Variable ND , long exposure problems

[Dave Gurtcheff]

I just tried using a variable ND filter to get blurred waves on the beach. The vignetting was because the filter is thick, and I was using a 24-70 lens at 24mm. But that is not my problem. The shorter exposures, such as 1/13 second, posted here are OK. But the long exposures such as 15 full seconds, also posted here, have broad vertical stripes. can anyone explain what is going on? Are they selectively "polarized"? I used a tripod, mirror lock up, and eyepiece blind. Camera was a D800E with Tamron 24-70 f2.8 VC lens with VC set to "OFF"
Thanks in advance. This technique is new to me; more difficult than I thought it was going to be. BTW to filter is a "cheapie".
Thanks in advance
Dave in NJ

[Dave Gurtcheff]

Here is the shot made at 1/13 second, which does not have the stripes
Dave

[francois]

Dave,
If you are referring to the dark areas (circled in orange in the attached shot), this is "normal". This is due, I believe, to the way your variable ND works. It uses polarisation to darken the scene. If you use wide angle lenses, it becomes very visible. FWIW, I've experienced this around 35mm and at shorter focals.

You're probably using more lens polarisation when shooting at 1/13 s than at 15 seconds. This makes the issue more visible.

[Dave Gurtcheff]

Thanks Francois. I may try again using longer focal length. I Aldo have a non variable ND which I may stack with a polarizer.
Dave

[bobfriedman]

i think unless you shoot in the shade you will get those polarization effects as that is the cancellation mechanism... especially with the sun behind you..

which variable ND are you using?

[stamper]

The Fader filters don't perform well at wide angles. Two or three stops of filtration is the best you can expect. At longer focal lengths you will get up to nine stops. Then again I suspect that few photographers do long exposure work at long focal lengths but restrict themselves to the widest lens they possess.

[PhotoEcosse]

...and remember to close the viewfinder shutter when making long exposures with a strong ND filter.

[EricV]

The variable ND filter uses two crossed polarizing filters to attenuate the light.  If the polarization of each filter was perfect, then you could block all light by orienting the polarization axes perpendicular to each other.  In your case, the polarization purity is not good enough to support the attenuation you are trying to achieve. 

If each polarizer achieved 99.9% perfect polarization, you could get a total attenuation of 9 stops.  But if the polarizers have sections which are only 99.8% perfect, those sections will transmit twice as much light, creating a blotchy image.  If you rotate the polarizers to produce less attenuation, then the transmission difference across the filter becomes relatively smaller.