Any idea what's going on here?

This image was taken with a Sony a7R2 on a tripod with a cable release.  The lens is a Canon 24-70 M2 on a metabones adapter.  In front of the lens is a Singh Ray Vari ND filter.  I took a fair amount of images with this set up and maybe 5% end up with corners like this.  Most likely operator error, but what is the error?

It's the vari-ND - it is out of it's rotation range or you are applying too much ND on a wide lens. The filter will only produce normal images in the range that is marked on the filter - rotate it outside that and you get this. With a wider lens (say wider than 35mm) you will likely get this sort of thing even within the normal range of the filter when you go beyond 5 or 6 stops of ND. It's a cross polarization effect from the way the filter is made. it's the reason I stopped using vary ND filters and instead just have a 6 stop and 10 stop for each filter size that I need. Other than that, the filter is too thick for the lens at the focal length setting you are using and you are getting vignetting. The cross polarization effect has nnothing to do with the lens or the camera - it's the way these filters work unfortunately. On a longer lens, you would not see this.

Check to see if it's only happening with very small aperture settings.

Thanks EJ! You are spot on again. I was pretty careful to keep the filter within the normal range. I reviewed the shots and everything at 35mm or higher was fine. The shots from 35 to 24mm had this effect to some degree. I have a 6 stop B&W ND filter but I find it difficult to use because the focus is tough to visualize.

In my earlier post I was addressing the question about vignetting in the corners. I believe vignetting will cut-off more sharply with small apertures than with larger ones, and hence be more noticeable.

Dave Kocher wrote:In my earlier post I was addressing the question about vignetting in the corners.  I believe vignetting will cut-off more sharply with small apertures than with larger ones, and hence be more noticeable.

Thanks Dave!  My apertures didn't exceed 7.1.

Charlie Woodrich wrote:Thanks EJ!  You are spot on again.  I was pretty careful to keep the filter within the normal range.  I reviewed the shots and everything at 35mm or higher was fine.  The shots from 35 to 24mm had this effect to some degree. I have a 6 stop B&W ND filter but I find it difficult to use because the focus is tough to visualize.

I know, I often use a 6 stop plus polarizer or a 10 stop plus polarizer.  I achieve focus first and then add the filter.  You absolutely must take AF off of the shutter button when you shoot with these things.  Or you could achieve AF, switch to MF and then put on the filter but it's too easy to forget to switch off AF and then as soon as you hit the shutter button your focus is lost so definitely go to back button focus when shooting with these things.  Another option that many use is to use a filter holder with slide in filters so it's easy just to slide it out, achieve AF and slide it back.

Maybe I'll try a slide in.  I've tried focusing then adding the filter, but when screwing on the filter I'm never sure if I've inadvertently adjusted the focus or not.  Canon put a zoom lock on the new 24-70.  I wish you could change it to focus lock.