Wide angle lenses usually vignette, especially wide open, and it can be quite severe e.g. 1 stop. So your experience is not unusual. But most people stop the lens down anyway, and that removes the effect. Even if vignetting is present, software can easily remove it, so it's not a big deal on digital.
I've also read many comments from people who suggest that full frame sensors can cause vignetting with wide angle lenses. The effect is supposed to result when light from the lens hits the sensor at an angle. Apparently the light does not hit the sensor directly, but rather it hits a layer of microlenses in front of the sensor. These are supposed to focus the light onto the sensor pits. But, if the light hits the microlenses at too steep an angle, it is lost. I don't know if this effect is real. Unfortunately there are too many bogus claims on the net to believe everything.
Leif
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