You don't have that many options I suppose, fix it in photoshop or buy a new lens? I don't mind a bit of vignetting, sometimes it adds to a photo. Digital photography in general seems to be wrapped in this quest to have technically perfect photos. No Barrel distortion, no noise, no vignetting! I actually like barrel distortion, noise and vignetting when its appropriate. I wouldn't worry too much as these darker corners will not make a break a good flick.

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betty banger