I have three Canon Lenses. 28-135 IS USM, 70-300 IS USM and the 18- 55 kit lens. I have polarizing filters and UV filters for all three. I have marveled at the waterfalls and sunsets taken with ND filters. I plan on buying three of these filters, one for each of the lenses probably. My question relates to the ND number of the lens. I know some people that primarily use an ND 8 and others use a 3 or 4. I realize there is no universality here. If you were going to buy a single ND filter for general use, what number would you consider?