Mike J has a good series of articles on sharpness, resolution and contrast on the Luminous Landscape web site. The one on contrast goes into bokeh a bit. He believes good bokeh is largely determined by how closely the tangential and saggital MTF curves follow each other. I expect he's come to this conclusion empirically, but it does seem to make sense. It certainly explains how some lenses that aren't very sharp have great bokeh (I had a cheap AF Sigma 400/5.6 that had to be stopped down to f8 or f11 to be even usable, but it had really nice bokeh) and why some sharp lenses have poor bokeh (although there certainly are lenses that are sharp and have great bokeh; the FA43/1.9 springs to mind). The article on lens contrast is at http://www.luminous-landscape.com/tutorials/understanding-series/lens-contrast.shtml
-- Mark Roberts www.robertstech.com Photography and writing
