The Formatt Hitech 82mm Combination 81EF/ND 0.6 Glass Filter, made with SCHOTT-Desag B270 crown optical glass, combines the effects of an 81EF light balancing filter with a neutral density filter.
An 81EF filter (the strongest in the 81 series) will more than replace the yellow missing from the image and place all skin tones on a distinctly "warmish" level. You might even consider using an 81EF at the beach to make a hot scene even "hotter."
Neutral density filters appear gray and reduce the amount of light reaching the film. They have no effect on color balance. They are very useful when you want to shoot high speed film outdoors on a very bright day.
Neutral Density Filters Have Four Main Uses
To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars
To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background
To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations
To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which could cause overexposure
Neutral Density Factors
ND.3 (exposure adjustment = 1 stop, reduces ISO 1/2)
ND.6 (exposure adjustment = 2 stops, reduces ISO 1/4)
ND.9 (exposure adjustment = 3 stops, reduces ISO 1/8)
ND 1.2 (exposure adjustment = 4-stops, reduces ISO 1/16