High-key photography at its simplest is purposeful overexposure. Madhu Manickam gets into the nitty-gritty in this article for Photography Life:
In short, high-key images are those that have few shadows and have the subject set against a very bright background. High-key images are often set against over-exposed backdrops. In high-key photos, the average of all the pixels in the image is very bright.
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