At some point or another, most photographers have wanted the look of a long exposure, such as moving clouds, blurred motion, creamy water, but are limited by the reality of exposure. If we take a typical sunny day, we know that our exposure at f/16 and ISO 100 will be about 1/125 of a second. No smooth blur there. The solution is a Neutral Density filter. And, if you want a big drop in light transmission, the go to answer is a Lee Big Stopper.
Big Stopper filters come in three ratings and multiple sizes. They are available at 6 stops, 10 stops and 15 stops. The one I purchased, along with it's frame and mount is the 10 stop variant. 15 stops may give me five more stops of slowdown, but it also means exposures that are much longer, with the increased risk of shake, movement, trucks going by or random asteroid impact. For my use cases, and your mileage may vary, 10 stops is a good working filter and as these critters are not inexpensive, I only bought the one. Folks really oriented to landscapes may choose the 15 stop and those looking for simply shallow depth of field for portraits in brighter light might choose the 6 stop.
Using a dense Neutral Density filter is a planned action. First, understand up front that you are going to want to focus before putting the filter on and then lock the focus for the shot. That means turning off autofocus, and using a piece of gaffer tape to prevent the focus ring from being moved while you are attaching the filter. The Lee frame mounts easily but there is always a chance to move the focus ring so a small piece of tape will be useful.
The other challenge is that you really cannot trust your light meter, because a big cut may push the light hitting the sensor below the sensor's sensitivity floor. My guidance is to set your exposure without the filter and then put the camera in manual mode and fix those exposure settings. This is very easy and takes little time.
The next step involved calculation (Horrors!!) to determine the exposure with the Neutral Density filter in place. Speaking to photographers, this appears to miss the left turn at Albuquerque all too often. Lee has stepped up with an app for your iOS or Android device called Lee Stopper. If you don't have iOS or Android and all these little apps sound helpful, well maybe its time to move on from Blackberry or Windows phone but that's entirely your business.
Lee Stopper has three modes. Little Stopper is for the 6 stop filter, Big Stopper is for the 10 stop filter and Super Stopper is for the 15 stop filter.
In the first screen capture we see how the app works, you set the pre-filter shutter speed on the left side and the app tells you the shutter speed to use on the right side. Why shutter speed? Well the idea is not to change the aperture because that would impact depth of field, and not to change the ISO because that would impact colour and dynamic range, and we really want to be able to slow down the shutter. You could manually do the math for the ten stop cut, but this is really much faster.
Another use is to be able to pull the shutter speed down in order to use a wide lens opening for very shallow depth of field. In this example, I want to be able to shoot at f/1.2 on a very bright day and also want to drop my shutter speed so I can use some fill flash without having to go to high speed sync. This is the Little Stopper. This works well especially when you set the flash to TTL exposure.
This last example is using the Super Stopper on a bright sunny day to pull a really long exposure to make a cascading stream really smooth. If there is no wind, the foliage will look nice, but perhaps a bit soft, but that water is going to be very creamy.
The last consideration is that the sensor's ability to capture photons also has a capture floor. It's not precisely like reciprocity failure in film, but below EV-2 most sensor acuity starts to fall off and you may find that you need a longer exposure than the chart might indicate in some situations. Just make a manual adjustment and you'll know very quickly because there is the JPEG image on your LCD screen to give you a hint.
Long exposures with Neutral Density filters are not hard, and the good folks at Lee make it easier with this tool.