ND Filter. 2? 4? 8?

[james2]

Need some help on choosing which ND filter to get. I see that there are 2, 4 and 8. How do we determine which to buy or use?

 

[jnet6]

1st qns. why do u need a ND filter...?(and using it for landscape or?)

for mi i will need a nd4, 2 stops. of setting that can play around.