Which Shud have better Bokeh at f/2.8?
1) Nikkor 85mm f/1.8 (http://imaging.nikon.com/lineup/lens/singlefocal/normal/af_85mmf_18d/index.htm)
2) Nikkor 105mm f/2 DC
(http://imaging.nikon.com/lineup/lens/singlefocal/Telephoto/af_dc105mmf_2d/index.htm)
3) Nikkor 180mm f/2.8 EF-ID
(http://imaging.nikon.com/lineup/lens/singlefocal/Telephoto/af_180mmf_28d_if/index.htm)
Added (1). I have 105 f/2 DC in my mind…
You do seem to understand that DOF work is a lens issue but what you don't understand is that any lens can do DOF work if you know how to operate the lens properly. There's no great secret to DOF and there's no particular lens that you can buy that's better at DOF than another. All you need to do is learn how to operate the lens you have.
By the way here's a good article on the subject.
http://en.wikipedia.org/...h_of_Field
Agree with above ans
Boke is the quality of the out of focus region. Just knowing the f-stop number doesn't tell you anything about boke. Boke is affected by the design of the lens. You need to see actual shots by those lenses and figure out which has a nicer looking out of focus region. A good place to start is doing searches on Flickr for those various lenses and see which lens has the most pleasing out of focus region.
Bokeh is the specific visual effect you get from defocused points of light in a photograph. There's no better or worse Bokeh, just different, dependant mostly on the construction and shape of the aperture blades in the lens. Many people would consider "good bokeh" to be prefectly circular, but some of us with toy lenses can pick, or even create our own aperture shapes (and hence bokeh).
The second point I'd like to make is that you are basing your judgement on which lens by their bokeh, but the "prime" lenses you are looking at are substantially different (focal length). You would normally do far better to figure out approximatly what focal length you need, and then look at the reviews for a lens of that focal length with a bokeh you find pleasing.
Every one is saying that you can't tell how good ones bokeh is from another. That would be true other than the fact that reviews can tel you a lot. And you are correct. The Nikon 105mm f/2 DC has the best bokeh out of any lens ever made. But it is a very complicated lens to use (yes this lens manages to be complicated) so you should take some time to learn it.
here are 2 reviews
http://www.kenrockwell.com/...-f2-dc.htm
http://www.photozone.de/...est-report