Key-to-fill ratio is a film lighting term which often baffles even very able cinematographers. It basically describes how much shadows your lights cause on your subjects – or your contrast ratio. (That’s a pretty basic description – here’s where it gets fiddly) The Key Light is your main source of light, but using just this one light will cause shadows in the unlit areas. Therefore you need a second light – a Fill Light to ‘fill’ in these shadows. A Fill Light will also soften the harsh effect of the Key Light.
The Key Light’s ratio diminishes as the Fill Light becomes steadily stronger giving you key-to-fill ratios of 16:1, 8:1, 4:1 and 2:1 for example. The higher the key-to-fill ratio number the more prominent the shadows, so a 16:1 ratio would be Film Noir while a 2:1 ratio would be softer, more natural lighting with few noticeable shadows. 1:1 would be totally flat lighting with no shadows. The numerical difference between these ratios is measured in stops.
‘Low Key’ movies would have a lot of Key Lighting but little Fill Light and so includes noirish films such as The Third Man, Blade Runner, and Memento. Alternatively, High Key lighting would eliminate shadows – examples of this would include light-hearted comedies and TV based items such as news bulletins, chat shows and game shows. Your average toothy Colgate advert would also be lit in a very bright, positive High Key way.
If that’s too much to soak up here’s a quick summary tip – The Key Light is the main light but it causes shadows, the secondary light is the Fill Light and it ‘fills’ in these shadows. So if Film Noir is 16:1 and natural looking lighting with very few shadows is 2:1 the ratio stops in between are easy.