Excerpt: Carl Zeiss Planar 50mm f/2 ZM is one of two 50mm lenses that Zeiss is currently offering in the Leica M bayonet mount format (the other lens being Sonnar 50mm f/1.5 ZM). Planar is one of the most venerable designs in the photo lens industry, originally designed by Paul Rudolph in 1896. The design is often called 'Double-Gauss' for its symmetrical layout of lens elements.
Summary: This Zeiss 50mm f/2 Planar ZM is optically excellent, but since an optically and ergonomically superior used LEICA SUMMICRON-M doesn't sell for much more, I'm unsure who'd buy this Zeiss lens.
Zeiss Planar T* 50mm f/2 ZM on Sony NEX - Review / Test Report
4 October 2010
Conclusion: The Zeiss Planar T* 50mm f/2 ZM may be an exotic choice for Sony NEX users but it's certainly a highly interesting one. The lens shows no major weaknesses but there's still room for improvement. The center resolution is on a very high level at large aperture settings and the borders are pretty good here. The sweet spot is reached somewhere around f/5.6 with a very sharp results across the image frame. Lateral CAs are well controlled and not overly relevant.