Variable neutral density filters
Today I thought I'd write a little piece on using variable neutral density filters. These are not to be confused with graduated neutral density filters such as these 
where the variation of the opacity is fixed and graduated from top to bottom in a linear manner.
Variable ND filters generally come in circular form, similar to a polariser and screw onto the end of your lens. The one I use is manufactured by a company called Singh Ray and looks like this:
One of the main reasons I use a variable neutral density filter is basically to slow down time. In other words, it's to get a much longer exposure than my camera would otherwise allow. By greatly reducing the amount of light hitting the camera's sensor with this filter I can lose up to an additional 8 stops of exposure. What does this mean and why on earth would I want to do it???
Imagine a waterfall, or waves lapping on a beach. If I were to shoot one of these scenes in the middle of a bright summer's day, then the sunny 16 rule would generally apply. This is an old technique from film days which still applies today - simply put, to get the correct exposure on a sunny day, set the aperture to f/16 and the shutter speed to the reciprocal of the ISO, so most of the time on my Nikons this would be f/16 and 1/200 sec. If however I wanted to blur the water and produce a smoother silkier water effect, then I would need to slow the shutter speed down to make a longer exposure. This is where the variable neutral density filters come in. Being able to lose up to 8 stops of light means that in this particular sunny situation, I could potentially slow my shutter speed down to 1 second and still properly expose the scene.
This is exactly how I made the image at the top. However this was shot at sunrise when there was significantly less light, so using the vari-ND I was able to slow my shutter speed right down to allow me to make a 30 second exposure. This produces a lovely smooth effect on the water and completely flattens out all the waves making a much more calming image. This is how the image before looked without the Vari-ND at 1/125 second.
There is one very important thing to remember though when making long exposures - make sure your lens and filter are spotless! When making a 30 second exposure in daylight every last spec of dust will show up ten times more pronounced on your image! I probably spent close on an hour spotting this image....