Neutral density filters in photography

Neutral density filters (ND filters) used in photography are one of the most indispensable filters. It could be black and white, color film or digital photography, ND filter is a must, when we have to control a bright light.

Exposure setting at lower end has no limit. For example, when we are shooting in a dim light, we can use any ISO setting and aperture, and to compensate the lower ISO value or smaller aperture, we can use longer exposure time. Now even if the light is less, we can extend the time, which could be in minutes or in hours, and that duration has no limit for extension. So far the camera is on a tripod and there is no movement in the subject, we will get an exposure, which is perfect for brightness and contrast.

On the other end, at higher settings, there is a limit. For example, when we are shooting in a bright day light and even at lowest ISO setting, suppose the shutter speed is 1/250 of a second and aperture is f-11, and if we need to use a slower shutter speed of say 1/4 of a second (to record the movement or flow of water), we will need an aperture f-90.

Or, if we want to use an aperture at f-2 (in order to blur the background), we will need a shutter speed at 1/8000th of a second.

In both these cases, we may have limitations. The camera lens may not have aperture of f-90 or the shutter speed may not have 8000 mark on the shutter dial. So the only option is to reduce the intensity of light.

ND filters do the same thing. They simply cut some light, depending on the density of a filter we use. They will not change the overall color balance but simply reduce the light reaching to the film (or digital image sensor).

These filters are available in different density or opacity. Lighter version of ND 0.1 is 80% transparent, with filter factor of 1 1/4, and requires only half f-stop opening. Darkest one of ND 5 is 0.01% transparent, with filter factor of 10,000, and requires 13 1/2 f-stop opening. Refer a manual table provided by the filter manufacturer.

Variable density filter is a pair of two polarising filters. We can turn one filter and as they cross polarisation between two filters, the light will fall. We can regulate the fall by adjusting the rotation.

Focusing becomes difficult, especially with an SLR camera as the image in the viewfinder also becomes dark. Auto focus will not work as polarising filters interfere with auto focusing. In such case, remove the filter and put it back after focusing the lens.

Go for good quality neutral density filters, as the cheaper one may give a green or a blue color cast in a photograph. Use a good quality heavy-duty camera tripod.

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