Carl Zeiss C Sonnar T* 50mm f/1.5 ZM

The Lens

The Zeiss C Sonnar T 50mm f/1.5 ZM* is a manual focus lens designed for 35mm rangefinder cameras. Introduced in 2004 as part of the Zeiss ZM lineup, it is a modern recreation of the historic Sonnar design. Unlike the Zeiss Planar 50mm f/2, which aims for technical perfection and flat field curvature, the C Sonnar is engineered to reproduce the specific aesthetic qualities of mid-20th-century optics while utilizing modern multi-coatings (T*) to suppress flare.

History of the Sonnar Design

The original Sonnar optical formula was developed by Ludwig Bertele for Zeiss Ikon in 1929 and patented in 1931. The name "Sonnar" is derived from the German word Sonne (Sun), symbolizing the lens's large aperture and ability to capture light.

At the time of its invention, lens coatings did not exist. Complex lens designs with many air-to-glass surfaces suffered from severe light loss and internal reflections (flare), which reduced image contrast. Bertele’s genius was to group massive glass elements together, cementing them to reduce the number of air-to-glass surfaces to just six. This allowed the Sonnar to achieve an aperture of f/1.5 with significantly higher contrast and transmission than rival designs of the era, such as the Leitz Summarit.

For decades, the Sonnar was the premium standard for the Contax rangefinder system. The modern C Sonnar ZM is a direct descendant of the 1932 Contax Sonnar 50mm f/1.5, adapted for the Leica M bayonet.

Optical Characteristics

The lens utilizes a six-element in four-group design. This asymmetrical construction results in specific optical traits that distinguish it from the ubiquitous Double-Gauss (Planar/Summicron) formula found in most 50mm lenses.

• Spherical Aberration: The lens is under-corrected for spherical aberration. At wide apertures (f/1.5 to f/2), this results in a gentle softness or "glow" around high-contrast edges and smooth rendition of skin tones, a quality highly valued by portrait photographers.
• Focus Shift: A notable characteristic of this design is "focus shift." As the aperture is stopped down (e.g., from f/1.5 to f/2.8 or f/4), the effective plane of sharpest focus moves slightly backward. Because rangefinder cameras do not see through the lens, the photographer cannot visually confirm this shift in the viewfinder. Zeiss originally optimized these lenses for f/2.8 to balance the shift, though later production runs were reportedly adjusted for f/1.5 based on customer demand.
• Rendering: By f/4 or f/5.6, the spherical aberration clears, and the lens renders with high contrast and modern sharpness. This duality allows the lens to serve as a character optic for artistic work and a sharp tool for landscapes or street photography when stopped down.

Comparison to Alternatives

The C Sonnar occupies a specific niche for photographers seeking "character" over clinical perfection.

Voigtländer Nokton 50mm f/1.5 Vintage Line (Type II)

The Nokton Type II is visually styled to look like a vintage lens but contains modern optics.

• Optics: It utilizes an aspherical element to correct aberrations. Consequently, it is sharper wide open than the Sonnar and does not suffer from focus shift.
• Rendering: While it has "vintage" styling, the rendering is cleaner and more modern. The Sonnar produces a more distinct, unstructured bokeh that vintage enthusiasts often prefer.

Voigtländer Heliar Classic 50mm f/1.5

The Heliar is a deliberate "special effect" lens.

• Correction: The Heliar is intentionally uncorrected to a high degree, showing heavy swirling bokeh, vignetting, and flare (especially in the single-coated version).
• Usage: The C Sonnar is a usable everyday lens that happens to have character. The Heliar is an extreme artistic tool that imposes a heavy signature on every image.

Leica Summilux-M 50mm f/1.4 (Close Focus Re-issue)

• Design: This lens is based on the first version of the Summilux (1959). It exhibits a similar "glow" wide open but uses a Gaussian design rather than a Sonnar design.
• Bokeh: The Sonnar tends to have "bubbly" or structured bokeh; the Summilux typically has smoother, creamier background blur due to the symmetry of the design.
• Price: The Leica re-issue commands a significantly higher market price than the Zeiss.

Leitz Summarit 50mm f/1.5 (Vintage 1950s)

The historical competitor to the original Sonnar.

• Glass: Vintage Summarits are famously soft and prone to glass fogging and cleaning marks (soft glass). They have extremely low contrast compared to modern standards.
• Contrast: The C Sonnar has modern T* coatings, resulting in images that have the "vintage resolution" but with deep blacks and high contrast that the original Summarit lacks.

Voigtländer 50mm f/2 APO-Lanthar

The technical benchmark.

• Performance: The APO-Lanthar is apochromatically corrected to eliminate chromatic and spherical aberrations. It offers perfect edge-to-edge sharpness and zero focus shift.
• Summary: The APO-Lanthar represents the peak of modern optical correction; the C Sonnar represents the preservation of historical optical flaws for aesthetic effect.