Nikon D40 - Looking for new portrait - lens

[enzodm]

Xander86: I think the kind of pictures you take are better described as street photography than portrait, with slightly different needs.

As you see, if speaking of portraits in strict sense, most of us will suggest you a fixed, fast lens, because typically a portrait will give you the time to choose lens, composition, move, etc (friends get bored, but is part of the work not to bore them). For me 35/1.8 is slightly short for portrait (except environmental portrait), but 50, 100, 135 are all usual lengths. At equal aperture, the longer the more blur (called bokeh).
In street photography you need to be quick, and so you give up to some picture qualities.
When you asked about how limiting is a fixed lens, in travel I bring with me more than one lens, including my two zooms. In cities I'll typically use one of them, but if I want to take some nice picture of my wife, I stop, I mount an old fixed lens, manual focus, and take all time needed to do it at most of my possibilities. So I do not feel constrained. I should also say that sometimes I voluntarily go with just one lens exactly to be constrained, but this is another story .