Hi Rich, The problem you are experiencing is called spherical aberration. Macro lenses are corrected for this, but it's not usually a problem 99% of the time since so few close ups contain straight lines. You can get around this by placing your straight line, in this case the barrel, so that it passes through the center point of the frame, in other words, it's essentially a radius. I know this sounds complicated, but give it a try.
Regards, Norm
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