What exactly are MTF charts? What do they represent and how do you read them?

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml
http://www.luminous-landscape.com/tutorials/understanding-series/lens-contrast.shtml
http://www.luminous-landscape.com/tutorials/understanding-series/und_resolution.shtml
http://www.luminous-landscape.com/tutorials/sharpness.shtml
http://www.luminous-landscape.com/tutorials/sharp.shtml

From what I now understand about the MTF charts (thanks for pointing me to them) then when used on the smaller digital sensors, we can ignore the right hand edge of the diagram, because of not using the full 35mm area.
I think with a 1.5x crop factor you'd only be using up to 0.6 on the horizontal avis, which may avoid the fall off in both resolution and contrast on some lenses.