|An ND filter held up to a lens,|
showing the light reduction.
Neutral Density (ND) Filters:
Unlike the polarising filter which changes the way the image looks, ND filters aim to remain 'neutral' and simply reduce the overall amount of light coming through the lens. Like dark sunglasses for a camera, by reducing the amount of light coming through the lens, ND filters allow the photographer to purposefully compensate by slowing the shutter speed down and achieving motion blur effects, even in situations of bright light. When elements in the scene are moving, such as a waterfall, waves, clouds or even people, a range of creative possibilities are opened up.
There are a number of ND filters on the market. Typically they come numbered (ND2, ND4, ND8 etc.) These numbers signify how much light the filter cuts out. An ND2 is supposed to transmit only 1/2 of the available light and is therefore the equivalent of 1 f-stop. An ND4 (twice as dark) will transmit Â¼ of the light (allowing 4x longer shutter speeds) and is equivalent to 2 f-stops. ND8 (twice as dark again) will therefore only let in 1/8 of available light (8x longer shutter speeds) and is the equivalent to 3 f-stops and so on. Depending on the situation, these filters can be stacked on top of each other to reduce the light even further. If shooting on Av (aperture) mode, the camera will alter its selected shutter speed accordingly, lengthening it as the darker filter is applied.
There is also the option of purchasing a variable ND filter. These screw onto the lens but have an adjustable outer ring (like the polarising filter design) which can be adjusted to darken the filter (i.e into an ND2, or ND4, whatever) depending on your needs. While less accurate than a fixed ND filter, it removes the risk of tunnelling and loss of image quality that can occur when stacking filters. (Tunnelling or vignetting, as mentioned earlier, is the problem that occurs with wide angled lenses when the filter starts to become visible in the corners or sides of the final image).
ND filters have limits, particularly in the variable ND filters. When pushed to the extreme (in bright light), when the ND filter needs to be exceptionally dark, the resulting image can sometimes become uneven. Occasionally the camera also assumes you're trying to take a dark image and you may have to adjust your exposure compensation a little until you get the settings you're after. Alternatively you could play around in manual (M) mode with test shots until you reach an exposure you're happy with.
Because everything looks so dark through exceptionally strong ND filters, you may find that a) it becomes impossible for your camera to auto-focus (AF) through it, and b) impossible for you to see your composition. In these cases, it's best to frame up your composition and pre-focus (using AF) before you put on the ND filter. As you'd be using a tripod anyway with such slow photos, neither the focus nor composition will change. To prevent the lens hunting for focus once the filter is applied, after you've pre-focused with AF it's best to switch your lens to manual focus (MF). This will 'lock' your focus. Alternatively, some lenses also have a focus distance scale physically displayed on the lens, allowing you twist the focus ring to approximately the right focus distance.