Stacked NDs = Color cast?

Hi! Been out this morning trying my new set of GNDs from Singh-Ray. Overall they were really nice but when stacked they produced a magenta cast. I handheld them so the filters probably were not parallell to eachother which I think might be the problem here. Anyone here had the same problem when stacking filters?

Used one at a time they were neutral...

Yes, but in our case with LEE filters...
Not extremely strong but visible, when using Velvia film, which will enhance the otherwise light magenta cast. It can be a problem when shooting against a white, cloudy sky.
No solution, that I could offer, though...
Georg

Magnus,

I have had similar experiences with a pair of Cokin X-pro GND's - perfectly neutral on their own but a magenta cast when stacking under certain circumstances. It tends to happen when the subject is lit directly by the sun as with alpenglow but I have not observed it during overcast or very hazy conditions. I always use the holder so I cannot comment on handholding.

I would be interested to know if this happens again for you, if you are handholding or using a holder.

After reading Georg's comment, I'm beginning to think there may be no perfect filters when it comes to stacking and it may come down to whatever combo does it the least.

Brad

Thanks for the responses. Well, it seems like it could be an optical problem here. I bought the GNDs to use also as a ND filter to get longer exposures. I will try and keep them tight together next time to see if there's any difference. I was shooting with a B/W preview so I didn't noticed the magenta cast until I got home.

Used as an ND, at least the cast is more easily removable in software (should be the same everywhere). To remove it when you've used them as GNDs, I suppose you'd have to do the correction on a layer with a gradient mask where the real filter line was. Doesn't seem too hard, but not what you want out of expensive filters...

I don't get a color cast stacking Singh Ray split NDs, though I'm using a holder. Makes me guess your problem was due to handholding.

I do get color casts when mixing brands of non-split NDs. I also get color casts when stacking two circ polas.

I did get color casts with stacked ND filters in the past. You can also get a yellowish color cast when using the Singh-Ray VariND at its extreme end of densities. My solution was to shoot the same scene with a colorchecker card in the scene so I could select the medium gray square in either Photoshop or Lightroom to set the white balance - color cast is all gone!

Dave

I have experienced colour cast when stacking Lee GND grads (especially the soft ones).

Magnus for obtaining longer exposures I use B+W ND with UV/IR as well making it 64x or Lee ND (digital) – I struggle more with the graduated ones personally when shoot 1 second or more.

I have experienced the magenta color cast with both cokin and Singh Ray. It's less dominent in the Singh Ray. You're better off with carrying different strengths because the more you stack the thicker the layer of plastic over your nice peice of glass will be.
It's very evident when the sky and horizon are slightly underexposed.

I should mention that I also had a polarizer in the setup. I builded up seven stops of ND to gain a 5 second exposure in daylight. I think these will work fine using as an ND even with the color cast but I hope I can figure out what's causing it so that I'm prepared. When using extreme ND I don't have a problem with losing a tad sharpness as I in most cases do want to create movement in the scene. I think it's quite nice to have the possibility to go from 2 stops to 5 stops with a set of GNDs. The dual purpose of them are nice.

Magnus,

Thanks for mentioning the polarizer. I was thinking of asking if you had used any other filters just to compare with my situation where I am typically using a screw-in circ. pol. + typically one GND and occasionally, two.

I've done some more testing here and the color cast is obvious with or without the polarizer. If you want to test this for yourself grab two GNDs and start with looking through one of them. Then slide in the other in front of the first one and you will experience a magenta cast.

I'm going to write to Singh-Ray and ask about this and if there is any solution it.

I've have experienced the magenta color cast as well with the Singh Ray filters. I was holding them directly together.