Colour cast from stacking ND+GND?

[Gengh]

I tried stacking a Tianya GND over the B+W ND110, and I seem to be getting a purplish cast where the filters overlap.

Examples here:

The WB is more or less adjusted for the lower half of the pics.

I have another pic taken with just the Tianya, and the purplish tinge doesn't seem to be there:

Pictures with just the ND110 also turned out fine after WB adjustment.

Has anyone come across this problem before and/or know which filter is responsible? I hope that it's not some issue with the ND110 that makes it unsuitable for stacking with other filters, as I think a GND can be useful for many long exposure shots....

 

[wildcat]

Interesting. Normally we get more brownish colour casts from B+W's ND110, rather than the purplish Hoya ND400. But I do get these purplish colour casts from ND110 also; probably a combination of cloud conditions and weather.

Don't use GND, so I haven't tried stacking one on.

 

[hanzohattori]

Probably because cheap GND filter. I tried Cokin GND before and get grey color cast. Now a happy Lee user, although I don't use it that much

 

[night86mare]

hi,

nd110 will warm up the picture usually. strong yellow/red cast - rather unavoidable due to ir contamination.

stacking will sometimes give cast, which is why i will almost always convert my daytime pictures that involve stacking the two types of filters... into bnw.

sunset it will not be so obvious, because of the huge variety of hues/tones...

if you must keep the colour, you can always use layers to correct the cast selectively. it's not that hard, but i think i would just prefer bnw unless there is a compelling reason, e.g. very, very nice blue sky that works well with the subject in question.

 

[Gengh]

Thanks wildcat, hanzohattori and nightmare.

I am not convinced that it's due just to the Tianya GND, since on the previous occasions that I have used just the GND alone, there's no colour cast problem. If the GND alone produces this problem, then I'll would have encountered this earlier, and getting the colours to be neutral both top and bottom of the pics would already be difficult.

I just thought that if the GND is truely neutral, then stacking it with the ND110 should also only result in a uniform cast due to the ND110, and that would be easy to correct. But I guess that's not the case, and I'll just have to work around it as nightmare advised. Pity, 'cos I was thinking of using it to brighten up reflections in situations similar to this (taken with just the ND110, reflection was brightened in PS, but I'm hoping to get better results in future with GND+ND110):

The actual scene already has a fairly large dynamic range, and I can't be 100% sure there wouldn't be any clipping in either the bright actual scene and dim reflection.

 

[night86mare]

taken alone perhaps it is not so visible, but when you have a combined whammy....

 

[ExplorerZ]

try 2x gnd, the color cast will be back, but most like reddish purple with my case of tianya

 

[wildcat]

Thanks wildcat, hanzohattori and nightmare.

I am not convinced that it's due just to the Tianya GND, since on the previous occasions that I have used just the GND alone, there's no colour cast problem. If the GND alone produces this problem, then I'll would have encountered this earlier, and getting the colours to be neutral both top and bottom of the pics would already be difficult.

I just thought that if the GND is truely neutral, then stacking it with the ND110 should also only result in a uniform cast due to the ND110, and that would be easy to correct. But I guess that's not the case, and I'll just have to work around it as nightmare advised. Pity, 'cos I was thinking of using it to brighten up reflections in situations similar to this (taken with just the ND110, reflection was brightened in PS, but I'm hoping to get better results in future with GND+ND110):

The actual scene already has a fairly large dynamic range, and I can't be 100% sure there wouldn't be any clipping in either the bright actual scene and dim reflection.

The colour casts from ND filters are generally attributed to long exposure to light which includes IR and light spectrum around the IR range, which your camera sensor is sensitive to and generally blocked by the AA filter but long exposures allow a lot more of such light in. So the Neutral isn't really exactly neutral when long exposures of few minutes are involved.

 

[Gengh]

The colour casts from ND filters are generally attributed to long exposure to light which includes IR and light spectrum around the IR range, which your camera sensor is sensitive to and generally blocked by the AA filter but long exposures allow a lot more of such light in. So the Neutral isn't really exactly neutral when long exposures of few minutes are involved.

I was wondering about that. So it's almost like reciprocity failure for film, except that in this digital case it doesn't affect the overall exposure, just the IR range. So this problem does not show up in short exposures with just the GND.

And that probably means that the GND blocks more IR than the ND110, therefore the sky area has less of a reddish tinge than the bottom half of the pic.