I've gone deeper into the CoC concept in other threads, which often just descend into circular arguments that are confusing.
I'll add to Jamesy's test... Here are two 'equivalent' images:#1: 135mm f/4 on a 5D#2: 85mm f/2.5 still on a 5D, but I cropped the center in post. The camera-to-subject distance was fixed. The key thing to note is that the Bokeh appears identical between the two focal lengths. #3 is the 135mm FF at f/2... Note that a 135 f2 has far more bokeh than an 85mm f2.5 cropped.(Unfortunately I didn't do an f2<>f2 comparison, and I ate the tomato)
Interesting - what made you select 85_F2.5 vs 135_F4 - did you just chimp until the bokeh looked similar or did you bust out the calculator?
...but have a hard time grasping that cropping an image in post changes DoF, too.
I have posted these once before here - they are far from perfect 'lab' shots but they show the rough difference in bokeh between a 5Dc/85_1.8 and my 40D/50_1.8, both at 1/3200, ISO100, F2. Clearly the 5D has way more bokeh.