Try the setup in the link I posted above, using the box. As I mentioned, I had better luck doing that than with a ruler/yardstick alone.
Did you shoot that raw or jpeg?
Try shooting raw and do some basic sharpening in DPP - something like unsharp mask setting the sliders to 4, 4, 2 or so. This works for me for a quick sharpening. There's an anti-aliasing filter on the sensor always causes some softness.
I'm still pretty new, so I may be slightly off on that, but that's what I would try.
50D - Canon 60mm Macro - Sigma 30mm - Canon 70-300mm DO IS - Canon 17-85mm IS - Canon 50mm 1.8 II