problem with varia ND filter

[Neilmac50]

I am having a consistent problem when using my varia ND filter for landscape photography and I am having trouble trying to correct it in the editing stages. I am attaching an image using it,unedited other than lens correction in Lightroom,interestingly when clicking on Lens correction it exaggerates the issue. I get an inconsistent exposure across the image,in the one attached the sun was behind me and to the left. The equipment and technical details are; Nikon D3s, Nikkor 24-70mm f/2.8 lens, Singh Ray varia ND filter, ISO 100, at 28mm, f/14, and 1.6 seconds. Does anyone have any ideas what is happening here please? Neil

[Black Pearl]

Variable ND filters use a form of polarisation to achieve their density - the issue will all polarisers is their effect on the sky varies depending on the position of the sun to the camera. What you are seeing is common with using a polariser with a wide angle lens.

[John 2]

That's almost certainly what you are seeing here plus some of these variable ND filters do have flawed transmission at the denser end of their adjustment.

[Mike Buckley]

What model is your filter?

[Neilmac50]

Thank you for your helpful replies, Mike it is the Singh-Ray Thin 77mm Vari-ND Filter. Bought the slimline model to prevent vignetting though I do find at full wide angle zoom, 24mm I do get some so I cannot use it then.

[Donald]

Neil

I have the exact same filter.

Firstly, you say that you can't use it at 24mm because of the vignetting. I use mine at down to 12mm (on a Canon crop camera), and that with it on top of a UV filter. All you have to do is compose so that you allow room to crop out the vignette that occurs.

I am concerned to hear that you're getting vignette at 24mm. Are you sure it is mounted correctly, would be the first (and maybe insulting) question.

Second question, are you sure you're using it between the parameters of the minimum and maximum settings. If you twist it outwith those parameters, you get the sort of thing that we see here (I know, I've been there!!).

[Mike Buckley]

Donald,

I'm considering purchasing this model. Is it your experience that using the filter within the minimum and maximum settings for which the filter is designed will produce the undesirable polarized effect with wide angle focal lengths mentioned by a couple people earlier in the thread?

[Donald]

Donald,

I'm considering purchasing this model. Is it your experience that using the filter within the minimum and maximum settings for which the filter is designed will produce the undesirable polarized effect with wide angle focal lengths mentioned by a couple people earlier in the thread?

Mike

I've never had that experience. As I've previously written, I think it is just the greatest tool in the bag.

The other Singh Ray user I know of on here is Colin. I don't think he has the thin version. But so far as I know, he has never experienced this problem.

[Mike Buckley]

Thanks, Donald. One of my goals for the year determined late last year was to learn how to use this filter in time for my upcoming trip to Norway's Lofoten islands. I may pass on that goal for several reasons that have nothing to do with the filter itself.

As for the issue being discussed here, I wonder if the following note from the Singh Ray web site is applicable: "The design of the Vari-ND Filter may introduce irregularities when used with very wide angle lenses, especially on full-frame cameras. Adjust your focal length and reduce the filter density setting until the irregularities disappear." Neil is using it on a full-frame camera. I don't know about Donald.

[Inkanyezi]

All polarisers have this issue when you get to the denser end. The simplest solution might be a combination of a regular ND filter with the vario, using it at a less dense setting. Of course this in a way defeats the purpose of the vario, but there really isn't any polariser that is completely colour neutral, as colours have various wavelengths, and different wavelengths behave differently when passing through the polariser. You can see this rather clearly if you look the wrong way through a circular polariser and turn it. The "circular" side is not neutral, reflections will vary in colour.

The vignetting problem stems from how far back from the filter thread the entrance pupil is and the angle of view, not really a sensor size issue. Wide angles often will vignette when a filter stack or thicker frame filter is used. The solution is simple but more expensive - a step-up ring and a larger filter.

[Neilmac50]

Thanks again for your most informative replies, Donald I do make sure that I do not go beyond the maximum setting on the filter ring and that it is screwed all the way in. I will next time take several identical photos gradually reducing the filter strength to see if this does have the desired effect.