Panasonic LUMIX G X VARIO 12-35mm f/2.8 ASPH POWER OIS - Review / Test Report - Analysis
Lens Reviews -
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Traditionally this is a bit of a weak point of MFT lenses because the system relies on an auto-correction here and as such it 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 throughout the zoom range.
That said it is still interesting to look a bit behind the scenes by using an "unsupported" RAW converter like e.g. RAW Therapee. This converter reveals the native barrel distortion level of about 5.8% at 12mm. This is excessive - and as such it is highly disappointing for such a lens. Please note that the correction of such an amount of distortion is lossy due to the required image stretching and interpolations.
If you move your mouse cursor over the image you can switch to the corresponding non-corrected results.
The vignetting characteristic of the lens isn't overly impressive. There's a high amount of light falloff specifically at 12mm and 35mm @ f/2.8. You have to close the aperture by about one f-stops to reduce the issue to an acceptable degree. The vignetting is less relevant in the middle range.
Regarding all the high praise on the web we were excited to look into the resolution chapter. The Panasonic lens showed superb results in the center of the image frame. Everything is tack sharp here (=high contrast + high resolution) straight from f/2.8. However, things aren't quite as breathtaking when looking at the border and corner performance at 12mm and 20mm. The image is still sharp (=good to very good) here - no doubts - but it's actually nothing beyond the ordinary. However, the good news is that the performance is very good across the image at 35mm.
Note: You may be surprised about the "odd" MTF curve where the center performance starts to decrease straight after f/2.8 or f/4. Please note that this is a diffraction effect. It is actually typical to see a peak resolution (center) at such apertures on (micro-)four-thirds cameras.
A nice aspect of the lens is the very low field curvature so you don't have to stop down to improve the borders (except for increasing the depth-of-field, of course).
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
Chromatic Aberrations (CAs)
Lateral CAs (color shadows at the image borders) are also auto-corrected on Panasonic MFT cameras but NOT on Olympus MFT cameras. CA correction aren't lossy so we welcome this feature and Panasonic users can happily ignore this chapter.
Not so Olympus users. Now you may expect that such a pricey lens is well corrected. However, while the issue isn't quite as pronounced as on the PZ 14-42mm or Olympus' 12-50mm it is far from being low. At 12mm you can easily spot some CAs with a pixel width of 1.5-2px at the image borders and it's not much better at 20mm. This will be noticeable at times. The effect eases at 35mm.
Here's a sample crop taken at 12mm which illustrates the issue: