Circular or linear Polarisers for Digital Cameras?

The Facts.

Most modern cameras utilise some form of 'splitting' some of the light for fucussing and exposure.

This is not only true in digital photography, but also is true when shooting with film cameras, particulary when using any Autofocus SLR camera / autofocus lenses.

Many cameras, and most, if not all digital cameras utilizes a beam within the optical path to adjust focus and /or to meter the light to determine correct exposure. With Digital SLRs this is usually done within the Prism of the SLR.

As all these systems rely on splitting the light, if you use a Linear Polariser this polarises the light, and can prevent some the light from being correctly split into the focus or exposure device.

Thus this can prevent the camera from correcly exposing, or misreading the point of focus, causing problems leading to the camera giving false readings on focus and/or exposure.

Circular PL or Linear PL?

A Circular Polariser has (to the most part) the same effect as a Linear Polariser, but circular polarisers do not affect the metering, or focus systems as a linear polariser does. Therefore a Circular Polariser can be used on any camera, whether digital or film, autofocus or manual.

ie: a circular polarizer will work properly under all lighting condition and on any camera. Therefore it is wise to only purchase Circular Polarisers (although they are more expensive) to avoid any such problems.

Quality of Filters.

Of Course like any filter there is different qualities of Polarising filters from different manufacturers. The better filters are more neutral and have less effect on color and image quality then cheaper ones. They also are optically superior and produce better image sharpness then cheaper ones due to higher quality glass, and better manufacturing.

Multi-Coatings

The glass surfaces used on a polarizer may be uncoated, coated on one or both sides or fully multi coated. Filters such as the HOYA PRO1 Digital have many coats, and are manufactured to high specification.

The better the coatings the less unwanted flare and reflections will be present, and give a better and sharper image overall than a cheaper filter. This helps increase contrast and color saturation and deliver a better image.

Thickness of filters

Polariser filters tend to be thicker, as they usually require a 'sandwich' of a polarised film between two pieces of glass, and have a double ring, enabling the filter to be turned.

Again some manufacturers offer slim line filters. This is particularly useful when using wide angle lenses to avoid vignetting within the lens - ie avoiding getting the edges of the filter in the picture on very wide angle lenses.

Conclusion

A polariser is a very useful and creative filter indeed, and is an important part of the photographic kit for enthusiasts and professionals alike.

Linear Polarisers are fine for Non autofocus cameras and if exposure is metered externally. Otherwise ensure you buy a Circular Polariser for digital SLRs, or for any autofocus camera to avoid any issues with exposure or autofocus problems.