Must have ND filters??
Nexu1 wrote: What are you trying to achieve with your ND filter? Are you trying to shoot a portrait outdoors with a f1.8 lens and just need to take a stop or two of light away because you're beyond your shutter speed limit?
Well, yes, that is one use: getting your aperture wide open for bokeh.
Or are you trying to take a bright raging river and slow it down to a silky, smoky stream? I think having something in the 9-10 stop range is fun if you want to get some of those 20-30 second shots around a lake where the water turns into almost a fog.
Indeed, another use for an ND filter.
In either cases, they are valid reasons for wanting an ND filter.
Otherwise a 3-6 stop does a pretty good job of taking a 1/500 or 1/1000 second shutter speed and a choppy harsh looking waterfall and smoothing it out quite a bit.
I personally don't see much use for a 1-2 stop ND filter unless it's that first example I gave above, trying to shoot outdoors at f1.8 and you camera is limited to a 1/4000 shutter.
I think that longer exposure times with good sharpness & great color saturation, or smoothing movement are very legitimate reasons for wanting one, just as is wanting to shoot wide open and getting background bokeh.
If you must buy an ND filter, get one with very thin glass. if the glass is 1mm thick, at the center, the light will be going through 1mm of smoked glass. at the edges, at a slant, it will be going through about three times as much. so of course you'll have vignetting with cheap chinese thick filters. you need one that crams that smoked layer into as thin a wafer of glass as possible. the problem is accentuated with filters that are 3mm thick. the density increase is linear, but because we're dealing with a visual plane, that difference is squared.
I don't know enough about the graduated ones to tell if they are very thick or not - but at least twice the thickness in all cases, as there are two glass panes.
Getting fixed number ND filters may not be all that bad: at equal quality, you have half the thickness. and it is more intuitive to work with just a couple of ND numbers, like 1/4 (nd4) and 1/8 (nd8) of the light, as it will be less to familiarize oneself with and get a better feel (develop instinct) for how this affects exposure times.
but whatever you do, do NOT stack them, as this will increase the vignetting effect.
that's just my 5 cents worth.