Gel wrote in post #13523883
One of those things I've never thought about until now.
Up until recently the longest lens I've had is a 200mm.
Now I've a 100-400L and a 2x Extender. With the 100-400L I'm finding that the amount of bokeh present at 5.6 is very similar to an 85L prime at 2.8
Bokeh isn't a quantity, it is a quality.
Do you mean that you are seeing a shallower depth of field?
If so, there are three factors of relevance here: focal length, distance to subject, and f ratio (typically called "aperture").
Search for "dofmaster" and you'll find a utility where you can see what the DoF will be for various combinations of parameters. You'll need to supply a fourth parameter though, and that will be determined by the size of the sensor in your camera.
If you mean something beyond that, the degree of background blur, that is something I don't have a good handle on myself, but I have seen an excellent analysis done here on POTN, so maybe a search will reveal it. The key factors I think are focal length and the relative distance of the background compared to the subject. Aperture could enter into it also, and doubtless crop factor as a way of standardising across different formats that require different rendering magnifications to reach the same scale.
Is this because by zooming to 400mm I'm pulling the subject matter to me, thus increasing the distance between foreground and background?
That is physically impossible, so no.