DIY variable neutral density filter

It turns out that for one of my projects I need to be able to create some motion blur in daylight while using a reasonable f-stop. The natural solution is a neutral density (ND) filter. I have a good quality ND4 filter, but 4 stops is not always enough to get the right level of blur with a short depth of field. So, it was either buy a whole stack of filters- or go the alternate route and make a variable ND filter. (Buying a Sing-Ray vari-ND filter would be lovely because it is thin, minimising vignetting, but at US$390 very expensive...)

The simple option is to buy a couple of cheap circular filters, and stack them with the polariser of the outer filter reversed as per the diagram.The linear polariser layers of the two filters are then opposed and you can change the light transmission simply by rotating the orientation of the outer filter.

This seems to work pretty well, with a couple of caveats:

• You need to watch for oblique angle light leakage through the edges of the filters. When you make the filters really opaque, even small leaks destroy your image. So careful tightening / sealing of the filter mounts and use of a lens hood are needed.
• As you would expect, there are some colour balance issues that you need to watch at higher density levels, but the test images below suggest it is not a big problem to manage.
• The camera's metering system will underexpose at high filter densities. I am guessing it may be due to IR transmission but the actual cause is irrelevant. It comes down to checking the exposure histograms and manually adjusting to "taste".
• Cheap filters and thick - and two layered ensures you will have some nasty vignetting at wider angles.

The test shows I can dial in up to -9EV of exposure increase with no major image quality problems.  I could not me more pleased with the results and look forward to some real world testing in my projects.