Blending objects

Artistic Support Compositing and Post Processing
1

How is it that in some renders the objects seem to blend with the background? They don’t have completely solid edges. How does this work?

Thanks.

2

I’m not sure exactly what you mean w/o an image

but it might be with DOF (depth of field Blur)

try here http://blenderartists.org/forum/showthread.php?t=66055

3

might be alpha transparency coming into play.