Why would you put a dark filter over your camera lens to let in less light? It can make autofocus difficult, complicates exposure calculation and makes hand holding the camera impossible because of the long exposure. So why do it–to let the camera record subject motion over time. Long exposure photography records movement in a photograph and the results can be artistic and pleasing if done properly.
Using ND Filters
A neutral density filter may compromise the point-and-shoot convenience of today’s automatic cameras. Because it cuts the light by as much at ten stops it is best to first compose, focus then turn off auto focus before mounting the filter. Check this YouTube video for more detail on using a ten stop ND filter.
Exposure calculation can be complex, but a modern DSLR or mirrorless camera will usually calculate exposures accurately up to thirty seconds. Set the ISO to the lowest setting and use a narrow aperture (f11 or f16). With the camera on manual and a fixed ND filter it is possible to manually determine the exposure without the filter. Then mount the filter and adjust the exposure time by the given number of stops. Guides are available on line to calculate ND filter exposure.
Variable ND Filter
ND filters come in a range of 2 to ten stops. Variable ND filters are also available. A variable ND filter consists of two polarizing filters one of which rotates above the other. It can reduce the light from about two to more than ten stops. The problem is that manually calculating the exposure for any given setting is virtually impossible.
One way to deal with this is to set the camera to aperture priority and let the camera choose the exposure time. This is a less than accurate method and will require testing and adjustment. Also, exposures over thirty seconds are not possible and must be done manually with the bulb setting.
Choose the Appropriate Subject
Flowing water, clouds and people in motion are subjects that can benefit from long exposure photography. Waterfalls and ocean waves make excellent long exposure subjects when done properly. Experiment with exposures of one to five seconds for waterfalls and streams and decide what gives you the most pleasing effect.
Longer exposures of ocean waves can turn the water into a smooth, flat almost surreal representation that stretches the bounds of reality. Done right, the results can be very artistic. The same goes for clouds which can turn the sky into soft, billowy streaks.
People in motion can also work for long exposures. People in a crowded square or busy intersection become streaks of color and interesting shapes. Given enough time, people can vanish completely, giving the scene a deserted look.
For more information on long exposure photography, check out this excellent guide in BWVision.
Camera Stabilization is Essential
Using a neutral density filter to increase exposure time in daylight to many seconds eliminates the possibility of hand holding the camera. Use a tripod or other sturdy alternative support to keep the camera steady during long exposures. HandlePod is a light weight, pocket size support that can be used effectively with ND filters for daylight long exposures described in this article by Mirrorlessons. A neutral density filter and sturdy camera support opens up a new realm of photographic creativity.