For many lenses, focus shift is by far the most important lens behavior in determining sharpness as a practical matter for real images. This is especially true with higher resolution cameras like the Nikon D800E, but it holds true at 22 megapixels and 12 megapixels, in fact more than most photographers realize.
Just published in Guide to Zeiss ZF / ZE Lenses : a study of focus shift with the Zeiss 50mm f/1.4 Planar vs the Zeiss 50mm f/2 Makro-Planar. This follows my in-depth focus shift study of the Canon 50mm f/1.2L.
Focus shift and what to do about it are discussed in detail in Making Sharp Images.