[stoppingdown]

I can't directly comment on the AF/MF thing, but I'd like to point you out that the 58f2, at full aperture (which I suppose you're using for the swirly bokeh), is sharp only in the center of the image. You can check it looking at the two lighthouse pictures posted here:

http://stoppingdown.net/blog/the-zenit-h...otography/

So, if you want to have a certain degree of sharpness of your subject, you should take care in composition, by keeping it near the center of the frame (if you don't like the composition, you can crop it later). I suppose that if you want swirly bokeh, it's natural to think of center-based compositions.

You might want to run a few tests yourself, because the behaviour at shorter focal distance might be different.

Also be aware that on an APS-C camera a good deal of the swirly bokeh is clipped out.