[shuttertalk]

Hey I thought this was a great idea - you can build your own variable Neutral Density filter - variable because you can adjust the amount of light let in by a simple twist - out of 2 stacked circular polarisers. The trick is to unscrew and flip the glass on one of the of the filters around, so that the effect is additive.

This method uses the properties of polarised light, specifically that two parallel polarisers will block very little light but two at 90 to each other will block nearly all light travelling through them.

http://www.instructables.com/id/Simple-V...ty-Filter/

To change the darkness of the filter just twist the front polariser while keeping the back filter stationary. A twist of 90° will take you from maximum darkness to maximum clearness. My filters gave me about a 10 f-stop range, from ~4 f-stops darker to ~14 f-stops darker.

Hopefully this is useful - I think last time someone was asking where to get ND10 filters - well here's a DIY answer!

[Wedding Shooter]

What a great idea!

[matthew]

The trick is to unscrew and flip the glass on one of the of the filters around, so that the effect is additive.

If you can find a linear polarizer, use it for the front filter.

[Toad]

I like these DIY projects. 2 circular polarizers seems like an expensive way to make a ND filter, though...

[shuttertalk]

Agreed, although it depends on where you get them...
http://www.dealextreme.com/search.dx/search.cpl

Or if you have some lying around that you aren't using...