The Zeiss ZM Sonnar is an unusual lens by modern standards. The manufacturer has reworked a 1930s optical design and added modern lens coating to create a fast, compact prime that really does ensure a love or hate relationship.

The Sonnar is designed for Leica M mount cameras but can, with the use of adapters, be used on a host of other systems including the Sony A7 and A9 series.

Making it even more unusual is the fact that Zeiss, as a manufacturer, is renowned for its offerings of high quality, upmarket and technically proficient lenses. The Sonnar may be upmarket, but it is not a technically perfect or near-perfect lens – unlike many other Zeiss ZM models in the M mount line-up.

In fact, it's a lens that can be incredibly frustrating.

The key issue is the same reason it produces a unique, creamy character. The rendering is simply beautiful and reminds me of the character of old 1960s fast 50s such as the Leica Summilux MKII – yet it does so with a crisper finish due to the modern lens coatings.

Now to the issue: the age-old optical design creates an age-old problem – spherical aberration leading to focus shift. Most users have reported two separate apertures where focus shift rears its head on this lens: wide-open at F1.5 and slightly closed at F2.8

My lens seemed optimised for wide-open shooting, which means that at F1.5 the subject of the images was dead-on, but as I closed the aperture to around F2.8, there was some slight focus shift. My preference for optimisation would be wide-open, because closing the aperture gives you more wiggle room in terms of depth of field anyway.

Zeiss itself states on its website that the Sonnar is more designed with portraiture in mind, where shallow depth of field and subject isolation are more important. The company points to its other 50mm offering in the ZM range for technical perfection, the slightly slower Biogon.

In everyday use, the focus shift issue was not a great problem – again because it was optimised for wide-open. The lens has a beautiful character, although this is dangerous territory for a reviewer because it is entirely subjective. With that caveat, this lens is a great performer for portraits and people photos, and these topics should be considered its strengths.For landscape, or any subject that requires a critical treatment, I'd recommend something else.

The ergonomics on the Sonnar  are fantastic. It's remarkably small for such a fast 50, even by Leica M mount lens standards, and the build quality, like all lenses in the ZM line, is exceptional.

The reason why you either love or hate this lens depends on what you're photographing and what you want to begin with. If you approach this with clear eyes as to its relative weaknesses, but unique character, then you will fall in love with it.

The Zeiss Sonnar 50 F1.5 C ZM carries a recommended retail price of $1,699.