Zeiss Sonnar T* FE 55mm f/1.8 ZA - APS-C Format Lab Review - Analysis
Lens Reviews - Sony NEX
Article Index
Introduction
Analysis

Distortion

Standard lenses are usually pretty much free of image distortions and this also applies to the Zeiss lens. A pincushion distortion of just 0.6% is very low. Interestingly this is marginally more than what we have seen in our full format test but then it remains pretty much irrelevant from a real world perspective.

Vignetting

Full format lenses enjoy a sweet spot advantage when used on APS-C format cameras. The rather extreme vignetting that we've seen on the A7R II remains mostly outside of the APS-C image frame. That being said, there's still a noticeable amount (1.1EV) of vignetting left at f/1.8. Stopping down reduces the issue gradually and it's quite negligible from f/2.8 onward.

MTF (resolution)

As already mentioned, Sony's 24mp sensor is a difficult being for fast and very wide lenses. We are pleased to report that the Zeiss Sonnar T* 55mm f/1.8 ZA lens does a very decent job to overcome the obstacles. The image center is already dead sharp at f/1.8 and nothing short of outstanding between f/2.2 and f/5.6. At max aperture the lens can feel the impact from the sensor. The borders are good to very good whereas the extreme corners slightly soft. Stopping down to f/2.2 pushes the quality a little in the outer zone but the real improvement happens at f/2.8 with a very good result across the image field. Images are breathtakingly sharp at f/4 and f/5.6. Diffraction kicks in at f/8 but it's nothing to worry about here.

The centering quality of the tested sample was good.

Please note that the MTF results are not directly comparable across the different systems!

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 1px at f/1.8 and it's negligible once you stop down past f/2.8.

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. In the full format review we reported about the "cat's eyes" at the image borders but you will only see traces of those in the APS-C format scope at large apertures.

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

Sample Images & Verdict

Sample images can be viewed in the corresponding full format review. If you crop the sample images to the APS-C frame, you should get a very good idea how the lens performs in this format.

Verdict

The Zeiss Sonnar T* FE 55mm f/1.8 ZA is capable of pushing Sony's 24mp APS-C sensor to its limits. That is easily true for the image center even at f/1.8 The outer image region isn't quite as impressive at f/1.8 and f/2.2 but once stopped down to f/2.8 to f/8 the results are stunning. The lens produces some vignetting at maximum aperture but it's mostly gone from f/2.8 onward. Lateral CAs are usually no issue nor are image distortions. The Zeiss lens is certainly a fast lens so the quality of the bokeh is a primary characteristic for shallow depth-of-field photography. The results are a bit mixed here. The critical blur in the image background is nicely rendered whereas the foreground isn't as pleasing (still good). Out-of-focus highlights are rather rough. A fair share of bokeh fringing is also present.

On the mechanical side we have nothing but praises. The Zeiss lens is small but it feels really solid. Combined with inner focusing, smooth controls and weather sealing this is about as close to perfection as it gets these days. The AF is quite fast although it won't win against similar DSLR lenses here for sure. Manual focusing is very precise and smooth. Some may disagree but we actually like the focus-by-wire system.

So finally there's the big question whether the high price tag is really worth the sacrifice. After all Sony is offering a dedicated E 50mm f/1.8 OSS for their APS-C cameras ... at a quarter of the price. The Zeiss Sonnar T* FE 55mm f/1.8 ZA is undoubtedly the sharper lens across the aperture range. The aggressive optimization of the design had an impact on the bokeh though which is where the less ambitious E 50mm f/1.8 OSS is more harmonious really. Furthermore the Sony lens features an optical image stabilizer which gives it some extra potential in very low light. If you are a purist or intend to upgrade to one of Sony's full format camera, it has to be the Zeiss lens but otherwise the E 50mm f/1.8 OSS can do most of the job really.

Optical Quality:
Mechanical Quality:
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