Sony FE 20mm f/1.8 G - Review / Test Report - Analysis
Lens Reviews - Sony Alpha (Full Format)

Distortion

The Sony FE 20mm f/1.8 G is surprisingly well-corrected for a mirrorless lens. Its native pincushion distortion is very mild at just ~0.6%. Image auto-correction can take care of "the rest" and the unavoidable loss in quality is marginal in this case.

Vignetting

Many ultra-wide lenses struggle when it comes to vignetting. Some of this can be contributed to plain physics but it's not just that. The Sony lens doesn't have an ideal characteristic here. A native vignetting of almost 3EV (f-stops) is hardly surprising at f/1.8. However, if you stop down the light falloff remains quite heavy even at f/5.6.

Image auto-correction "shaves off" more than 1.5EV so it's a much lesser issue in this case. Remember that this signal boost comes at the cost of increased corner noise though.

MTF (resolution)

The resolution capabilities of the Sony FE 20mm f/1.8 G are much more impressive again. The broader center quality can be considered to be pretty much PERFECT across the aperture range. You may expect that the outer image region is miserable at f/1.8. Unsurprisingly, the borders/corner are weaker indeed but the quality is still in a range that we classify as "good". Stopping down improves the quality and the results are best around f/4 - including very good borders/corners. Diffraction is a limiting factor beyond f/8.

The centering quality of the tested sample was very good. Field curvature is not an issue and the astigmatism is very low.

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)

Image auto-correction will take care of lateral CAs (color shadows at the image borders). It's a lossless procedure so it doesn't have any negative impact.

However, the native CA characteristic is excellent anyway. An average CA pixel width of 0.3-0.4px at the image borders is barely visible in the first place.

Bokeh

Usually, we don't really discuss the quality of the bokeh of an ultra-wide lens in detail. However, the Sony lens is much faster than most in its class so let's have a look.

Below is a sample crop of a bokeh scene where most ultra-wide lenses would struggle due to harsh contrasts. Surprisingly, the Sony lens manages this scene just nicely with smooth transitions to both the image background and foreground.

Out-of-focus highlights are also very nicely rendered with a smooth inner zone and marginal outlining. This is magnitudes better than on most wide-angle lenses. The more edgy aperture shape is just visible at f/2.8 but the deterioration isn't overly obvious even at f/4.

Perhaps even more baffling is the fact that the Sony lens can maintain the circular highlight shape at the image borders/corners. We'd like to see such a characteristic in a tele lens ... but we haven't so far ...