Hello, was looking for a 2-stop Graduated Neutral Density filter for some beach sunset . But not quite sure what is the 0.3, 0.6 & 0.9. Can anyone help me to gain more knowledge on this. Need to know which one is how many stops & what it means. Also is it neccessary to get the HT one, which is way more expensive. would like to know too the difference between Graduated Neutral Density & ND filters. your answer is much appreciated. Thansk!