This image illustrates how the vignetting on the Panasonic Lumix 20mm f1.7 destroys image detail. One review site measured 1.7 stops vignetting.

Notice the bushes, great sharpness in the center, especially considering it's taken at f 1.8. Now take a look at the bushes closer to the edges. Noise from is destroying the detail, turning the fine detail into hazy noisy mush.

Processed in Raw Developer, no sharpening, no noise reduction. Default settings otherwise.