A good amount of vignetting (dark corners) is expected with a super zoom and the Pentax 18-270mm is no exception. We shot the sky at wide-open aperture, stopped down one stop, and stopped down two stops at (or close to) 18mm, 35mm, 70mm, 135mm and 270mm.
At 18mm the field of view is so wide that the sky isn't evenly illuminated vertically, but you still see the effect of vignetting in the test photo. Vignetting is the least prominent at 70mm.
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|F-max||One stop down||Two stops down|
If you don't stop this lens down by at least two stops, chances are you'll see clear vignetting in the photos, especially if they include evenly-lit surfaces.
Although this lens generally produces quite pronounced vignetting, depending on what you're shooting, it may not be a showstopper. Plus, most post-processing programs do a very good job of removing vignetting with little effort from the user.
Overall this lens does a bit worse than the 18-55mm kit lens and 18-135mm WR in terms of vignetting.