I posted/corresponded with another RFF'er who was looking for comparisons between the Zeiss 50 Planar and the Summicron. The following is an edited version of what I sent him.
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I have a tabbed 1980's Summicron and the ZM 50/2. I wish I had something definitive to say about the image quality between them. I don't. The Zeiss is scary sharp in the center of the frame, as are the modern Summicrons. Finish quality goes to the Summicron, as the CV-Zeiss black paint started wearing through to the brass within weeks of purchase, whereas my 1980's Cron has seen use every week since 1994 and doesn't have a mark on it (and I bought it used). Hood and lenscap on the Summicron are easier to use; the Zeiss lens cap is pretty fiddly. I have just run two side-by-side images at f:2. You can see fairly subtle differences in the way the two lenses render an image, for instance in the out of focus areas (slightly smoother with the Summicron -- but we are talking VERY hard to see). In focus highlights have a slight "bloom" to them with the Zeiss that is a marginally better controlled with the Summicron. Both lenses show a little CA off axis in the test image wide open, but you have to view the images at 100% to see it -- there is sliiiightly more CA with the Zeiss lens, to my eye. I know it is a cliche, but you really can't go wrong with either lens. . . . I purchased the Planar hoping that it would match the quality of the G-Planar 45/2. It turned out to be its own beast, very good at making images, but not a substitute for the G-Planar. The better "value" seems to be the Zeiss on the face of things, but I think you'd really have to check in in 20 years to know for sure. What I mean by that is that it is pretty clear to me where the extra money went for the Summicron (higher quality finish, better cap/hood design, more screws in the Cron's mount, Cron is slightly lighter in weight (1 oz.) -- but in terms of image quality, I don't know whether you would notice this relatively minor stuff).
Finally, I would note that my own crude results differ slightly from other reviews I have read, including Sean Reid's excellent comparison of 50's on his paid review site. This may mean that there is some sample variation in the lenses that are out there, or it may mean that in real world situations, you probably would not notice any practical difference in image quality between these two excellent lenses.
You can listen to folks' opinions all day long, but at the end of the day you have to see images made with each lens to choose. FWIW, I have a 50/1.5 Canon Sonnar clone and it has a totally different look than the Summicron/Planar wide open. Tons of spherical abberation, lots of 'glow' -- It is in no sense "better" than either the Summicron or the Planar . . . but if you want that look you can't beat it with a stick. FYI, camera position was identical for [my test pictures]. The camera was mounted on a tripod and only the lens changed. I did have to change the tilt of the camera though, to focus. . . . I would give the off-axis edge to the Summicron. . . but like most differences in performance of lenses of this caliber, it may come down to personal preference.
Ben Marks