I spent some time this past summer playing around with a Variable Neutral Density filter.
For those of you who are not photography nerds, a Neutral Density filter is a piece of dark glass that you fasten over the end of your lens to reduce the amount of light coming in. This allows you to use longer shutter speeds to achieve various effects – including the one where you turn bumpy fast-moving water into smooth silk.
Neutral Density filters come in various strengths, and if you are hard-core you will buy several. If you feel like spending a lot of money, you can get some very fancy Variable Neutral Density filters that rotate from weak to strong. If you are not hard-core, and just want to fool around a bit without spending obscene amounts of money, you can get cheapo Variable filters; they do exactly the same thing as the expensive ones, but they tend to add a bit of colour cast – mine (made by Neewer) has a warm golden cast which is actually rather attractive – but, in any case, the whole issue is of no consequence if you convert to black and white.
Why would you want to use a longer shutter speed? To capture images you otherwise can’t. If your filter is strong enough, you can take a photo of a busy downtown street and make it look deserted – the camera will record only the things that don’t move. If you want to make water look smooth (like in this photo) you need to keep the shutter open for several seconds. (The exact length of time depends on how fast the water is moving, among other things.) Use of a Neutral Density filter almost always involves a tripod as well, since there’s no way to hold your hands perfectly still for that long.
For the record, this photo took ten seconds to shoot.