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Sony FE 85mm f/1.4 GM - Review / Test Report - Analysis |
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Lens Reviews -
Sony Alpha (Full Format)
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Distortion
Normally - thus with activated image auto-correction - you won't really notice any image distortions. However, that doesn't mean that the Sony FE 85mm f/1.4 GM is a full corrected lens. In RAW mode, you can spot a pincushion distortion of ~1.3% which is fairly high for a prime lens.
Vignetting
Image auto-correction also comes handy with respect to vignetting. The RAW image reveals a light fall-off of 2.4EV (f-stops) at f/1.4. While not unusual for such a full-format lens it will be visible in many scenes. Please note, however, that some photographers actually prefer to have some vignetting at this setting anyway. Stopping down to f/2.8 solves most of the issue and it's gone by most standards from f/5.6 onward. If you don't want to bother the image auto-correction decreases the vignetting to 1EV (f-stop) which is quite acceptable.
MTF (resolution)
The resolution characteristic of the Sony FE 85mm f/1.4 GM is impressive - as you might expect. The image center is already very good at f/1.4 and the outer image region is on a good level. Stopping down to f/2 boosts the center to an excellent and the borders/corners are also lifted. The peak performance is reached around f/4 with an outstanding center and very-good to excellent levels at the border/corner. Beyond diffraction is limiting the quality. This remains pretty much irrelevant till f/8 but it's getting more noticeable from f/11 onwards.
The field curvature is minimal. The centering quality of the tested sample was only Okay with a slight tilt of the focus plane at f/1.4.
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)
Once again - on Sony cameras, you don't really have to worry about chromatic aberrations due to image auto-correction. However, even if you prefer to disable this for whatever reason, the CAs are very low at around 0.6px on the average at the image borders.
Bokeh
You do, of course, expect such a lens to deliver a high level of sharpness but the real reason for investing that much money is the shallow depth-of-field potential. And the bokeh (rendition of the out-of-focus blur) should better be good. The Sony lens does not disappoint us in this respect.
The background bokeh is truly silky and the foreground is just slightly less perfect as you can see below.
Highlights are also rendered very nicely. Short of STF lenses, this is about as good as it gets from a conventional lens! The Sony lens also features no less than 11(!) aperture blades thus even if you stop down a little, the circular aperture shape remains intact.
Image highlights aren't quite as nicely circular in the image corners though. Our test scene below illustrates the usual "cat-eye" rendering and it never really dissolves when stopping down. This is typical for pretty much all lenses though. Technically it is a vignetting effect due to the limited diameter of the lens barrel.
Bokeh Fringing / LoCA
Bokeh fringing describes chromatic aberration on the Z-axis with greenish halos beyond the focus point and purple halos in front. There are lenses capable of correcting the issue but the Sony lens does not qualify as you can see below. The fringing is easily visible at f/1.4 and traces are visible till f/2.8. This is usually no drama but it is an imperfection nonetheless.
If you look closer you may spot that the focus point shifts towards the back when stopping down - these are so-called "Residual Spherical Aberrations". On DSLRs, this would be an issue but Sony cameras auto-focus at stopped-down aperture so usually you won't notice this in real life. You may also spot that the lens is fairly soft near this minimum focus distance albeit that's not surprising - this applies to many lenses.
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