What is the range in stops for this ND filter?
Here is handy reference for ND density.
Vivitar is variable so you get a range. You can see it work when you rotate through your in camera meter. I'm thinking officially it is .2 density to 1.0. Which is 2/3 to 3 1/3 stops which is plenty for most settings. Their marketing dept probably came up with the ND2 To ND1000. The biggest concern is it really neutral. So far good. However, guys say the singh-ray is the best and really expensive. Hope this helps.