Help me correct color cast (from B+W 10-stop ND filter)

[Brian Ellis]

I just visited a local expert w/ a color meter, which provided some useful information about this rather un-neutral B+W filter.

According to the Sekonic, the B+W filter caused a 970 K drop in temperature (more red). This was even more than I suspected, and I’d call it significantly un-neutral. This reading also suggests there’s a little more magenta in the actual film than appears in my scanning effort above (and confirms John’s B+W magenta cast in post #4). Even more technically, we had a chart that equated this change w/ a +40 nm shift.

BTW, for comparison I also had a Nikon 2-stop and Tiffen 3-stop ND filter, and we measured these, too. The Nikon produced a 160 K rise in temperature (more blue). The Tiffen produced a 340 K rise (also more blue). These changes, of course, would not be as noticeable to the naked eye (if at all). Much more “neutral,” especially the Nikon.

Then we measured some “correction” filters that might be a good match for the B+W. The B+W’s 970 K drop suggested an 80d filter (which typically raises the temperature about 1300 K or so) would be a good place to start. We had a Lee resin 80d, and the Sekonic said it actually added about 1,000 K…

The Lee 80d – maybe a good filter to start experimenting with, though I think correcting the B+W will be a little more difficult (make that a lot more difficult) than simply adding a common filter like this one.

However, since I like a little warmth, I’d much rather find a single filter that adds less than 970 K (instead of a little more like the 80d) to leave some warmth, and see what happens. Any common filters out there that do this?

If it's any help to you, I spent about 30 seconds in Photoshop playing around with the color balance of your Surf2. By adding some blue (+33) and cyan (red minus 20) and fiddling around with the sky I got this result, which on my calibrated monitor is pretty close to your original. Another few minutes of effort would probably duplicate it, except of course for the water that your long exposure changed dramatically. I have no idea whether there's a color balance filter out there that is the approximate equivalent of adding blue and subtracting red (which adds cyan) in Photoshop but maybe this will give you some idea of a direction to try.

[Heroique]

Brian, that’s a super job. Including the water. And especially the sky.

Thank you.

I hope I can find a single correction filter that can approximate what you’ve done in PS.

[Mark Sampson]

Maybe you should consider using a Kodak Wratten #96 ND (gel) filter of the appropriate density. The Kodak filter book says they are "for uniform light attenuation throughout the visual spectrum". I've never used any of them with color film but it's worth a try.

[brian mcweeney]

I still feel like there's more than a simple color correction to this problem. Here's what I came up with. First image is the original shot, second is with the ND, and the third is my photoshop work. Channel mixer, slight curve to get snap back, and slight overall green and blue added in color balance. Still not perfect match though.

[dsphotog]

Although not the same...... But could be fun...
You might enjoy shooting the scene at night.

[Heroique]

That’s another fine PS conversion. And like mrladewig’s post #14, your job suggests a touch of green may be part of the solution.

I realize field tests are the only way to be sure, but I want to get the filters that make the most sense for a first try. I’m not sure how to translate the PS work (by both Brians) into on-location filters – but as discussed above, the 80d is one I’m sure to bring.

A couple of others might be, say, a CC cyan and CC green, but I’m curious which strength one might recommend. For example, below is a set I might start with.

1) 80d, 2/3 stop, adjusts color temp +1300 K (or +1000 K if it’s the Lee 80d we tested)
2) CC40 cyan, 2/3 stop, absorbs red (perhaps in place of the 80d)
3) CC20 green, 1/3 stop, absorbs red and blue (w/ 80d, or w/ CC40 cyan)

I hope I don’t have to use more than one filter for correction, but you never know…