Friday, 3 August 2012

Neutral Density Filter

A Neutral Density filter or ND filter as it is commonly called acts just the same as sunglasses to our eyes. We use sunglasses to lessen the intensity of light, similarly Neutral Density filters are used to reduce all colors in the light spectrum evenly.

ND filter with 77mm filter thread
ND filter with 77mm filter thread

Neutral Density (ND) Filters reduce the amount of light entering the camera. When used wisely, these filters allow the photographer to capture a scene that would be virtually impossible to photograph otherwise.

But why would anyone want to reduce light, often we hear photographers screaming for more and more light, not less. Yes there are situations when we wish to take out a couple of stops from available light.

Some situations that call for the use of Neutral Density Filters are.

  1.     Blurring water, capturing motion (capturing movement of water in waterfalls, rivers, oceans).
  2.     Reducing depth of field in very bright light (need to use f/2.8 or similar while shooting outside on a bright sunny day).
  3.     When using flash or strobe in combination with direct sunlight, without ND filters, your options are quite limited by the sync speed of your camera, usually not more than 1/250 of a second.
  4.     To capture motion blur in brightly lit scenes.
  5.     To facilitate use of long exposure in a bright scene.

So ND filters enables the use of longer exposures (to emphasize motion), and also facilitates the use of larger apertures (shallower depth of field), on bright days, outdoors. There is also the favourite trick employed by landscape photographers all the time, the one with long exposures combined with small apertures.

ND filters appear gray or even opaque to our eyes, as a result the viewfinder will appear very dark, so it is a good practice to compose you shot and achieve focus before placing the ND filter in front of the lens. ND filters are specified by their light-reducing ability, where stronger filters appear as darker shades of gray. Some manufacturers specify their filters as ND-2, ND-4 and ND-8. These admit half, quarter and one-eighth the light – easy to interpret as 1 stop, 2 stops and 3 stops reduction. Others use the density 0.3, 0.6 and 0.9 descriptions for the same filters.

ND Filter Compensation

ND Filter % of Light Transmitted Increase in exposure needed
0.1 80% 1/3 stops
0.2 63% 2/3 stops
0.3 ND 2 50% 1 stops
0.4 40% 1 1/3 stops
0.5 32% 1 2/3 stops
0.6 ND 4 25% 2 stops
0.7 20% 2 1/3 stops
0.8 16% 2 2/3 stops
0.9 ND 8 13% 3 stops
1 10% 3 1/3 stops
2 1% 6 2/3 stops
3 0.1% 10 stops
4 0.01% 13 1/3 stops

Two filters can be used together but the filter factors must be multiplied, not added, together. So if you use a combination of one 2 stop filter and one 3 stop filter, the resultant reduction in f stop value will be 6 and not 5. There are special ND filters specifically designed for high end industrial use such as solar photography and imaging the interior of furnaces and other high temperature processes.

A general advice applicable when buying any filter is to buy the filter with thread size that matches your largest lens and buy cheap step down rings to use the filter with smaller lenses; this way you could do with just one filter for all your lenses.

In the next article we will discuss about Graduated Photography Accessories - Filters - Graduated Neutral Density Filter

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