Those aren't black eyes in the heading image - they're a pair of variable ND filters; 67mm on the left and 58mm on the right.
They're alike except for a few small things:
a. The 58mm can be turned to successfully exclude light - the 67mm cannot.
b. The 58mm turns smoothly and freely - the 67mm is stiff and difficult to work with.
c. The 58mm has a smooth overall darkening pattern - the 67mm has a central greyish band and two polar patches of bright light.
The small one was purchased to go in front of a Fujinon 18-55 f:2.8-4 zoom lens. It was used at a recent dance show to provide smooth fade-in and fade-out for video recording of portions of the show. When the recording was started, it was slowly turned with one finger to the open position - when the segment finished it was rotated again to darkness. The final cut-off point was dark and precise enough to give a definite black.
As this was so successful I asked a friend if I could borrow his 67 Variable ND to test on my Fujinon 18-135 f: 3.5-5.6 lens. This has a smooth zoom and quiet focus motor - a good video lens. I'll be returning the filter to him next week at our hobby club meeting with thanks - because the loan has taught me a valuable lesson; you either gets what you pays for or you gets what you deserves.
Ron Frank used to tell me this but I frequently ignored his advice. Well, he was right.
The 58mm is a Hoya filter. Made in Japan by a top-line firm. It cost a reasonable amount, but delivered the optical goods straight from the filter case. It stays in the video bag from here on in. Not a landscape filter, as these sorts of devices sometimes fight with the natural polarising effect in the sky - but perfect for stage work.
The 67mm was apparently an eBay purchase at about 1/4 of the cost of the Hoya. The package has no identifiable maker's name, nor does the filter. The wrapper instructions are written in the sort of English that suggests a scam phone call. The black null point is not black, the odd light and dark pattern is always present, and you can hardly turn it in the first place. It is a product that is made to sell, not to use...
Lesson? The video idea works, and I'll eventually get a 67mm Hoya from CE to extend it to the longer zoom lens. Or I may find a NiSi or a B+W or some other reputable brand. I will not try to buy precision optics for $ 25 from a drop shipper on eBay.