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

[Jim Michael]

Although it's been more than a few years since doing a lot of color corrections, the difference in the 2 images looked more blue/yellow to me than cyan.

[mrladewig]

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.

I think the filter is adding magenta. All the notes (from DSLR users) I've seen of problems with the B+W 10 stop have indicated a magenta shift. If so, a green filter may be more effective than a cyan. But in your example it almost appears to be adding a tobacco cast with Astia and I'm not sure how you would want to correct that.

One thing about Astia 100F that may be coming into play is the 4th layer (MCCL) technology which is supposed to help correct white balance under various lighting. Perhaps shooting something else like Provia without the 4th layer tech might help provide a better prognosis.

[Sevo]

If the filter had a constant and uniform wide band cast, they'd simply add green in the production - it must be some more complex flaw.

[mrladewig]

If the filter had a constant and uniform wide band cast, they'd simply add green in the production - it must be some more complex flaw.

That reminded me to go look at the posts I've seen on the B+W filters. Seems these filters are not always cutting IR and the magenta cast in digital cameras has to do with their IR sensitivity. Normally the cut filters in front of the sensor reduces the IR component relative to the visible light sufficiently that the IR isn't noticeable, but when 10 stops visible is cut without a proportionate cut in IR, some shift is developing.

Example discussion

So, it looks like a magenta cast is a dead end for a film user.

I'd still look at the 4th layer function of Astia though. Perhaps it is interacting in an unusual manner with the ND filter. I recommended Provia as a test because it is one of the best films for daylight long exposures. It should have no reciprocity issues at the time you indicated and doesn't employ the 4th layer. From there you should be able to accurately determine what color cast is emerging and set about correcting it.

[Heroique]

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?

[Mike Anderson]

If you have a digital camera, why don't you repeat the experiment with that?

...Mike

[Heroique]

Well, that would be convenient indeed, but I think I'm one of the few in this forum who, irony of ironies, has no digital camera, or even access to one! But I like your idea. If someone else w/ a digital camera can make a similar examination, their results would be quite interesting to hear about, and clarify even more what type of "correction" might be good to experiment with in the field.

[Bob Salomon]

We have never had a complaint with the heavy ND filters from Heliopan regarding a color shift from any user or any dealer.

[brian mcweeney]

Heroique,
For what it's worth, I took both of your posted images into Photoshop to try and make a "simple" color correction. I found that it wasn't just a matter of more/less magenta or blue/cyan. I ended up going into channel mixer and adjusting color amounts in each channel. I got close but never matched your original image. I don't know if this helps any, but it seemed the color shift was across many areas and not a simple single color filtration solution.

[Heroique]

Thanks Brian! That gives me a little more information to go on, esp. in view of my limited PS skills. Now it's time to start some field tries w/ this or that filter, and see what happens. Even if the B+W filter continues to forbid Astia-100F's more normal palette w/ longer exposures, just removing that strange golden-brown tint from the sky would be satisfactory.

(BTW, if you'd like to show your resulting image, that would be fine by me.)