Nikon lens rebates expiring this weekend. I caved and bought the 85F1.8g

The differences between the f/1.8G and f/1.4G are many, but subtle.  Here is a full-res D800E comparison that I did recently at f/2, using my car as a model:

http://actionphotosbymarianne.com/Tech/Lens/G85_18_f2a.jpg;

http://actionphotosbymarianne.com/Tech/Lens/G85_14_f2a.jpg;

If you study the background bokeh, you will see the f/1.4G is smoother, but the difference is not overwhelming.

One of the problems of doing such comparisons, is duplicating the focus precisely.  The method I use, is to manually focus-bracket, moving the focus ring in the smallest steps possible with my fingers (less than 0.1mm movements).  Still, these aren't a perfect match - the f/1.4 sample is focused slightly further back - but it makes a good comparison for background bokeh.

For comparing other details in the photo, especially foreground, you might find this alternative frame more suitable, which is focused slightly closer:

http://actionphotosbymarianne.com/Tech/Lens/G85_14_f2b.jpg;

A few observations of my own -

At f/2, the f/1.8 model appears to have a bit more DOF (look at details in the pavement around the focus point).  This is a good indication of a sharper lens.  At such wide apertures, the f/1.8G has a little sharpness advantage over the f/1.4G, however it also exhibits some coma (or possibly astigmatism - more testing is needed here) in the corners at wide apertures, which the f/1.4G does not.

There is a little difference in the color balance of the lenses.  The frames are valid comparisons, as the camera was set to manual WB (5200K).  Transmission efficiency is a little higher for the f/1.8G in the center, but not in the corners; the f/1.4G exhibits less vignetting.

The f/1.4G foreground bokeh is harsher (a side-effect of having smoother background bokeh; you cannot have both smooth simultaneously) and this leads to some odd effects.  Compare the horizontal joint between the car's sill and quarter-panel just forward of the rear wheel:  The f/1.8G shows this clearly, but the f/1.4G seems to be filling in the dark gap with some extraneous red blur.

At the upper-right edge of the rear wheel against the tire, the f/1.4G has less CA than the f/1.8G.

Ultimately, at moderate apertures and close to moderate distance, the f/1.4G offers better corner-to-corner performance, although surprisingly, my old 135/2 DC does even better in the corners at f/5.6, at test-chart distances.

At very long distance such as astro and distant landscapes, the best 85mm corner-to-corner is the 70-200 VR II - yes, it's better than both primes!

--
Qualities possessed by God in infinite proportion: Love, Grace, Power, Righteousness, Wisdom, . . .
Qualities possessed by humans in infinite proportion: Ignorance.
- Marianne