Your guide to essential filters
Types of FilterRound or Square, glass or resin - filters come in differing forms. Glass versions are round and cater specifically for a set lens diameter, with better ones being coated.Coatings improve light transmission and reduce reflections which can cause flare. Single coatings loses typically 3% of the light transmitted through them; multi coatings reduce this to about 0.5%, wheras multi-resistant coatings (B+W terminology) also protect the filter from scratches and are water and dirt repellent. PolarizersIf you ever buy one filter for your camera, make it a polarizer. This is the one filter that has yet to be replicated digitally, and is just as useful now as is has ever been.With the help of a polarizer, pale blue skies can become saturated almost to an inky black, obscuring glare can often be removed completely from water and other shiny surfaces, and it's also an effective neutral density filter, absorbing around two stops of light - ideal when longer shutter speeds are needed. How do they work?Essentially a polarizer is much like an incredibly fine sieve - one which allows only light vibrating in a certain direction to pass through. All of the filter's incredible abilities stem from this one property.Most light in nature is unpolarized, meaning that as it travels in a straight path it does so as a wave which undulates or vibrates in all directions. However, if it should meet a shiny surface (apart from those of metals and mirrors) some of this light can be reflected in a set orientation becoming 'polarized'. Though its unique filtering action, a polarizer can be rotated to selectively block this light, which in turn reveals the reflecting surface's natural color and detail. Polarizer in action
Another popular use of polarizers is to darken or enrich a blue sky. This is possible because polarized rays exist naturally in the heavens where the light reflects from suspended particles. The effect is strongest at ninety degrees to the sun. 'There's a useful and equipment-free method for finding this zone. Point your forefinger at the sun and raise your thumb as if it's the hammer of a gun. Now rotate your wrist right and left so that your thumb has marked out a complete hemisphere from horizon to horizon. This is the area in which the polarizer will have most effect. Near to the sun, changes will dwindle to nothing; similarly, with the sun at your back the effect will be just as mild.' Linear Versus CircularThere are two types of polarizing filter. Both have similar effects on the image, but linear polarizers work best with manual focus cameras and hand-held light meters. If attached to a modern DSLR they're likely to confuse the auto focus sensors and internal light metering systems as they cause the light that these sensitive mechanisms depend upon to vary.A better solution with DSLR is a circular polarizer. Through clever construction these filters restore the light inside the camera, allowing correct metering and accurate auto focusing to take place. Interestingly, some photographers still prefer to use linear polarizers, despite their drawbacks, as they believe that they simply do a better job. Neutral Density Graduated FilterExcept at night or on the odd thundery occasions, the sky is almost always brighter than the land. At certain times of the day, often when the sun is low, the difference can go way beyond what a single exposure is able to capture, and there are other instances too when the sky is overcast and filled with highly reflective and featureless cloud. In both these circumstances you can make a big difference to your result by reaching for that other essential landscape photographers filter - the neutral density graduated filter (or 'ND grad', for short).Grads in Action
How do they work?Grads are half clear and half covered with a light-blocking tint. As the name implies, this is distributed in subtly decreasing density from the top to the middle, where it becomes clear.Neutral Density grads are similar to polarizers in that they block light, but do not introduce color. Colored grads, on the other hand, change the picture's hues as well as blocking light. (These can also be useful in some circumstances, to enhance a scene, but are often misused). It's also possible to obtain something called a 'gray grad'. These are hypothetically ND grads but may not be graduated to be free of slight color cast. ND grads work by reducing the light entering the lens from one half of the frame, in order to balance its brightness level with the other half. Most often this is the sky, but with some creative thought, ND grads can be handy for many other situations of high contrast too. | ||
