Light is the language of photography. To express yourself well and communicate you need to be able to speak with light.
Light determines what you see (and what you don’t see); the mood of an image; colour and tone. Light can be loud and brash or soft, gentle and soothing. It can wrap around something or cut across it as hard and sharp as a Samurai sword.
People have probably been writing about the qualities of light and how to use it in relation to photography from the moment the first print was made. Google “light in photography” and you will get millions of hits (well actually 68,800,000 hits to be entirely accurate).
