NDs and visible light

[reality]

Very trippy!

If I take two ND 1.2 filters stacked and look at some black fabrics under a tunsten light, they appear a very strong maroon (brighter than the dark red on this web page).

So is the IR problem really an IR problem or is it just that NDs have such a strong shift that they block blue and green really well but red not so well?
Or is it that deep red / near IR is the weakest?

The same shot with the RED One looked pretty neutral and normal compared to what my eyes were seeing through the ND 1.2s.

Can someone else try this with their NDs?

 

[Steve Phillipps]

What make NDs are they? Some do have tints rather than being truly neutral. I think it's one of these cases where you get what you pay for and so the more expensive the make the more neutral they tend to be.
Steve

 

[Andrew Walker]

Its been talk about.

http://www.reduser.net/forum/showthread.php?t=9412

 

[Steve Murray]

And here:

http://www.reduser.net/forum/showthread.php?t=12516

 

[reality]

I know it's been talked about before but I'm talking specifically about your on eyes seeing the IR also when you use an ND.

 

[reality]

What make NDs are they? Some do have tints rather than being truly neutral. I think it's one of these cases where you get what you pay for and so the more expensive the make the more neutral they tend to be.
Steve

Formatt filters
I tested .3, .6, .9, 1.2 and combinations.

This wasn't an across the board color shift but was specifically with the black fabrics turning red just by looking through the NDs (no camera, just my eyes).

 

[Mike Prevette]

Yes your eyes can perceive the near IR as well. If you take a Rosco gel swatch book and combine the deep blue gells with the deep red gells you get a IR pass filter and if you squint enough you can see near IR.

Look here: http://amasci.com/amateur/irgoggl.html

 

[reality]

I was surprised at how bright the red was on the black fabric. It didn't seem like a very little red but very strong red.

What I'm curious about is if our eyes can see the deep red or near IR so well, you'd think that a neutral density filter would at least block to this 'visible' part of the spectrum. Even the IR cut filter I held in front of the fabric did almost nothing to the transmission where it works perfectly on other cameras.

Mike, can you try this with an ND 1.2 + 1.2 or something like that? I'd be very curious to hear your opinion.