Variable ND Filter vs Non-Variable ND Genustech variable neutral density filter or fader lets you easily control the amount of light entering your camera. With a twist of the ring, you can reduce the light from 2 to 8 stops. This can be useful if you want to shoot at slower shutter speeds during the day- like a waterfall, ocean or crowded street or if you are shooting video and want to keep your shutter speed around 1/50 of a second and control your depth of field both lending to a more cinematic look.[himage]02_1-15 sec at f - 2201_25.0 sec at f - 22[/himage]Rollover to see the longer shutter speeds possible with an ND filterThe filter is a glass and aluminum construction, very well made with smooth rotation and the optical quality is excellent. Clear markings on the filter ring indicate the strength of the filter. I have been using the 82mm size for my 24-70 f/2.8 lens. You can stack additional filters with the front threads accepting 86mm filter size - larger to avoid vignette. One downside to the larger size, your lens hoods and lens caps no longer fit. They do offer additional sizes down to 52mm. B&H has the full list of sizes.So this sounds great - one affordable high-quality filter that gives you great versatility in the amount of light you block. There is a drawback and this is true of all variable filters, at certain strengths you can get a very noticeable X pattern in the sky. This makes these filters unusable in some situations.[himage]01_30.0 sec at f - 18I drew the white lines (in case that wasn't clear)[/himage][himage]02_15.0 sec at f - 18I drew the white lines (in case that wasn't clear)[/himage]The alternative is a non-variable ND filter which comes in various strengths. I generally recommend 3 or 6 stop for photographers looking to use in a variety of situations. You can stack ND filters though if you really want the ability to stack you should consider something like the LEE system which has very versatile setups. I have a link below to recommended sizes and strengths and will be talking about the Lee system after my Iceland trip.Recommended 3-stop filters (also labeled ND8 and 0.9)These will allow you to block 3 stops of light i.e 1/250 of a second to 1/30Budget: Tiffen 0.9 ND Filter (link to 77mm, make sure you purchase the correct size for your lenses)Premium: B+W 77mm 0.9 ND 103 FilterRecommended 6-stop filters (also labeled ND64 and 1.8)These will allow you to block 6 stops of light i.e 1/250 of a second to 1/4 of a second.Budget: Hoya 77mm Pro 1 Digital Neutral Density 64x Filter (link to 77mm, make sure you purchase the correct size for your lenses)Premium: B+W 77mm 1.8 ND MRC 106M FilterIf you want seriously long exposures in the middle of the day- the BIG STOPPER 10-stop ND filter is best.B+W 77mm 3.0 ND MRC 110M FilterAnd if you want control without using a variable - the Lee or Hitech filters are the way to go. They offer stacking and provide an easy way adjust graduated ND filters.More about the Lee Filter System at B&H Photo VideoBuy the Genustech Variable ND Filter from B&H Photo VideoScantips has an easy calculator to determine differences in stops. 

Don't be a LAZY photographer

Many photographers want ND filters to get those more dreamy looking waterfalls or smooth water beach photos and if you find yourself at your location and you still can’t get long enough shutter speeds to get the look you desired you are probably being lazy and trying to photograph during the middle of the day when you have massive amount of light and the quality of the light is harsh and flat. Don’t be lazy - get up earlier when the light levels are lower and the quality is softer and more directional. Or just stay out till evening. For many years, I photographed loads of longer exposure waterfall photos without an ND filter.Learn more about circular polarizers - sometimes useful as an ND filter too.