Zeiss Sonnar T* FE 55mm f/1.8 ZA ( SEL55F18Z ) - Full Format Review / Test - Analysis
Lens Reviews - Sony Alpha (Full Format)

Distortion

Standard lenses are usually pretty much free of image distortions and this also applies to the Zeiss lens. A pincushion distortion of just 0.3% is absolutely negligible.

Vignetting

At fully open aperture, the light falloff reaches a very hefty 2.6EV (f-stops) - this is immediately noticeable. To be fair, this is also the typical amount of vignetting for such lenses on full format cameras. Stopping down to f/2.8 results in a substantial improvement (1.2EV) but it's best to stop down to f/5.6 from where on the vignetting isn't objectionable anymore.

Please note that vignetting can be auto-corrected both in the camera or via various RAW converters out there. Due to the signal amplification this comes at cost of increased sensor noise noisy in the corners though.

MTF (resolution)

Mirrorless cameras (sensors) require excellent optics to make the most of their potential. The Zeiss Sonnar T* 55mm f/1.8 ZA lens is generally capable of delivering impressive results although it's not perfect. The center zone is very good at f/1.8 but the outer image field just makes it across the "good" mark here. Stopping down to f/2.2 boosts the dead center into outstanding quality. The "near-center" as well as the outer image zones are improving more gradually the more you stop down. The best results are achieved around the f/4-5.6 marks. Even the extreme corners are very good at these apertures. Diffraction degrades the quality from f/8 onward but the results are still sharp at f/11.

The centering quality of the tested sample was good.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

The Zeiss lens produces a very low amount of lateral CA (color shadows at harsh contrast transitions) starting with an average CA pixel width of 0.8px at f/1.8 and decreasing down to half that amount at medium aperture settings. This is impressive on a 42mp sensor.

Bokeh

The bokeh describes the rendition of out-of-focus blur in the focus transition zone - thus the area just beyond the depth-of-field. This is sometimes misunderstood - it is NOT about the blur of the far background. If the focus spread between your main object and the surroundings are big enough, pretty much any lens can produce a smooth blur.

The Zeiss Sonnar T* FE 55mm f/1.8 ZA is somewhat complex regarding its bokeh characteristic.

Out-of-focus highlights have a circular shape at f/1.8 to f/2.8 but the inner zone is very nervous. It is likely that the 3 aspherical elements have a fair share in this issue. As usual the highlight discs deteriorate to "cat's eyes" towards the corners but this is a rather normal behavior in the full format scope. Stopping down helps to tame this aspect to some degree.

On the positive side, the rendition in the critical focus transition zone is very smooth and buttery in the image background. The rendition in the less critical foreground is somewhat more rough though.

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

So-called bokeh fringing is an effect that occurs around the focus point (on the Z-axis). It's visible as halos of different colors in out-of-focus areas - magenta (red + blue) in front of the focus point and green beyond.

The Zeiss lens exhibits a significant amount of bokeh fringing at f/1.8 and it's still obvious at f/2.2 and f/2.8. An improvement can be seen at f/4 although traces remain visible even at f/5.6.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/1.8 f/2.2 f/2.8 f/4 f/5.6