Lens for Painting photo

I have Canon 5D MarkII and 50mm f1.8 lens. I take photos of artworks. I find that 50mm f1.8 lens has a little bit of wide angle effect on the frame edges. It makes the straight lines to look curvy like it was taken by fish eye. It is not too noticeable at first, but I have to photoshop all my images to straighten the curvy edges of the painting photos. I was wondering if there is any other good sharp lenses by Canon.
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Bat