First of all, ND (neutral density) filters are not to be confused with graduated-ND filters which are typically used to darken skies in landscape photography. ND filters are used to darken the ENTIRE image. The top photo is an example of an image that I made recently using a 10-stop ND filter on a trip to Denali National Park. This image was made at about 11 AM on a cloudy, but fairly bright, day. The exposure with the ND filter was:   ISO 400,  f/16, 15 seconds        

The second image is a “normal” photo, without the filter, of the same subject made with this exposure:   ISO 400, f/11, 1/250 second

 My intent in using the ND filter was to create an image of the rocks surrounded by soft, dreamy looking water that contained virtually no detail. The long shutter speed made this possible.

One thing you’ll discover when using an ND filter, is that it is not always possible to predict what the final image will be ahead of time. I don’t mind this and think using one is fun and interesting. The process becomes mostly trial-and-error to get the result you want. Longer exposures  produce softer features with less details of the moving elements, while faster exposures, produce more details which may be desirable, depending on the situation. You choose which is best.

 The other thing that you will discover in using the 10-stop ND filter is that you cannot see through it. So composing and focusing is done BEFORE the filter is in place. Of course, this is annoying and requires some care in making sure that the camera is not bumped or that focus or zoom settings are not changed when mounting the filter. This is not a problem when using a 3-stop ND filter since composing and focusing can take place with the filter in place.

 If you buy a ND filter, I recommend getting a quality one made by a reputable manufacturer. Cheaper filters may not be truly neutral and may impart an unwanted color tint. I have a 77mm B+W filter which works very well along with a step-down adapter so I can also use it on a 62mm diameter lens.  Manufacturers may list their ND filters using the “Optical Density” number. A 3-stop ND filter has an Optical Density of 0.9. A 10-stop ND filter has an Optical Density of 3.0. For viewing and photographing the sun, an 18-stop ND filter (Optical Density of 5.4) is used.