A most excellent discussion about edges on the blog of Stapleton Kearns - and the (to some) surprising fact that they aren't simply observed and faithfully recorded, but in fact designed.
Discovered through this tutorial thread on Conceptart, which includes this handy edge guide:
Edge basics 101:There is a scale of edges, just as there is a scale of values. It goes from hard > firm > soft > lost. Just as with value, you can use the whole scale in one picture or just a piece of it. The careful manipulation of edges is one of the most overlooked, but most important, tools an artist can use to create form, atmosphere, and believability.
In general, edges are:
- Harder in the light, softer in the shadow
- Harder in bright light, softer in dim light
- Harder in focused light, softer in diffused light
- Harder in the foreground, softer in the background
- Harder on smooth forms, softer on textured forms
- Harder on hard forms, softer on soft forms (Duh, but really)
- Harder on flat forms, softer on rounded forms
- Harder on thin forms, softer on thick forms
- Harder on still forms, softer on forms in motion (on moving forms they are harder on the leading edge and softer on the trailing edge)
- Harder at the center of interest, softer as you move away
The above are additive. So a kitten, far away, in the dark, would be really soft.
Of course, any of these guidelines can be ignored/modified for pictorial effect.
Read more: http://www.conceptart.org/forums/showthread.php?t=51913#ixzz2tVvy0lol