Most ND filters are labelled ND2, ND4, ND8 and so on. It means that your exposure needs to be 2, 4 or 8 times longer. The common ones for long exposure are ND400 and ND1000 from Hoya and B+W respectively.
Suppose your shutter speed is 1/200s. With ND2 it will be 1/200s * 2 = 1/100s. With ND1000 it would be 1/200s * 1000 = 2s. So you can work out what you need. Only you will know what aperture you prefer to shoot at and what kinda of shutter speed you normally work with. But usually even with a ND1000 you're not gonna get 30s in bright daylight unless you are using a tiny aperture.
cool! thks for educating me on the formula.. now i knw how to choose the nd filter to get..
no need expensive filters ma. any rectangular ND filter placed smack nxt to the lens will eliminate the vignetting.
Yah ryan, agreed to a certain extent but i m looking form TS standpoint that he might embrace the idea of handholding the filter for a prolonged period of time..Coming from past experience, it really take a toll on ur already achy hands from the setup u already on, much worse if we to do long exposure..
Originally Posted by giantcanopy
of course for very long exposures u cannot be handholding it always, and i tried using blue sticky tags to place the filters and lens in close proximity before
( so much so for taking care of filters and lenses )
I did ponder doing that before i got my foundation kit (holder), but the thought of the residue left behind from the blue tags on my Lee filters kinda puts me off lar..