A neutral density (ND) filter is a piece of glass that fits over the front end of a lens to reduce the amount of light entering the camera. Now, you might be asking yourself when and why would anyone would ever want to do this?
These filters are commonly used when shooting time lapse videos in bright daylight conditions. If it's the middle of the day and you want to capture a nice shallow depth of field you'll need to have the aperture wide open. Also if you're looking to get some motion blur as your subjects move around the frame you'll need a lower shutterspeed. Both of those settings can be hard to properly adjust for in bright daylight conditions. That's where ND filters come into play. Let's check out the explanation below by Olivia Speranza to get a better handle on all this.
Sounds pretty simple right? You're just reducing the amount of light to get a shallower depth of field and/or to blur the motion you're capturing in bright lighting conditions. These filters are referred to as neutral density because they don't block out one wavelength or color of light more than others, all light is equally darkened, unlike other filters that block only certain colors. ND filters are typically sold in varying degrees of darkness from about 1 level of f-stop reduction to all the way up to 13 or darker. There are some special and more expensive kinds called variable ND filters which allow you to simply rotate the filter to adjust the amount of light filtering. Here's a picture of one to give you an idea.
Photo courtesy of Shutterstock
When purchasing your filters make sure your lens has a thread on the end that can support the addition of a filter. If it doesn't, you can get a special ring adaptor to mount the filter.
Now for some inspiration. Check out the video below that Adonis Pulatus shot on a cruise-liner ship with the use of ND filters among other pieces of equipment.
Notice how the waves of water are blurred as they race away? That effect was achieved in part with a slower shutter-speed which he was able to expose at since he used an ND filter.
[challenge: There's a lot of room for exploration here with this relatively inexpensive accessory. Get on out there and try it yourself. Make a video where you use ND filters and share it with us here-, url: http://vimeo.com/groups/challenges]