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Sony FE 50mm f/1.8 ( SEL50F18F ) - Full Format Review / Test - Analysis |
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Lens Reviews -
Sony Alpha (Full Format)
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Page 2 of 3
Distortion
Standard lenses are usually pretty much free of image distortions and this also applies to the Sony lens. The numeric figure (below) determined by Imatest is quite impressive for sure.
Vignetting
At fully open aperture, the light falloff reaches a very hefty ~2.8EV (f-stops) - this is immediately noticeable. To be fair, this is simply the typical amount of vignetting for such lenses on full format cameras. Stopping down to f/2.8 results in a substantial improvement (1.1EV) but if it's an issue it's best to stop down to f/4 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)
The resolution is a bit of mixed bag. The good news is the very impressive center performance. It's very good at f/1.8 and that's including high (local) contrast. The near-center is still good but the outer image region is downright dismal with heavily smeared corners. Stopping down to f/2.2 has little effect here. The first substantial improvement is visible at f/2.8 resulting in an excellent center and very good borders whereas the extreme corners remain very soft. f/4 provides a massive gain in performance across the image field - including very good corners. The peak quality is reached around f/5.6. Visible diffraction effects reduce the quality perception from f/11 onward again.
The centering quality of the tested sample was acceptable.
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 Sony lens produces a very low amount of lateral CA (color shadows at harsh contrast transitions) with an average CA pixel width around 0.6px at the image borders. 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 Sony FE 50mm f/1.8 has a fairly decent bokeh quality.
Out-of-focus highlights have a circular shape at f/1.8 to f/2.8 (just) with a very smooth the inner zone albeit slight outlining. The outlining effect is emphasized when stopping down.
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 and somewhat less pronounced than usual.
Stopping down helps to tame this aspect - at the expense of more edgy highlight discs.
The general blur is reasonably smooth. The two sample crops below (background vs foreground) are positioned to the left and right of the image center respectively. If you look closely, you may spot that the blur is smoothing towards the center rather than symmetrically (visible at the black clubs and red diamonds). There is some smearing in the image corners 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 Sony 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 substantial improvement can be seen at f/4 and the effect is essentially gone at f/5.6.
On the downside you may spot that the focus point moves to the back when stopping down. These are so-called residual spherical aberrations or in plain English - focus shifts.
Some readers were a bit astounded by the following image from our sample image section.
Please note that this was a STRESS TEST with extreme contrasts!
You may spot the rather excessive degree of fringing in there - greenish in the background and purplish in the foreground.
This is primarily bokeh fringing/LoCAs although there's certainly also a hint of "purple fringing" in there (a blooming effect).
Please note that you can push many lenses into this behavior if you just want to.
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