10 Stop ND - Sloooooow Photography

Some time ago, I bought myself a 10 stop ND filter.  For those who don't know, this is a filter which is equivalent to a 10 stop attenuation of the light level - in usage terms this means the shutter speed needs to be around 1000 times longer than if the filter is not fitted.

What this allows you to do is take really long exposure shots in the daytime - for example, if a normal exposure of a scene was 1/100th sec for the appropriate aperture, with the 10 stop ND fitted you would need the shutter to be open for 10 seconds to exposure correctly.  In reality, the filter is never perfect and the attenuation is not exactly 1000 times so you will need to experiment a little to get the best exposure.

This image of a jetty at Sandbanks  was taken at f14 for 190 seconds - notice how the long exposure smooths the water and brings out the reflection of the jetty.

Things to watch out for:

There are a couple of things to watch out for when using the 10 stop ND filter.  First, like with any filter, if it's used on with a UWA (Ultra Wide Angle - about 17mm or less on full frame sensor or about 12mm or less on a cropped sensor) lens, you are likely to get vignetting (darkening of the corners of the image).  This can generally be corrected, but like with any post-processing operation - especially lightening of the image - you can enhance the noise.  If the vignetting is severe, the increase in noise in the corners of the image could become an issue.

The second thing to look out for is to do with imperfections in the neutrality of the filter.  It is very difficult to make a less dense (e.g. 3 stop) filter with absolutely no colour cast (i.e equal transmission of light at all frequencies), but to make a 10-stop filter like this is pretty much an impossibility.  The filter I have is not a cheap filter (it cost me around £100), but it still has a slight brown colour cast which becomes more apparent with longer exposures which is typically what you are using this filter to take.

In reality, this cast is not normally much of a problem.  There are two options - to either adjust the colours of the final image to remove (or in some cases enhance if it adds to the image) the cast, or convert the image to black and white, thus removing (most of) the effect of the cast.

 Uses for the filter

There is no right or wrong use for a filter like this.  Maybe it's not so good for portait shots, although I have used it for these to get a "ghostly" figure in a shot.  Typically, the filter is used to smooth water or sky, or accentuate motion in something slow moving like the sky.  You can also use it to remove motion - for example the shot of the M3 motorway in Hampshire below is around a 25 second exposure, and the rapid motion of the cars causes their image to be removed (except for a few subtle headlight trails and indicator flashes).  This image also shows how you don't need to convert every image to B&W - I left the brownish colour cast on this shot as I thought it worked with the image:

The image below shows the capturing of slow motion by extending the shutter speed - this is a shot of a car ferry which goes from Sandbanks in Dorset - notice how the water is very smooth, and the motion of the ferry is captured as it makes it's way across the harbour.