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Sharpness, contrast, low light operation, close focusing (due to a floating element).
Size and weight. When used on a DSLR or CSC via an adapter, its shallow dof when wide open can present focusing problems.
This has to be the best Zeiss lens I have ever owned, even better than the beautiful 85mm 1.2 AE and MM Planar lesnes. Let's be honest, it needs to be the best to justify the frightening cost of what is essentially nothing more than a fast, standard lens. Less than 1000 were made, produced to celebrate the 100th Anniversary of the Planar optical design. When seen next to the celebrated Zeiss 50 1.4 Planar, the size of the 55mm become very apparent but when used on a camera like the RTSIII or AX, it is beautifully balanced; with an Aria or S2b, it feels very front heavy and you may need a deep tripod spacer if shooting that way.
Where it comes into its own is in low light, especially for natural light portraiture when its very shallow depth of field at f1.2 can be a huge bonus, throwing all but the eyes out of focus. If you wish to use the shallow depth of field on a bright day, you either need to use a 77mm ND filter or be able to use a top speed like 1:32,000 that you can find on the Fuji X-T1.
Illumination across the frame is remarkably even for such a large piece of glass and edge definition is oustanding. One other nice touch is found in the lens hood - the inside of this dedicated metal hood is covered in a lovely black felt material - no risk of light reflection at all.
If you have strong wrists and deep pockets, this is a magical lens to own. Is it worth more than 10x the cost of the 50 1.4 Planar? Not for general photography but if low light - as with indoor sports or ballet - or a shallow dof is essential, it's worth its weight in gold.