Shadow Edges: How Soft the Light Was

The sun is not a point. From here it covers about half a degree of sky, which is roughly the width of a pea held at arm's length, and that width is what keeps a shadow from having a clean border. For every meter a shadow travels away from the object casting it, the fuzzy band along its edge widens by about a centimeter. Where a chair leg touches the floor the shadow is a hard line; a meter up the wall the same leg has already gone vague.

Change the size of the lamp and the whole picture changes with it. An overcast sky turns the entire dome into one soft source and shadows go missing, which is why photographers keep working on grey days and stop pretending it is a loss. A low winter sun through bare branches does the opposite and prints the branch pattern across the snow sharply enough to count.

So the softness in a photograph is evidence about the weather, not a filter someone applied afterward. Once you notice it, you start reading the edge before the subject: crisp means open sky, feathered means cloud or distance. On a wallpaper it is the part that survives being half covered by windows, since an edge stays readable long after the thing that cast it has been hidden.