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Distortion
The M.Zuiko lens produces only a slight degree of pincushion distortion (~0.7%) which is rarely noticeable in field conditions. This applies also to the uncorrected RAW results which are just marginally worse (~1.0%).
If you move your mouse cursor over the image you can switch to the corresponding non-corrected results.
Vignetting
Atypically for a large aperture lens - in a positive sense - the lens shows only a slight vignetting of around ~0,4EV (f-stops) at wide open aperture with virtually no edge shading. Stopping down reduces the light fall-off even more to irrelevant levels. Furthermore, the uncorrected RAW figures are negligibly worse due to the missing distortion correction.
The chart below shows the vignetting characteristic based on camera JPEGs as well as RAWs converted by Raw Therapee V4.0, which does not apply any correction.
MTF (resolution)
The lens delivers very good resolution in the image center straight from the maximum aperture. The borders are easily on a very good level whereas the extreme corners are "only" good. Stopping down to f/2.8 lifts the center quality beyond the excellent barrier and the corners improve substantially. The peak performance is reached between f/4 and f/5.6 with very sharp results across the image frame. Stopping down beyond f/8 reduces the quality considerably due to diffraction effects.
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)
Lateral CAs (color shadows at harsh contrast transitions) are pretty much negligible throughout the aperture range as they stay below 0.5px on the average at the image borders. The CAs of non-corrected RAW files are marginally more pronounced at 0.7px wide open and dropping noticeably below 0.5px from f/2.8 onwards.
Bokeh
The quality of the bokeh (out-of-focus blur) is naturally a major aspect for such a lens and the Olympus doesn't disappoint us here. Out-of-focus highlights are very evenly rendered and the focus transition zone is pretty smooth. Bokeh fringing (colored halos in the focus transition zones) are visible at max. aperture but not overly disturbing.
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