And/or longer focal length and/or the lens at or near minimum focus distance (assuming the MFD is close enough to begin with). Bokeh is a function of all three of those - you can shoot, for example, with a 24mm lens wide open, like at f2.0 (ex. the M.Zuiko 12/2), a group shot of about ten people in two rows, and if it was a lesser lens, chances are you're more likely to blur parts of the people at the edge of the frame than whatever's behind them. At the same time a good quality 85mm or 105mm lens at f4 will still produce enough background blur on a head-shoulder portrait without the issues of using some cheap, older 50mm lenses where wide open the background blur could have been good, but the lens sharpness really drops going off center that only the eyes and half the nose are in focus in the whole frame, and the tip of the nose looking like foreground blur.
This one was taken at 400mm, f6.3.
Edited by ProtegeManiac - 10/23/13 at 12:06am