Thanks!
You'd use a neutral density filter any time you want a longer exposure than you can get without one. Essentially, it is just a dark filter that reduces the amount of light hitting the sensor. Sunglasses for your camera, if you will. The one I use is a variable one you turn it to make it lighter or darker. There are also simpler ones that offer a set amount of darkening.
I'd have to look at the EXIF data for the photo to see just how long the exposure was. In this particular case, that is less important. I use a cable release and hold the shutter open for a large group of bursts. It helps to have seen Wishes many times and have a feel for when the big groups of bursts are coming and when they end. I would imagine that exposure was probably anywhere from 45 seconds to a a minute and a half. The time itself is less important in this case because you're exposing for the fireworks, and each burst doesn't last that long.
Prior to determining that an ND filter was the answer for a photo such as this, I had tried reducing my ISO to even lower than its rated minimum (which was ISO 200 in my case) and using the smallest aperture I could set (f/22 or even smaller) with a relatively short exposure of around 20 seconds, to no avail. The result still looked like a flash-bomb going off over the castle a brilliant white blast of light that was way too bright. The ND filter makes all the difference.
In the daytime you can use such a filter to give you a long exposure that will make running water have that silky look you've probably seen in photos. You can also use it in the right situations to make people moving through the frame effectively invisible. If you're exposing for the overall scene (as opposed to for things such as fireworks), day or night, it is best to calculate how much time you need to add to the exposure.
SSB
