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.