Compared to the f/1.8D
The real issue here is value. The f/1.8D is about one-third the cost of the f/1.4D. You obviously lose two-thirds of a stop in light gathering, the question is do you lose anything else? My answer might surprise you. Not nearly as much as the price suggests. Indeed, if anything the f/1.8D is a little sharper wide open than the 1.4D at f/1.8 on my FX bodies. This is especially noticeable in the corners (the f/1.4D may beat the f/1.8D in some apertures in the center, but barely). The f/1.8D vignettes less wide open, too. Chromatic aberration and distortion are very similar to the f/1.4D, too, which is to say very good. Nope, the only other thing you really lose besides that two-thirds of a stop aperture loss is, wait for it, bokeh. Yep, there it is: it just doesn't quite have that "don't see it" bokeh of the f/1.4D. What to attribute that to, I don't know, but it isn't quite the same. That's not to say that I find the bokeh objectionable on the f/1.8D--it's good. But if I look closely, I can tell the two apart. So, it comes down to two-thirds of a stop and bokeh that can't be beat for an extra US$800 or so. If you're buying an 85mm for a general purpose lens that will sometimes be used for portraits I'll surprise you and say no, it isn't worth the extra money. Even if you're on FX and shooting only portraits with it, I think you need to really examine your shooting to see if f/1.4 and better bokeh are really worth the extra money.
