ND filter and White balance

[oxfordy]

I used a NDx4 filter on my Nikon D80 for the first time today (needed a long exposure in bright light). Why does a ND filter affect the white balance? This was what came out of the camera ...

Fortunately, I was shooting in RAW and was able to correct the WB in post processing, though I had to bump the temperature right up (about 8700).

My question is, why does a ND filter affect the white balance like this and is there anything I can do about it?

oxfordy - puzzled.

[Palladius]

I'm puzzled too! I thought the 'neutral' in neutral density meant it didn't affect the colour of the light passing through. Does the filter have a blueish tint when you look through it?

[xxpinballxx]

What brand of filter are you using?

[MikeM]

ND filters are supposed to be plain old gray and not affect the color at all. I assume you had you WB set to auto? Have you duplicated the problem? Perhaps that scene just really threw the Auto WB for a loop. Or, perhaps your filter does have some color tint to it (though it shouldn't).

[oxfordy]

OK, I'm glad it's not just me who is confused :-) The filter is marked "BOWER DIGITAL ND4 62mm". As far as I can tell, it doesn't seem to have a blueish tinge and seems to be plain ol' dark grey when you look through it. I had a set of three bracketed exposures of the scene which all came out the same. The sun was so bright that I didn't notice on the review screen and I didn't use the filter again that day. Perhaps I will try and see whether it can be reproduced.

[oxfordy]

Further experiments have ensued ... my ND filter does indeed introduce a blue/green colour cast though nothing as gross as the example that sparked this post, MikeM may be right - the scene probably threw the WB out (perhaps coupled with the long exposure?).

Nevertheless the blue/green is there (and I don't think it should be) but a minor adjustment in post processing does remove it - unlike the the shot above that required major surgery. So, lessons learned ...
- Maybe my Bower filer is not all its cracked up to be (if indeed it isn't a cheap rip off of a Bower)
- When using the ND maybe I should switch off the Auto WB

Any other thoughts?
oxfordy

[candleman]

what was your original WB setting? Auto?

i seldom use autoWB. try the Kelvin scale to get it the way you like it. OR, when you attach ND filters just create a custom WB off some white paper. or get an expodisk

[kencaleno]

Originally Posted by candleman

what was your original WB setting? Auto?

i seldom use autoWB. try the Kelvin scale to get it the way you like it. OR, when you attach ND filters just create a custom WB off some white paper. or get an expodisk

A cheap alternative to an expensive expodisk,and the only good use I can think of for a uv filter: Cut a circle of coffee filter to fit against the glass/rim of a uv filter and you have your own expodisk-and it works just as well!
Regards, Ken

[navcom]

Originally Posted by kencaleno

A cheap alternative to an expensive expodisk,and the only good use I can think of for a uv filter: Cut a circle of coffee filter to fit against the glass/rim of a uv filter and you have your own expodisk-and it works just as well!
Regards, Ken

Now that's a fantastic idea! Cool advice!

Definately turn off the auto WB. At worst, use a preset for the conditions you are in.

[candleman]

Originally Posted by kencaleno

A cheap alternative to an expensive expodisk,and the only good use I can think of for a uv filter: Cut a circle of coffee filter to fit against the glass/rim of a uv filter and you have your own expodisk-and it works just as well!
Regards, Ken

aaaah, that sounds clever... nice one Ken.