Some notes to myself I made while doing value studies.
Top: The difference between hard and soft light. Hard light is when you have a very focal, strong light source, such as a spotlight or (less extreme) direct sunlight. Changes from light to dark tend to be very sudden. It really catches your attention because of this heavy cotrast and you see it for instance in those dramatic lightning portraits. Soft light is a lot more subtle. You encounter it outside on cloudy days or from indirect (window) sunlight. Changes from light to dark are smooth, with many midtones. Although not as attention-catching this allows for much more detail.
Bottom: Some sloppy practical application of the bove principles. A focally lighted object on dark bg looks better with more contrast (a wider range of values) than a diffusely lighted object on a lighter bg. The type of light source decides the range of contrrast while the properties of the object itself (object luminnce) decides where in the value spectrum that range begins.