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

[Heroique]

I’ve heard about the color cast w/ B+W’s 10-stop ND filter, but now I see it!

My main question is what on-location filters might help “correct” for this cast, so I can “get it right” on the film? (I’m less interested in using PS for correction; I mainly view my slides with an analog projector.)

BTW, these Epson 4990 scans, on my calibrated monitor, re-produce – very closely – what my slides look like when they’re back-lighted w/natural light…

— The first shot is w/o filtration, Astia-100F, 1/30th @ f/22.
— The second shot is also Astia-100F, w/ B+W’s 10-stop ND filter, 34 seconds @ f/22. I think the film suggests exposure/color compensation isn’t necessary for this exposure, and my experience mostly confirms this.

Now, I like warming my daytime shots in conditions like this. Usually, I’ll go w/ an 81-series filter, and sometimes when I want stronger warming, I have an 85c. But this result is, well, “strong & wrong” for my tastes. The slightly hazy sky’s “brownish” cast is especially displeasing to me. (Makes one curious why B+W puts “neutral” in the name of this filter. )

Would CC20 cyan, or CC40 cyan be a good place to start experimenting? Or maybe the 80 or 82 series? Other ways to experiment?

[John T]

Are you sure this is a color flaw of the filter or reciprocity failure of the film. My guess is that the color shift of the film is partially to blame. Astia has much less reciprocity problems than other films, but 22 seconds will affect the color. Since you are willing to experiment, do 2 shots one with the 20 and one with the 40

[Heroique]

Hi John,

Yes, I've used Astia-100F w/o filtration at longer than 20 seconds, and the daytime colors, though slightly different than when taken at quicker exposures, don't change as dramatically as they do here. I don't think I've used over 40 seconds yet...

I should add that the two shots were taken within moments of each other in the same light.

Also Tachi 4x5 w/ Schneider XL 110mm/5.6.

[John T]

The interesting thing about the color of your image was that my B+W was slightly magenta. That, plus the color shift of the film was really difficult to clean up. (Scanning did make that a non-issue later on.)

Your color shift IS pretty strong. Unfortunately, I'm viewing it on a laptop so I don't trust the color on my end. Have you tried to use the cyan filters as viewing filters? It may get you in the ballpark.

[Darin Boville]

I get a shift with dark B+W ND filters using digital--no biggie to correct digitally, of course. But the shift seems is real...

--Darin

[Jim Michael]

Why don't you take a reading in PS from a neutral area (like a cloud) of an image you like and one you don't like, and see what the difference looks like? If the cast is f(t), then determine the slope of the line and calculate the approximate shift per unit of time. That should get you in the ballpark.

[mandoman7]

Going beyond 1 sec. introduces a big variable, if you're trying to nail down the color. Film companies make no promises for color accuracy of daylight films at such long exposures. For testing purposes, I would shoot 2 sheets at close to the same speed before making any assumptions about the effect of added filters.
Having said that, in that past when I used to do a lot of copy work for artists, I came to the conclusion that nearly all filters seem to have a bit of coloring, if you looked closely. Frustrating, but it rewards the worker who has tested his equipment and knows their characteristics, which is as it should be.

[Brian Ellis]

If you have to put a second filter on top of the ND filter to fix this problem (assuming there is another filter that will fix it and assuming it is in fact your ND filter that's causing the problem) I'd get rid of the ND filter. Sticking two filters on a lens generally degrades the image more than I like (just a personal opinion, I know many people do it).

[Eric James]

I brought the filtered picture up in PS and applied a cyan "photo filter"; if the PS filter is an accurate representation of the cyan filter you hope to use, I suspect that you will not be happy (read: horrified) with the result. Do you know someone with a dSLR who would investigate the color cast vs color shift question? FWIW, I've used Astia-F for 20-second exposures and have never seen this sort of color perturbation.

[Heroique]

Thanks for these suggestions to get “in the ballpark.”

Looks like I have some old-fashioned “try & try again” experimentation ahead of me. To start, I think I’ll find some cyan-ish filters, like John says, and hold them behind the transparency — though Eric’s cyan filtration in PS suggests a different color may be better! I may even try Jim’s “f(t)” examination in PS for additional ideas...

Then it’s back to the field – I mean the beach – to get some more results.

If I didn’t include the yucky sky, the golden-brownish color cast wouldn’t bother me so much. Ideally, I hope I can find just one additional filter to “correct” this color cast & “create” the warm color I do like … at the same time. And, maybe a separate second filter to bring the image closer to Astia-100F’s normal palette.