make shadow blurry on the further object?

hi guys, i would like to know how to make shadow blurry depending on the object distance?
like in this image

111247-2494231542_074f0f3200_z.jpg

i hope you guys understand what i mean, i use directional light btw
thanks in advance

You could take a look at distance field soft shadows or use a translucent material on your foliage.