Variable ND filter?

I am looking at purchasing an ND filter - mostly for slowing down water (ocean waves/waterfalls) when I might have too much light and can't sufficiently slow down my SS. Fortunately, my lenses are all 77mm thread, but mostly the filter would be used on my 12-24mm f4.

Is a Variable ND the way to go? (Hoya / Tiffen?)

Will there be vignetting on this lens? Thanks!

Yes there will be vignetting on that lens. At least there is with the polarizing filter. They do sell a thin polarizer but I don't know about a ND filter.

Vari-ND sure beats the heck out of a whole stack of fixed ND which will guarantee vignetting and be a general pain when determining the right amount. I have the Tiffen version and have no vignetting issues to speak of. However, while I have had great results with mid to longer lenses, the results with wide angle (24mm on full frame) have been problematic. The effect is not uniform across the entire frame with the wide angle. Not sure if this is a Tiffen thing or just a vari-ND thing. We know that polarizers can give non-uniform results with wide angle lenses so maybe this is a similar phenomenon. I'm sure there are those more knowledgeable than I on this who will chime in. If you're using your lens on full frame body, I'd be willing to bet you will surely see vignetting and the thin versions won't help under 20mm.

Thanks guys - forgot to mention this is on the D7100 so it's a cropped sensor. I haven't really noticed any issues when using a CPL aside from some banding

I use the Singh Ray Vari-ND filter. It's expensive, but you can dial in the number of stops from 2-8 stops depending on what you need. I've not had any issues with color cast, banding, or other problems with it impacting the images. Because of it's thickness, there is a bit of vignetting when shooting at 16mm. It works very well in mid to low light situations when you want to slow down the shutter. In bright light, it's not as effective as it doesn't really provide the full 8 stops of ND filtering that it claims - not sure why. I have a friend that uses the Lee system with the large glass filters. This generally eliminates the vignetting at almost any focal length & their big stopper ND really works well in any light.

Variable ND filters are essentially two polarizers placed together in opposite directions, you can make your own by taking one polarizer out of the metal frame and reversing it (it still works as a regular polarizer) and then stacking the two polarizers. You'll then be able to cut off as much light as you want.

If you stack two and use extreme wide angle lens, you'll have a greater chance of vignetting.