thank you very much. i forgot to ask another question that is, there is such thing as a reverse ND filter. in which situation would you use this one?
another question. is it better to use a single filter instead of a combination of filters? for example is it better to use a 3stops instead of a 1 stop and a 2 stop put together?
To answer your first question (reverse GND), Singh-Ray do offer such filters. You can find these here
(scroll down to "Daryl Benson's Guide to Reverse Neutral Density Graduated Filters").
As for your second question, a single 3-stop is better than using 1-stop & 2-stop together. I have not made tests and I guess that in most cases, you won't be able to tell the difference between the one filter set-up vs the two filters combo (provided your GND are not scratched, dirty etc).