The Formatt Hitech 40.5mm Combination 85/ND 0.6 Filter, made with SCHOTT-Desag B270 crown optical glass, is a combination of an 85 color conversion and neutral density filter.
The 85 series will produce natural colors when shooting with tungsten film outdoors. Using tungsten film in daylight will otherwise produce a bluish cast in the photograph.
Neutral density filters have several uses and offer the possibility to achieve otherwise unachievable results. They appear gray and reduce the amount of light reaching the film.
85 Series Conversion Chart
An 85 decreases the color temperature from 5500-3400 Kelvin
An 85A decreases the color temperature from 5500-3100 Kelvin
An 85B decreases the color temperature from 5500-3200 Kelvin
An 85C decreases the color temperature from 5500-3800 Kelvin
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)