The ND Anomaly :)

I've decided to make a little test with the ND filters. Since a while now I wanted to find out how 2 (or more) ND filters would act together. What will we get if we put 2 ND8 filters together? Will it be ND16 or ND64? Or maybe something else?

Well finally I found the first clue on E-Bay in the form of a cheap chinese ND8 filter for the Cokin P holder. So spent a few quids on it and tonight I did the test-run myself.

The result is what I expected, which is somewhere near to ND64 which is 6 stops.

Also it turned out that the cheap chinese ND8 filter is about a half stop weaker than the Cokin one and also makes the pictures slightly softer.

But I found out something else as well :) As you can see on the pictures above the 3 ND8 filters together is nowhere near the same colors as the one without the filter. The picture is way more reddish. It's the same thing as I experienced previously with the vario-ND filter, which is the ND filters do not block the IR light. To test it I only needed an IR sensitive camera and as we know most households have one so does mine. So I grabbed my modded Panasonic FZ50 and made a quick test. The result is the Cokin ND8 filter does not block ANY IR light, while the cheap chinese filters reducing with approx. one and quarter (1.38 to be precise) stop.

Now this leaves me to some new ideas in IR photography, as we could use the ND filters stacked together as IR filter in a way... so basically we can let the IR light get to the sensor along with _some_ colors as well, a bit similar to the IR filters that cut around 720 nm, just they will let in _all_ colors instead of just a bit of red.

Just for fun I included my good ol' 11 DIN welding glass. The result is with the K10D it did what I've expected, which is a bit more than 11 stops, well closer to 12 stops (that means it equals an ND4K filter) and it cuts the IR light almost entirely.

Now what I am curious about is: are the other ND filters act like this as well or only the Cokin system ones? I mean what about the Hoya ND400 filter which is about 8.5 stops quite similar to the one where I put the 3 ND8 filters together*? Maybe that's why I only found B&W shots with the ND400 filter? Unfortunately the ND400 filters are expensive so I can't try it myself but if someone has one could share the experience with me :)

* - Theoretically the 3 ND8 filters together should be equal to an ND512 filter.

And now... the test shots: