Metering a makeshift variable density filter

[qleak]

I'm planning on making a variable density filter by reverse coupling to circular polarizers together. Long story short: I purchased a nice 72mm B&W circular polarizer and got another cheaper Tiffen polarizer for free when I got my newest lens.

I'm currently waiting for the 72-72mm reverse coupler to ship from china.

So why mount in reverse you ask? Well... a circular polarizer is really a linear polarizer with a depolarizer behind it to make the light go in circles again (for your AF's beam splitter). Two linear polarizers will make a variable ND filter when stacked. However, a depolarizer between them negates the effect.

How would you meter the number of stops reduction at maximum?

I'm fairly sure at minimum it is 1.5 stops ( the 2nd filter has a negligible affect).

Best,

-Q

 

[tirediron]

Mount it up, put the camera on a tripod aimed at a fixed light source, put it in 'Auto' and watch the metering change as you increase the ND effect.