ND filters and red bias

Serena wrote on 7/13/2006, 4:55 AM

This is something I didn't know, so maybe news of interest to a few others on this site:

"lookout for a shift in the reds, especially in the blacks, when using front of lens ND."

Note from Tiffen:
Organic dyes as are used in the more popular ND's generally do not absorb in the far red/infrared to the same extent that they do through the rest of the visible spectrum. This is not noticeable until you get into the denser filters, usually above 0.9. The reason is that the 'spike' in transmission in the far red is a smaller part of the overall transmission (T) when that overall transmission is higher. As that overall T gets smaller, the spike is a higher percentage of it.

Partial mirrors like Pancro generally produce a straight-line transmission through the spectrum of interest, and do not exhibit the increase in red. There's just the highly reflective surface to deal with, but they work well in the right situations.

Ira Tiffen
Basking Ridge, NJ

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Comments

Serena wrote on 7/15/2006, 6:49 PM

Hi Glenn, our difficulty in discussing this is filter characteristic is that we don't have an example to anchor our ideas for correction. So each has argued according to our own ideas of what sort of colour correction may be necessary, so the range of our responses covers the ground quite well. So should any of us find ourselves using a "dodgy" filter, then there should be something of use in this thread for correction.
I think Bob is correct about the limited range of WB. I haven't dragged the camera out to check, but I remember playing with WB to see how far I could push it to see what sort of images would result under normal lighting (ie colour distortion). The effect wasn't dramatic. Certainly WB copes with the range 2800K to 10000K (home incandescent to blue sky illumination) and may go down to 2000K (flame). This is a much greater range than Bob's warm cards.