see also:

• portrait lenses
• fast 50mm focal length lenses
• Olympus 45mm f/1.8 comparison with Canon EF 85mm f/1.8 and Nikon Nikkor AF-S 85mm f/1.4G lenses
• copy variation in MTF charts of various short telephoto lenses

• this all depends on which camera you have, how you are going to use the lens, what subjects and what is most important to you
• the Zeiss Otus T* 85mm f/1.4 ZF/ZE lens gives nice bokeh excellent sharpness (both of which are now available in many modern lenses) and the least CA and longitudinal CA of any of these lenses if that is important to you, but it is very expensive, manual focus only and doe not have weathersealing, so most photographers will go for more versatile and affordable options.

• when shooting full length shots of people at some distance, the 85mm f/1.2 lens is a winner as it is most able to separate your subjects from the background although one would be hard pressed to see a substantive benefit over a 85mm f/1.4

• you need sufficient depth of field (DOF) to ensure the subject's ear to tip of nose is in focus, this means ANY of the above lenses will provide sufficient depth of field (DOF)
• important problems include:
  • AF accuracy:
    • dSLR systems by their nature have inaccurate AF systems which require microcalibration particularly when using such shallow depth of field (DOF)
    • this is not an issue with mirrorless camera systems using CDAF
    • some lenses have spherical aberration resulting in pronounced focus shifts when stopping down and this also causes inaccurate focus
    • furthermore, when dealing with narrow DOF, moving the camera so the AF region is on the subject to lock AF then re-composing will result in inaccurate AF
    • mirrorless camera systems with CDAF allow AF region to be selected almost anywhere in the frame, not just the central region as with most dSLRs, and nearly all now also have face detection AF
    • Olympus cameras go one better with eye detection AF and allow you to select WHICH EYE the camera should detect and lock AF on - this is a truly awesome feature, although it does rely on the subject being relatively still for about 0.5sec
  • AF speed:
    • generally not a major issue for most portraits, but the Canon 85mm f/1.2 lens takes 0.5secs to AF and this could be problematic and make it difficult to use
  • edge-to-edge sharpness:
    • other than the Cosina f/0.95 lens, most of these lenses are very sharp wide open in the centre
    • but you generally want the sharpest part of your image to be on your subject's eyes and these are rarely in the centre, hence good sharpness towards the edges allows more creative compositions
    • the MFT options were generally the best in this regard
    • the Sony 50mm f/1.8 OSS was terrible, and even the super expensive Leica Summicron had soft edges wide open
  • close focus:
    • the ability to focus even closer than the usual 0.85m allows added versatility
  • camera shake:
    • this is particularly a problem when shooting in low light or, with fill-in flash which may limit shutter speed to 1/180th-1/200th sec in many cases
    • the addition of image stabiliser significantly increases the versatility in either of these scenarios to ensuring camera shake is minimised
    • very few prime lenses under 300mm focal length have optical image stabiliser, although the Panasonic Leica DG 42.5mm f/1.2 Nocticron lens does have it
    • Olympus cameras give you the world's best image stabiliser with ANY lens including any of these prime lenses
  • vignetting:
    • most of us love to add a bit of vignetting to our portraits so our subjects take the emphasis in the image
    • BUT we DON'T want optical vignetting as this means our subject who is often not in the centre, will be darker than the centre, and then we have to boost the exposure to compensate, adding digital noise in the process
    • most of the above lenses have a LOT of vignetting (almost 2 stops) which can be very annoying
  • bokeh:
    • most of the above lenses have nice bokeh although some have a little bit of nervousness
    • some have troublesome longitudinal CA (LoCA) which rears its ugly head in the foreground or background blurred imagery
    • some have troublesome lateral CA which is problematic on bright highlights - personally I found this to be an issue with my Canon EF 85mm f/1.8 wide open and it seems to be a problem with the Zeiss lenses
    • remember too, that the degree of background blurring and transitional blurring (as distinct from depth of field (DOF)) is more dependent upon the actual aperture not just the full frame DOF effective aperture

• fast moving subjects are still more easily tracked with AF using a dSLR than with a mirrorless camera systems although this is rapidly changing
• if C-AF tracking is your style, then perhaps a pro dSLR or pro mirrorless with a f/1.4 or f/1.8 lens is the way to go

• astrophotographers value low optical aberrations so that their stars have a nice shape, this means minimal astigmatism and coma⁸⁾
• preliminary reports suggest that the Panasonic Leica DG 42.5mm f/1.2 Nocticron lens has these aspects very well controlled and thus it may well be the best of these lenses for this type of work, although, historically, astrophotographers do not like lenses with optical image stabiliser as these elements tend to degrade the image, and the strong vignetting will require correction frames to be shot.
  • if you are going to relatively “lose” 1.5EV due to vignetting, the Olympus m.ZD 45mm f/1.8 portrait lens will probably give similar aberrations and exposure with sharper images when shot at f/2.5 compared to the Panasonic at f/1.2 or f/1.4.