away from the point of maximum effect. The same goes for any panoramas you’re making, as the effect will obviously increase and decrease across the width of the image. One solution to this is to not use the filter at its greatest effect, which can stop pictures looking unnatural anyway. Or you can use a longer focal length to hide the fall off. Cropping the edges of the image in processing will also reduce the patchy effect.
NEXTMONTH ALL ABOUT WHITE-BALANCE AND HOW TO USE IT CORRECTLY
POLARISING FILTERS AND FOCAL LENGTHS
Because the amount of polarisation is relative to the position of the light source in the scene,
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