I'd use FoCal pro. Even with a manually focused lens like the 15mm, you will be able to analyze the sharpness at various apertures. This is often a indication of issues, if the lens does not perform as expected at various apertures, and it will do a great job on the 24-70L as well. They keep adding more analytical tools to FoCal, so its a great tool to have, and gets better. Its only been for sale since the first of 2012, but its already very popular.