Was at a shop yesterday that sells photographic accessories and while looking over their large collection of 3rd party Cokin/Lee style filters noticed that as knock-offs they did not have a 10 stop ND - only 2, 4, 8. There were also a huge variety of coloured filters.
Anyway noticed a ND 16.
A 10 stopper is already pretty intense and people seem to get good results from it.
Under what circumstances would one want to use a 16?
About the rating - does the ND number equate to the actual number of stops or is there some other formula?