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I'm buying a new lens and am interested in determining how the out-of-focus effects will turn out with it.
How is a lens's bokeh determined by its construction, and can one get an approximate idea of what to expect from the lens's specs?
This is a topic not often covered by lens reviews, unless I'm missing some major site.
For the most part, there is no way to predetermine the quality of a lens's bokeh simply by looking at its construction, and even if you look at sample images you may be flummoxed, because the quality of the bokeh can change with the aperture setting as well as with the subject and background distances.
For example, the whole "number of blades" thing where folks run down how the EF 50/1.8 II is crappy and gives you pentagon out-of-focus highlights because it only has five blades, and not those nice smooth circular blobs is only true at specific aperture settings and distances. You can easily get circular out of focus highlights with an EF 50/1.8 II if you avoid them.
Bokeh C/A (aka purple fringe, or longitudinal chromatic aberration) can also disappear once you stop a lens down.
Double-ringed, or harsh bokeh may only happen at given subject distances, while the same lens at a different distance or setting may give you creamy smooth bokeh. See: http://neilvn.com/tangents/bokeh-vs-shallow-depth-of-field/ which has an example of harsh bokeh shot with a 50/1.4D. The same lens with which Nikon boasts about "beautiful background blur" on their website and posts these sample images).
We know that swirly bokeh shows up in Petzval lenses because they don't correct for field curvature, but most folks don't want swirly bokeh and most modern lenses do correct for field curvature. :) And, of course, lenses with extremely large max. apertures (f/1.2 or larger) can exhibit catseye bokeh when used wide open due to optical vignetting. But it's not like you can avoid that.
Generally speaking, the more experienced you are with lenses, the more you realize that to really learn a lens's bokeh, you have to use the lens. Perhaps the only thing in a lens's specs, if it's there, that might guide you to some general idea of a lens's bokeh characteristics is to look at the MTF chart and see how close together the meridonial and sagital lines are to each other. The closer they get, the smoother the bokeh will be.
That's pretty much it.
Arrangement of optical elements, level of correction for abberations such as Secondary Color / Chromatic Abberation and Spherical Abberation, shape of the aperture diaphram, a narrower-than-ideal lens barrel which partially occludes the exit pupil, as inkista says your chosen aperture and shooting distance. There are any number of factors which determine the quality of bokeh. It's something I've done a ton of reading about and still have quite a limited understanding of.
Any good lens reviews (such as at http://photozone.de/) will address bokeh. But it's often in limited scenarios and it's a subjective topic. So the short answer is, become an optical engineer or find samples which demonstrate the kind of shooting you do. Or, just rent the lenses you're considering from LensRentals - which is probably a good idea anyway as you can assess various other factors of image quality as well as handling and operation.
Lens Bokeh can be very subjective. Some reviewers do give examples. The best way to determine what to expect from a lens is to look at test photos either online, or from your own camera as you test the lens.
