What is a Neutral Density filter?

[TMR Design]

Can someone please explain what a Neutral Density filter is and when to use one?

[cool_dude07]

ND filter cuts the amount of light reaching the filter. It is used when you want to achieve longer shutter speeds. Like when you want to make waterfalls appear smooth on sunny days..etc.

Ty

[Briancd]

In addition..... it has no impact on the colour unlike a circular polariser filter.

[Jon]

In addition..... it has no impact on the colour unlike a circular polariser filter.Actually, a circular polarizer should be colour-neutral too. What it will do is cut out scattered, non-imaging, light that's being reflected off atmospheric particles or molecular irregularities on painted surfaces. THis lets you get cleaner, more saturated colour, but doesn't change the colour of the subject.

[Pixel9ine]

Actually, a circular polarizer should be colour-neutral too. What it will do is cut out scattered, non-imaging, light that's being reflected off atmospheric particles or molecular irregularities on painted surfaces. THis lets you get cleaner, more saturated colour, but doesn't change the colour of the subject.
It should be noted that some cheaper CP's I've bought for my A95 (Tiffen, for one) have shifted my WB towards the brown side-- nothing a Custom WB can't fix, but it's still annoying. My HOYA, however, is almost completely color neutral.

Back on topic, the other main feature of a ND filter is the ability to open up your aperture (by reason of cutting down on available light), useful if you wish to shorten your DoF (for portraiture, for instance) while shooting in bright surroundings with an upper limit of 1/2000sec..