Is there a *general* nd filter, or one that is most commonly used? I'm wanting to capture movement in daylight. Primarily landscape shots with clouds. I'm not sure which filter to buy. I'm sure there is a time and place for each, but I can only afford one to start. Then add more over the next couple of weeks. So, do I go for ND2, ND4, or ND8. I know there's others, but those seem to be the most common of the group. I've also seen them listed as 0.3, 0.6, etc. I can figure out the conversion part if someone can point me in the right direction.