lens - How do I interpret an MTF Chart?

As part of how Canon (and other lens makers) give technical information about their lenses, they supply an MTF (Modulation Transfer Function) chart. How do I read and interpret what the chart is telling me?

Here is a sample MTF Chart for the 16-35 f2.8 L II (one of my favorite lenses for walkabout photography). What do the various lines mean? What are the axes?

Answer

There is a really good tutorial that explains all the details at luminous-landscape.com.

If you don't want to read the whole article, this section covers the basics:

Here are some rules of thumb for reading a chart...

— the higher up the chart the 10 LP/mm line is (the thick lines), the higher the contrast reproduction capability of the lens will be.

— the higher up the chart the 30 LP/mm line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.

— keep in mind that the black lines show the lens wide open while the blue lines show the lens stopped down to f/8, so the closer these sets of lines are to each other the better the performance of the lens when used wide open. The very best lenses will have the black and the blue lines close together.

— generally speaking a lens whose thick lines (10 LP/mm) are above .8 on the chart should be regarded as having excellent image quality. Above .6 is regarded as "satisfactory". Below .6 is, well, below.