The C4 color correction filter cools the ambient light entering the lens by 350° kelvin (-40 mired shift) by adding blue to the overall exposure. This filter helps fine-tune the color or look of your scene to match your film's white balance. The Hoya C4 filter is helpful when: You are using tungsten-balanced film and the light source is warmer than the traditional 3200° K and you need a specific amount of color correction to match your film type. Candles and low watt incandescent bulbs are examples of warm light sources. You are shooting at sunrise or sunset with daylight-balanced film (5500° K) and the overall scene is too warm. The filter will help cool the scene for a more natural look. HOYA Multi-Coat (HMC) filters are recommended for enhancing the performance of today's multicoated lenses. These popular filters minimize reflection, reduce flare and ghosting, and result in over 97% average light transmission.