Dark spots when using a 52mm variable neutral density filter [duplicate]

Question

I recently bought a variable ND 52mm ProMaster filter for around $70 but I got some strange results when I tried to take pictures with it.

This is a shot I took at 18mm when the filter was at its darkest.

Here's another shot with more light coming through, but still at 18mm.

I'm not entirely sure if the issue is with my filter being defective or if it's just a crappy one. I really want to take some landscape shots with it and would love it if there is an easy fix or alternate filter that I can use at 18mm.

Answer 1

This happens with all variable ND filters, good and bad. It's due to the partcular angle of the two polarizers when approaching max density. There's nothing that you can do apart from avoiding the use of that setting. Note that it will vary slightly depending on the focal length of the lens in use.