What is a ND filter?
In short it is a filter that blocks out light uniformly without any colour bias. A 10 stop ND filter effectively blocks out light akin to a sunglass. Can you imagine, 1 stop reduction is half the amount of light . 10 stop is.... alot of light being block!
I use Big Stopper from LEE and you probably get equivalent from other manufacturer such as HiTech.
You can find an excellent reference on ND filter from the following link.
Credit : The Ultimate Easy Guide To Neutral Density Filters by Peter Hill.
http://www.redbubble.com/people/peterh111/journal/4421304-the-ultimate-easy-guide-to-neutral-density-filters
The biggest challenge in using an ND filter is the exposure and composition because the light blockage would render the camera focussing system useless. In many instance it may be visually impossible to see the subject clearly.
The best way is to mount the camera (without the filter) on a tripod, focus (NB: set the lens focus mode to MF after the focus is locked), compose the frame and meter the scene. The best mode of exposure is Manual exposure mode. After metering, without the ND filter, mount the ND filter and the re-exposure with compensation. The easiest way to compensate for the ND would be to use a free application such as "Longtime Exposure" from Apple Store.
The application is free and easy to use. First choose the filter (how many stops ND filter) that you would like to use in the picker. Then choose the metered exposure time without the filter. The 'compensated' exposure time will appear. Set this new' exposure time on the camera and your exposure will be correct. Lastly mount the ND filter onto the camera.
If the new compensated time is longer than the maximum that you can set on the camera (example 30 secs) you'll need to use the BULB mode. Happy shooting. "CLICK"
http://itunes.apple.com/us/app/longtime-exposure-calculator/id362297743?mt=8
