Originally posted by Abbazz Ben, if you are talking about the Nikon 135/2 DC, then technically it's not a soft focus lens but rather a lens with "variable bokeh," as it allows the user to control the amount of spherical aberration in the out of focus areas. For further details, you may want to see
Ken Rockwell's web page or
Dante Stella's web page about the 105/2 DC.
I know that lens, I have used it myself.
Also, just let's have a look at Nikon's official description, which seems to match exacly, what I would expect from a soft focus lens:
"DC
AF DC-NIKKOR lenses — unique NIKKOR lenses for unique portraits
AF DC-NIKKOR lenses feature exclusive Nikon Defocus-image Control technology. This allows photographers to control the degree of spherical aberration in the foreground or background by rotating the lens’ DC ring. This will create a rounded out-of-focus blur that is ideal for portrait photography. No other lenses in the world offer this special technique."
This is a very similar effect to any other specialised soft focus lens, except that the Nikon one allows the spherical aberration control independently of the aperture (quite similar to your Tamron zoom,, after which I was chasing some time ago...), whereas most other SF lenses will have an out-of-focus effect very much dependend on the aperture. What we commonly call "soft focus" lenses always produces a sharp "core" image, overlayed with an image softend by spherical aberration or a multi-opening aperture (the Imagons of this world).
You may argue with some right, that then the term "soft focus lens" is simply wrong. That's true. On the other hand, it is just a convention, to use that term.
Ben