Neutral density filters are primarily used for balancing exposures. By balancing exposure, I mean adjusting the amount of light that enters the lens so that the scene appears uniformly lit across the frame and every inch of it is exposed properly. A properly balanced image will show a histogram that is clustered more towards the middle of the graph. Too much on the right and you have an over-exposed image and too much on the left means you have under-exposed.
Technically speaking, an under-exposed image is better than an over-exposed one, because you would still be able to salvage some details from an under-exposed image. You would need to shoot in RAW for that. However, getting the exposure correct in-camera is the best option by far.
The reason these filters are called as neutral is because they stop light across the spectrum equally, without any bias. Consider them as shades for your camera lens.
Multiple camera shots with different expose combined in HDR photo
If you are just starting out in digital photography, you have probably never heard about auto exposure bracketing (AEB). If you are an enthusiast photographer, probably you have heard about this but never quite mastered the concept to use it with confidence. Regardless of your expertise in digital photography this article can help you master an extremely important feature of your DSLR, i.e.; auto exposure bracketing (AEB).
Cameras like the Canon 5D Mark III and the Nikon D3X have completely changed the playing field when it comes to low light photography. In the earlier days photographers would have no way to change the ASA (ISO as it was called in those days) once they had loaded a film in their cameras. They were pretty much stuck with the film sensitivity until they changed it. That had a limiting effect on their creativity and it ensured that they were careful about their selection of film before heading out.
These days, digital photographers have a much easier time. They could change the ISO (light sensitivity of the sensor) just with the flick of a button. Cameras like the D800 comes with the added advantage of a greater dynamic range, allowing photographers to compose and make images that are closer to what they see with their naked eyes.
Yet, there are certain time-tested and useful tips that you need to know in order to nail good exposures in low light conditions. These are, in no way, alternatives to the ability of your camera to shoot in low light. Rather, these are complimentary to a high ISO capability.
Image before white balance correction
Are you plagued with the problem of strange color casts in your images? Many a times after taking a picture with our little Point & Shoot cameras the results fail to impress us. Having taken a picture, say under a dominating source of fluorescent light or for that matter a tungsten light bulb, the images appear to have a strong color cast. To be a little more precise a tungsten light source will have a yellow color cast and a fluorescent light will have a bluish color cast. Why does this happen? This happens because every light source has a color temperature and that is what is captured by the digital sensor when the image is made.