In practice ND filters are not perfect as they do not reduce the intensity of all wavelengths equally. This can sometimes create color casts in recorded images, particularly with inexpensive filters. More significantly, most ND filters are only specified over the visible region of the spectrum, and do not proportionally block all wavelengths of ultraviolet or infrared radiation..   NB!! Dangerous if you have one fitted taking images with full sun whilst you view the scene in your viewfinder.. 

That is quite a leap from ND2 to 8 as stated it would be good to see the exif data, was it not possible to lower the iso and  use something like f32 or whatever the lens you had was capable of?

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt