Thank you on that shot - that actually IS a long exposure shot, however it was taken at night, so didn't require any filters. I do a lot of night shooting, and mounting the camera on a tripod and using long exposure times is usually the best way to get nice crisp details.
I have a 9-stop ND filter which I do occasionally use when I want to do daytime long exposures - not just for making smooth water, but also for getting cool motion blur effects with people, and if you make the exposure long enough, you can even 'erase' people from a shot if they stay in motion and walk all the way through the frame before the exposure is over. Here are a few daytime long exposures I took using the 9-stop ND filter:
This is a very helpful post, especially for a novice like me. I have some more questions if you don't mind. Since I have a kit lens (18-55mm), I suppose I won't be able to use the filter if I decide to upgrade to a better one in the future?
Actually, you can - you buy the filter with the filter threads that match the lens typically, so if your kit lens has a 49mm thread, that same filter can attach to any lens with a 49mm thread. But a better method I usually recommend is to buy a filter in a much larger thread diameter - such as a 66mm or 72mm. It will be a little more expensive, however because of the much larger threading, that filter could be used on many more lenses - any lens with a filter size all the way up to that number. All you do is buy some cheap 'step up rings' - which allow you to put a larger filter on a smaller lens. If you buy a filter in a large size like 66mm, you could buy the cheap step up ring and use it now on your kit lens...and then if you buy another lens later with a larger thread diameter, the filter can still be used with it - you just get another step ring to that size. Step up rings are cheap - $5 - 10 or so, so getting one expensive filter and a bunch of cheap step rings is much cheaper than having to buy multiple expensive filters.
Would a 3-stop ND filter help me do the job? I'd like to shoot during daylight and get the silky smooth feel or would I have to should at or after sunset?
Well a 3 stop ND filter would certainly help in lower light - say, dusk or early dawn, or in a deep forest, etc. In bright daylight though, it probably wouldn't be enough much of the time. 'Stops' are measured in the shutter speeds by doubles or halves...so to know how much shutter speed you can gain or lose with a '3 stop filter', go outside and set your aperture as small as you can, then snap a shot. What shutter speed did your camera choose? If it was 1/200 for example, then you can cut the number in half for each 'stop' the ND gives you - 1 stop would be 1/100, 2 stops would be 1/50, and 3 stops would be 1/25. 1/25th of a second wouldn't be very long, so it probably still wouldn't stop flowing water or give you much motion blur. You can always test it by taking test shots in the conditions you expect to be shooting in, and seeing how many stops you need to lose to get the shutter speed into the seconds.
I guess no ND is needed for night shots?
Right - in fact often at night the light is so low that you need to even open up the aperture, set the camera on a tripod, and expose for many seconds, even 20, 30 seconds, to get a well exposed shot.
One final question, can I mount two 3-stop filters on top of each other and still get good quality pictures?
Yes - the quality may be slightly reduced, but so small that likely you wouldn't notice in most cases. As long as the filters are of a decent quality to begin with - some el-cheapo third-world filters you buy off eBay for $3 may not be very good and could be more damaging to your quality.
Sony DSLR-A68 / Sony 18-250mm / Minolta 50mm F1.7 / Tamron 150-600mm / Minolta 300mm F4 APO
Sony A6300 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / FE70-200mm F4 G OSS / FE70-300mm F4.5-5.6 G OSS / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses