Pentax-DA 560mm F5.6 ED AW
Vignetting is darkening of the corners. Most if not all lenses exhibit some vignetting at the larger apertures. The 560mm does have some vignetting, but it is less than 1/3 stop at F5.6, and for all practical purposes it is gone by F8. Even the vignetting at F5.6 is nothing to worry about, it will not be noticeable in most real-life photos. In the few cases where it may be noticeable, it is easily corrected in post-processing or in-camera.
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Verdict on Vignetting
When used with APS-C cameras, vignetting is a non-issue in practice with this lens. On full-frame, it is still difficult to observe dark corners in real-life photos, but there is significant vignetting at F5.6 and F8.