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Distortion
Technically distortion is a bit of a weak point of MFT lenses because the system relies on an auto-correction and as such minimizing the issue isn't really a design priority. You may argue, of course, that this is not relevant from a user perspective because all the applied corrections are done "under the hood". This is also something that we can confirm - standard JPEGs or conventionally converted RAW files don't show a significant amount of distortion. At 12mm, there is only a slight degree of barrel distortion (~0.5%) and it's negligible beyond.
Nevertheless, it is still interesting to look behind the scenes by using an "unsupported" RAW converter. This reveals a native barrel distortion level of disappointing 8.5% at 12mm, which is on the extreme side. However, at 25mm it isn't overly relevant while the 1% of pincushion distortion at 40mm is a moderate issue.
Vignetting
The vignetting characteristic of the lens is pretty disappointing. There's a high amount of light falloff at 12mm @ f/2.8. and even stopping down reduces this issue only slightly. Fortunately the vignetting gets better beyond 12mm, but its still on a pretty decent level for a “pro” lens.
The next figure illustrates the mentioned vignetting issue at 12mm.
MTF (resolution)
Following the disappointing vignetting results, we proudly present the highest average resolution we tested so far on a MFT zoom lens. The centre is incredibly sharp (high contrast + high resolution) trough out the zoom range with only a slightly swab at 40mm. Additionally to that the border and corner performance is continuously on a very good level with a very low field curvature so you don't have to stop down to improve the borders.
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 the image borders) are also auto-corrected and as CA corrections aren’t lossy we welcome this feature. However, uncorrected RAWs show a different situation and even if the issue isn't quite as pronounced as on the Panasonic 12-35mm it is still visible with values between 0,7 and 0,8px in average and at 40mm you can even spot some CAs with a pixel width of 1.3px at the image borders.
Bokeh
Another important aspect of such prime lenses is the quality of the bokeh (out-of-focus blur) and the Olympus does an acceptable job here. The seven aperture blades deliver a round bokeh at wide-open with smooth transitions delivering a pleasant out-of-focus blur. However, in critical scenes there is a risk of an onion-like substructure and when stopping down out-of-focus highlights show a bit of outlining.
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