This is an "advanced topics" tutorial, on par with the existing article about diffraction.
Warning: this tutorial can at times get technical
Camera Lens Quality: MTF, Resolution & Contrast
It gives an overview of concepts relating to lens quality, contrast and resolution, in addition to how to read and compare MTF charts. There's also a little on the "sweet spot" of a lens when it comes to sharpness/contrast. It's quite long compared to the last couple I've added, so there's lots to read...
As usual, any corrections/feedback/suggestions/etc are welcome.