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I've worked up a direct comparison using various image crops from the three 85mm prime lenses listed below.  I started at F/2.8 for the purpose of comparing the two very expensive lenses with the very cheap Sony 85/2.8 SAM.  If you want to see how the Sony CZ and Sigma do against one another, go to the Sigma 85/1.4 HSM review and scroll to the bottom of the page. 
 
The Sony A900 was used for this comparison, along with a tripod, remote shutter and manual WB.  Bracketed manual focusing was used, with the resulting absolute sharpest shot used as the base for the entire set.  The distance from the sensor plane to the stamp subject is 11' 3.5" or 3442mm, which is almost full body portrait distance (on FF), and allows it to be more useful than shooting a close focus test chart.
 

Sony 85mm F/2.8 SAM   More info
 
 
Random observations;
 
Sharpest centers; Sigma by a hair, mid-sections; Sony 85/2.8 SAM (f/2.8-4), corners; easily the Sony 85/2.8 SAM (2.8-4).
 
If you're thinking most of the crops are the same, try looking at the detail of the bell body, and the text outline sharpness at the lower left of the bell, (it's part of the 'twenty first class stamps' that's been cropped out).
 
The Sony 85mm F/2.8 SAM is almost at its best at F/2.8, which is stunning performance when you factor in the $250 price tag!  Since the diminutive and inexpensive Sony performs very well wide open, it makes a great walk around lens for hand-held low-light shooting.  You might think a larger aperture is always better for dark conditions because it allows you use lower ISOs right?  Have you every tried photographing subjects (or worse, moving subjects) in low-light at 85mm, F/1.4?  It's mostly a game of luck, the depth of field is very slim; it's hard enough to focus properly in daylight!  I normally use F/2-2.8 for hand-held low-light shots using a focal length around 50-85mm, it gives me a little extra DOF to play with.  Additionally, you'll spend $800-1200 more for the larger, heavier and faster F/1.4 versions.  Although two stops of light is huge, there are also huge compromises in designing those lenses, that's why you pay big bucks and get only marginal (if any) performance gains.

 

85mm centers

 

 

          Sigma 85mm F/1.4 EX DG HSM

          Sony CZ 85mm F/1.4

       Sony 85mm F/2.8 SAM

F/2.8

endof2011/sigma85ctr28.jpg
endof2011/scz85ctr28k.jpg
endof2011/sam85ctr28.jpg

F/4

endof2011/sigma85ctr40.jpg
endof2011/scz85ctr40k.jpg
endof2011/sam85ctr40.jpg

F/5.6

endof2011/sigma85ctr56.jpg
endof2011/scz85ctr56k.jpg
endof2011/sam85ctr56.jpg

F/8

endof2011/sigma85ctr80.jpg
endof2011/scz85ctr80k.jpg
endof2011/sam85ctr80.jpg

 

Center sharpness is marginally better at all apertures with the faster lenses, but this probably wouldn't be noticeable unless displayed side-by-side as I have here. 

 

 

85mm mid-sections

 

 

          Sigma 85mm F/1.4 EX DG HSM

          Sony CZ 85mm F/1.4

       Sony 85mm F/2.8 SAM

F/2.8

endof2011/sigma85mid28.jpg
endof2011/scz85mid28k.jpg
endof2011/sam85mid28.jpg

F/4

endof2011/sigma85mid40.jpg
endof2011/scz85mid40k.jpg
endof2011/sam85mid40.jpg

F/5.6

endof2011/sigma85mid56.jpg
endof2011/scz85mid56k.jpg
endof2011/sam85mid56.jpg

F/8

endof2011/sigma85mid80.jpg
endof2011/scz85mid80k.jpg
endof2011/sam85mid80.jpg

 

The mid-sections look fairly close in resolution at all apertures, however, I do see a very sharp F/4 crop for the little Sony 85/2.8 lens.

 

 

85mm corners

 

 

          Sigma 85mm F/1.4 EX DG HSM

          Sony CZ 85mm F/1.4

       Sony 85mm F/2.8 SAM

F/2.8

endof2011/sigma85corn28.jpg
endof2011/scz85corn28k.jpg
endof2011/sam85corn28.jpg

F/4

endof2011/sigma85corn40.jpg
endof2011/scz85corn40k.jpg
endof2011/sam85corn40.jpg

F/5.6

endof2011/sigma85corn56.jpg
endof2011/scz85corn56k.jpg
endof2011/sam85corn56.jpg

F/8

endof2011/sigma85corn80.jpg
endof2011/scz85corn80k.jpg
endof2011/sam85corn80.jpg

 

Clearly the Sony 85/2.8 produces the sharpest corners, especially compared to the mushy Sigma crops.  As you close the aperture past F/4 the little Sony starts to show some slight degradation, along with a little color fringing as well. 

 

That's it for this comp, thanks for visiting!