Using a Neutral Density filter - Attaching a neutral density filter to your camera lens will bring you several benefits says Peter Bargh of ePHOTOzine.
Words & Pictures Peter Bargh
One filter that is in the camera outfit of many professionals is the neutral density filter, or ND filter as it's better known. The filter is less appreciated by amateurs and the reason it probably has little appeal is its looks - a plain, dull grey filter. Not colourful, no star effect, no gradation, no multiple image glass...just plain grey. And what does it do? Reduce the exposure? Hmmm I can do that with my camera...it's pointless! Well actually it isn't, and that's why the ND filter is a necessity for the professional and often found in the enthusiast landscape photographer's camera bag.
The ND filter may be plain grey, but it's a neutral grey so whatever light it lets through isn't affected in colour, just in brightness. So why would you want a filter that reduces the exposure when it can be done using your camera's shutter speeds or apertures? This depends on a few things. Firstly you may have a fully automatic camera. If so, the ND filter will give you a small amount of manual versatility. Or you may have been taking pictures in low light using fast film and then ventured out into bright sunlight where it was physically impossible to take a photo because your camera's fastest shutter speed was flashing, even though the aperture was adjusted to the smallest setting.
The ND filter will reduce the light and allow the shot to be taken. Using an ND filter in this way is not its primary benefit though. Have you seen those shots of waterfalls that look ethereal with blurry cotton wool water? Well the chances are an ND filter will have been used. Here the filter is used to reduce the shutter speed so that blur occurs. If you are out in a bright location the shutter speed will be at least 1/125sec and ideally you need 1/15sec or slower. So pop on the necessary ND filter and you'll gain the effect you're after.
Architecture photographers have a useful technique that often needs an ND filter to work. When photographing famous landmarks you often have problems with tourists getting in the way. If the shutter speed is slow enough it will be open long enough to ensure the moving people are so blurred they cannot be seen on the image. A 1/2sec exposure may record a streak of someone walking while a 4 sec exposure will make them vanish. Simple, but very effective.
A tourist was looking around this historic ruins. Left: A half second exposure has picked up the person's body as a blur. Right: two seconds and the figure has gone!
It's not just the shutter speed that you may want to improve. If you are shooting in bright conditions you may find the aperture the camera is selecting is small and the resulting picture will have far too much front-to-back subject sharpness known as depth-of-field. This is often the case with portraiture or flower photography where a distracting background ruins the photo. Using an ND filter will help you open up the lens and provide shallow depth-of-field.
Another use for an ND filter would be when using flash. You can often reduce the exposure of the flash using auto settings, but for close ups that may not be possible. The ND filter will provide the key to this essential barrier.
There are several types of ND available - screw in ones come in various filter thread sizes and slot in ones fit into a holder that screws onto the lens. They are available in different strengths too, known as the filter exposure factor as follows:
ND filters and their exposure factors
2x one stop
4x two stops
8x three stops
64x six stops
Using an ND filter is simple, you just either screw on or slot into a holder and leave the camera's automatic exposure system to work out the filter factor. If it's an 8x, for example, the camera will reduce the shutter speed from, say, 1/125sec to 1/15sec to compensate for the three stops extra light required. Or the aperture will be opened up from f/22 to f/8.
Tip: use a polarising filter
A polarising filter offers a similar light reducing option as a Neutral Density 4x filter. Add two polarisers together and you can get an infinitely variable ND from 4x to very strong (almost black) by rotating the front one.
