I thought people might be interested to see this graphic of a set of MTF (modulation transfer function) measurements. I got it from Manfred Lai's Gallery. What's interesting about it is that by linking it to a representation of the 35mm frame, the illustrator makes it more clear than usual what the MTF chart is representing.
On this chart, they call image height "distance from center," which is more intuitive, and, helpfully, they distinguish the distance from the the optical axis ("center") to the edge of the film, 18mm, and the distance to the corner, which is 21.63mm (the hypotenuse of a right triangle with sides of 18mm and 12mm). On both graphics here, the difference between the two is a bluer, less purple color, allowing you to see clearly what part of the MTF chart records the transfer performance into the corner of the frame. You can see why the corners are often considerably less sharp than the edges.
The word "radial" for sagittal also helps a bit—this is the line that shows the contrast transfer for line pairs placed parallel to the radius in the test target.
Another point about MTF not often acknowledged is that to really describe a lens, you would need charts for several apertures and several distances. As far as it goes—giving information for 10 and 30 lp/mm at infinity and ƒ/2.8—this particular chart describes a very good lens indeed, although one important piece of information is missing—just what lens it is!
Featured Comment by Jan: "If you're into Canon lenses and MTF charts, the book EF Lenswork III is a great resource. It not only describes the design and intended segment of each lens Canon makes, but also has MTF charts for all of them, including wide-angle and zoom variants for zoom lenses."
Mike replies: I second Jan's recommendation. The section on optics in EF Lenswork is one of the best short treatments of the topic I know of, and I've seen a lot.