I did some tests this morning using custom white balance (CWB) with my various K&F concept ND filters. These were all conducted at the same aperture and ISO and under identical lighting conditions using an 18% grey card to set white balance without any filters fitted and each time I fitted a different filter. I was particularly interested to see if there was a shift of colour cast at each of the variable ND filter settings.
First a CWB shot taken without any filter for reference.
Now with the variable ND 8-128 filter @ ND8, ND16, ND32, ND64 and ND128. Custom white balance taken at ND8 setting and applied throughout.
Now the ND2-32 variable + CPL filter set at ND2, ND4, ND8, ND 16 and ND32 with no changes made to the CPL. Custom white balance taken at ND2 setting and applied throughout.
Lastly the ND1000 filter which is underexposed. I manually focussed before fitting the filter as it was too dark for the autofocus to lock.
With 1 stop exposure correction
With 1.5 stops exposure correction
I’m quite happy with these tests, the variable filters seem to be pretty consistent in terms of any colour casts across their setting range. There are some fairly minor changes to exposure/saturation but nothing that I can’t live with and one can set a custom white balance for each filter. I’ll happily use auto white balance all the time with the exception of when I have an ND filter fitted. Another thing I was looking for was if there was any noticeable decrease in sharpness and, if it is there, I think that it is minimal to say the very least, I can’t see it.
I should just add that this isn’t in any way a criticism of their filters, I’m happy with them and, in my opinion, they’re good quality filters at a fair price and there are far more expensive makes out there that also exhibit a colour cast. The important thing is finding a reliable method to correct for it, in the field when there’s not anything white in the frame that can be used to sample white balance in post processing..