Hoya Variable ND Filter - What's Happening?!

[orioncroft]

Hello Everyone,

After many months frequenting this website, I decided to post.

I have been experimenting with slow shutter as of late and have two questions about ND Filters.

QUESTION 1: I am getting a weird effect with my ND Filter. Why? Examples below.

Here's what is used in these example photos: Canon 5D MKII, 24-70 F2.8L, Hoya Variable ND Filter, Remote Controller.

First (Good) Photo Settings: 50mm, ISO160, F18, 0.5 Sec Shutter
Second (Bad) Photo Settings: 50mm, ISO160, F18, 30.0 Sec Shutter

It's obvious that it's something to do with the shutter speed ...but isn't that the point of an ND Filter? I want to take long exposures ...but every time I do, I get this weird blue smudge.

QUESTION 2: How do you expose properly with an ND Filter? The exposure meter does not get effected when I place an ND filter on ...So how do I compensate exposure accurately when I put the filter on?

 

[wickidwombat]

variable ND filters are two polarising filters stacked this cross effect happens at a certain point just around maximum strength on all of them. you should have a min and max mark on the filter this effect will happen around the max point just back it off a bit till it goes away. its not a problem with the filter.

to get exposure correct the in camera meter should still be fine with a variable ND since they are not too dark. just shoot manual and use the meter scalein the vf

 

[Loswr]

It looks like the uneven polarization you get with a CPL and a wide angle lens (a variable ND is just stacked and crossed linear and circular polarizers - that issue is why I done have a variND). The effect gets worse as the filter is darkened. I'm surprised to see it at 50mm, though, unless you've set it beyond the max marking.