Friday, 3 August 2012

Graduated Neutral Density Filter

A graduated neutral density filter, also known as a graduated ND filter, split neutral density filter, or just a graduated filter, is a filter in which the darkness of the filter progressively changes from dark to clear. One end of the filter will be dark and the other end clear.

Graduated ND Filters
Graduated ND Filters

Graduated ND filters are commonly used to adjust the brightness levels (contrast) in certain areas of the scene to acceptable levels so that a cameras digital sensor could record them properly. These are mostly used by professional landscape photographers.

Neutral Density Graduated Filters are broadly classified into two; “Soft” and “Hard”, depending upon the type of transition it has. In case of hard filters, the transition from dark to light will be in the form or a sharp cut. And in case of soft filters, the transition from dark to clear will be gradual. Further there are three variants each of both hard and soft filter, they are classified according to their density (amount of light restricted) into L, M, S (Low density, Medium density and Strong). The density of filters are expressed in values as 0.3, 0.6, 0.9, etc. or the light loss ND-2, ND-4, ND-8 etc.

Although ND grads come in all sizes and shapes the best among them by way of design and practical usefulness are the squares or rectangles. These are not screwed on to the lens like conventional filters. They come with an adapter with a holder to hold the filters. The adapters fit to the lens. The main advantage of this type of filters is that the filters and the filter holder can be adjusted, not only to put the graduation strip into the correct part of the scene, but also by rotating the filter to accommodate hillsides or any time the composition puts the horizon on the slant.

Rectangle Graduated ND Filters
Rectangle Graduated ND Filters

Unlike glass filters, grads are not coated to reduce reflections and flare which could cause serious drop in contrast and saturation of images, so it is important to effectively shade them when used in situations that could cause flare. There are also coloured variations of graduated filters which were once very popular (during the times of black and white photography). But these days they find limited to no use due to advancements in digital imaging and post processing techniques.

There are also a couple of very specialized versions of graduated filters, they are;

Reverse Grad

Reverse grad as the name suggests go sharply from clear to grey and then gradually back to a lesser grey. They are mainly used to photograph sunsets or such scenes where in the mid portion in the scene will be the brightest so you have your special filter with clear for the foreground which will be very dark, grey for the midsection (usually horizon) which is very bright in comparison and lesser grey to cover the sky which will have an exposure value somewhere in the middle of the foreground and horizon.

Curved Filter

Specialized filter made to shoot the mountains. It got the name from the curve in transition it has.

Centre Spot Filter

Centre spots are ND graduated filters that are slightly grey in the centre and are clear at the edges. These are used for special effects or to compensate for light falloff that is natural with large optics.

In the next article we will discuss about Photography Accessories - Filters - Polarizing Filter

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