Try one of these before buying something expensive.
There is also the cheap Fotga variable ND filter. Seen good and bad reviews. Tested it the other day and it didn't have terrible affects to the image, it just went slightly more yellow/brown, like with sunglasses. I did notice a little lost detail, but had to pixel peep for it. Still want to test it more but I think it will replace my ND8 filters which I previously assumed would be better (if on a budget), which are very limiting.
Perhaps it really depends on what one is shooting? I usually shoot in Asia on the street or tropical landscapes, so the Fotga's color is acceptable as it is like adding a touch of tempertaure to the image, which I often do anyway for that golden hour look. There are Youtube tests with really expensive ones which drastically change the color in various ways like red or blue. So paying more in this case is not necessarily going to mean a better filter.
Therefore I think Clint's suggestion is a good idea first (or the Fotga, which may even be the same one rebranded). Because if they do work for you then you can get one for each lens size. Rather than an expensive highly rated one like the SLR Magic, which you would just get in a large one size fits all, then deal with the inconvenience and bulk of step up rings.
The other night I tried videoing a sunset with the Fotga and turned the filter to brighter as it got darker. Wasn't perfect as it was my first try, but if you have a steady hand it might be an easy skill to pick up. Plus it saves one having to look at buying cine lenses if you do video.
