The 105mm Rotating Polariser
This Polariser is attached to the filter system by an Accessory Ring (sold separately), which fits to the front of the filter holder.* This allows the Polariser to be rotated independently of any other filters in the holder. It is the ideal solution for landscape photography, where a combination of graduated filters and Polariser may be required.
Why use a Polariser?
Polarising filters are used to manage light at wavelength scale, and if magnified could be a likened to a series of microscopic slats.
In use, as the filter is rotated, it either transmits or blocks the waves of light that are vibrating in particular orientations. When light from the sun bounces off a flat non metallic surface, such as glass or water, it becomes polarised - i.e. all of the reflected light waves vibrate in the same plane. This reflected glare can be removed by the polarising filter.
Rotation of the filter will be needed to find the optimum position of the Polariser. In photographic terms this can render water or glass transparent, and gives the impression of saturating colours in a scene, because much of the reflected glare from the subject is removed by the Polariser.
On a clear sunny day, much of the light in the sky is also polarised, and the filter will give a very strong blue effect when used at 90 degrees from the sun - any white clouds will stand out impressively.
Linear or Circular?
There are two types of polarising filter: linear and circular.
These terms do not refer to the shape of the filter, but rather the way in which the filter modifies the light waves that pass through it.
The type of filter required depends on the camera. If you use an autofocus SLR (digital or 35mm) in, for example, spot metering mode, you will need a Circular Polariser. This is because a Linear Polariser will interfere with the complex metering and AF systems of modern cameras.
If you use a manual focus camera, whether 35mm or medium format, you can use either a Circular or a Linear Polariser. If you are still unsure of the type of Polariser you require, check your camera's instruction manual.
A polarising filter will increase your exposure by 1⅔ stops.
*(Filter holder sold separately)