For a given lens/camera, will a ND filter always need same WB correction in all conditions?

Question

I've taken a few photos with a new B+W ND 6-stop filter and found that it leaves them quite warm. I'm able to correct this in ACR using temperature/tint sliders. For the few photos I've taken so far, the same temp/tint settings seem to work for each one.

I've heard that color cast from a given ND filter can vary between cameras and lenses. But presuming I continue to use this filter with the same camera and lens, is it safe to assume my WB correction will be the same every time, for any photo? Or will it be dependent on other conditions like lighting and exposure/aperture?

Note that I ask, because if the color cast varies based on some conditions, then I will probably take no-filter companion shots for any filter shots that are important, so I'll have a reference point for matching WB later. Though, it would be nice to not have to always do this.

EDIT:
To clarify my question, here is a concrete example. Please excuse the horrible photo.

Here is a RAW file shot with no filter. I used auto-WB, and ACR says that "as shot" the temp is 5400 and the tint is +13.

And here is another RAW file, shot with my 6-stop ND filter. Same tripod, same composition, same lighting (different exposure settings though). Also used auto-WB. Resulting in temp of 5700 and tint of +43. It is noticeable warm. If I adjust temp to 4300 and tint to +10, I think it is a pretty close match to the first one.

So, I needed to drop the temp by 1400, and the tint by 33, to correct it. My question: can I make these exact same drops to every shot I ever take with this filter/lens combo, to make it match what would have been the result if I hadn't used the filter? Or will it vary?

Answer 1

For a given lens/camera, all images taken with or without any filters will need WB correction to account for the varying shooting conditions. Even when no filters are involved, one can't use the same WB settings for an image taken in 2700K light that one would use for the same scene taken under 6400K light and expect the colors of the objects in the scene to look remotely similar.

And here is another RAW file, shot with my 6-stop ND filter. Same tripod, same composition, same lighting (different exposure settings though). Also used auto-WB. Resulting in temp of 5700 and tint of +43. It is noticeable warm. If I adjust temp to 4300 and tint to +10, I think it is a pretty close match to the first one.

Your issue may have absolutely nothing to do with whatever color cast is being introduced by the ND filter.

When you use Auto WB you're at the mercy of the camera's programming or ACR's analytical engine. It may well be that your camera is designed to render long exposures of very dim scenes warmer than shorter exposures of brighter scenes.

Looking at the two example images¹ you linked, it appears the metallic roof on the lean-to next to the barn as well as the white building near the left edge are fully saturated in the filtered shot. The filtered shot is also much brighter overall than the unfiltered one. Any time you have a completely saturated area in the image, the way the camera or a raw conversion application does Auto WB can be thrown off. Exposing slightly dimmer to prevent full saturation anywhere in the frame can significantly shift the color temperature and WB correction the camera or raw conversion application uses to render the preview image of the raw file. So can selecting things such as "ambiance WB" or "highlight WB" in cameras or raw conversion applications that offer such settings.

The only way to check to see what is the effect of the filter is to set color temperature and white balance correction manually to the same settings for both the unfiltered and filtered photos and then compare them.

_{¹ Of course it is difficult to tell much from the example images when we can't even tell what image format they are in as delivered to us by google drive, much less see any EXIF info. Are these JPEGs you exported before uploading? PNGs? TIFFs? Or did you upload actual ARW files and google is converting them with whatever unknown default settings it uses to interpret what we are seeing?}

Answer 2

The filters should be replaced. I bought some B+W ones too at some point and found them unusable due to their horrible color-cast. Got rid of them and bought many Hoya filters that are perfectly neutral. I recommend you safe yourself the headache and do the same or anything good brand like IRIX, Zeiss or Lee (Although I did not try them myself but they are highly regarded, so are the B+W but I have no idea why!).

Color cast simply occurs because a filter does not uniformly affects light of all wave-lengths. As you say, it can be compensated in software but it does not depend on anything but the poor quality of the filter itself. You may see a slight difference with certain camera and lens just because the lens themselves sometimes affect colors and the interaction between all these will cause different shifts.

In summary, the cast you see from a poor quality filter is mostly due to the filter but how that cast interact with any optical imperfections in the lens or filter-stack in from of the sensor means that you can get slightly different color shifts with the same filter when used on different lenses and camera combinations.