If it is one of the focus charts which you put at 45 degrees to the lens, and at a short distance, don't let Hoppy hear you say that!
With my 70-300, I thought I had an issue with the lens. Real world pics just weren't coming out too sharp. And when I used a focus chart, at a small distance, it did seem to be a problem. However, the minimum focusing length for this lens is 1.5m, so I put up a piece of A4 at 10m, and took a shot of that. Came out quite crisp. Must have been my technique that needed improving, not the lens