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Distortion - Head-2-Head Micro 4/3 Lens Review: Olympus Zuiko Digital ED 7-14mm f4 vs. Panasonic Lumix G Vario 7-14mm f4
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posted on May 13, 2010 at 4:24AM

Head-2-Head Micro 4/3 Lens Review: Olympus Zuiko Digital ED 7-14mm f4 vs. Panasonic Lumix G Vario 7-14mm f4


By Patrick Singleton

H2H ROUND-4: Distortion

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Distortion (Panasonic)
Ultra-wide zooms have plenty of distortion. Some of it is inherent – foreshortening and perspective effects make familiar subjects look odd. What's interesting about the Panasonic and Olympus 7-14s is how the distortion varies. As much as we work to publish solid quantitative results to back up (or disprove) the impressions we get from field tests, sometimes, there's no practical way to generate numbers. The Olympus 7-14mm behaves in a way that we can't measure with quantitative software, but we can demonstrate it in a way that can be published.

At its 7mm setting, the Olympus lens shows linear distortion that changes as the focus setting changes. We noticed the phenomenon as we watched the camera seek focus while we shot. The image goes from noticeable pincushion (horizontal and vertical lines bend in toward the center) to noticeable barrel (the lines bend out) as focus shifts from near to far. It also appears that the distortion isn't smooth across the frame – as if there's a little barrel distortion in the center when there's pincushion at the corners. Imagine watching a tattoo on a belly-dancer's stomach – the image waving around.

We set up sequences of shots from both the Olympus and the Panasonic to compare the two. Both were set at 7mm, and were manually focused in several steps from near (a foot or less) to infinity. Given a small aperture, depth of field was good enough that poor focus isn't a distraction.

We see two practical issues raised by this result: 1) it's very hard to predict what kind of distortion you'll get in a given shot, because it varies with both focal length (which we expected) and focal distance (which was a surprise), and 2) the distortion is complex, and therefore hard to correct in post processing. This is not the lens for architectural shoots where off-kilter lines will be distracting. Even a seascape or other scene with an uninterrupted horizon would be problematic.

Imatest, our testing software for image quality, measures distortion across small areas of the image, so it doesn't capture the variation across the frame that we see in our field images. We used the software to evaluate regions toward the middle of the frame, so the data do not reflect edge and corner performance. Our data is output as a percentage of deviation from a perfectly straight line. In our tests, the Panasonic delivered 3.36 percent barrel distortion at its widest setting, while the Olympus delivered 1.31 pincushion. In the middle of the zoom range, the Olympus improves slightly with 1.11 percent pincushion and the Panasonic improves to 1.3 percent pincushion. 

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