David H. Hartman Prolific Poster, jul 01, 2006; 04:39 p.m.
For a half body most photographers want about a 55 to 60mm lens on the 16x24mm format. A maximum aperture of f/2.8 is probably good enough, f/1.8 is better. The actual focal length of the 50/1.8D AF Nikkor is 51.6mm. The shorter focal length and relatively small aperture, thats small in millimeters will limit how much background blurring you can achieve. You probably want more DOF for a half or three quarter portrait so this probably wont be a problem.
I dont see a need for the 50/1.4 here. Also the 50/1.4D AF Nikkor does have a good reputation for bokeh. If you had the budget for the Zeiss 50/1.4 ZF (AIS) this would be a much better choice from early and reliable reports.
For a tight head shoot, I guess thats what you mean when you say, full face you want longer focal length. Many want to throw the background out of focus for shots like this. To do this you need a longer focal length combined with a large maximum aperture. These combined blur and enlarge background details. Also because of the narrower angle of view a longer lens allow easier selection of the background content. You might want a dark background to contrast with the subject of you might find soft pastel colors that look attractive in your background. An 85/1.8D AF will make a good lens for tight head shots. The 85/1.4D AF Nikkor is probably the in this focal length from any lens maker just now.
The distance from the subject to the lens determines perspective and the perspective normally desired in portraits and candids is pretty well set by social norms. Closer looks more intimate until it looks distorted while farther looks more aloof. If you have to err then generally using a longer lens is better than using a shorter one. If you are trying to setup a studio in walk-in closet you might want to ignore this advice.
The lens Id really like to recommend doesnt exist in a Nikkor. Id like to recommend a 70/1.4D AF. A 70mm lens would be ideal for head and shoulders portraits and candids on the 16x24mm format.
Given that you dont care to sell a kidney I think a rather safe purchase would be the 85/1.8D AF. You really need both so if the budget will allow a 85/1.8D AF and 50/1.8D AF then Id buy those two.
Best,
Dave Hartman.