Modulation Transfer Function
The MTF is one of the best ways of decribing a telescopes quality. It shows the difference between a perfect telescope (black line) and the telescope under review. The X-axis shows the resolution going from 0 to the maximum a telescope can discern (depending on aperture).

STREHL 0.8
The above PSF/MTF shows a telescope (red-line) with 0.25 L.Spherical aberration. Notice that the peak of the PSF is at the diffraction limited level (0.8 strehl). The below image shows more or less the same PSF with also a peak at nearly 0.8. Still the MTF tells a major difference the lower telescope performs better at higher resolution. That is due to the fact that the lower telescope had a 30% obstruction (no other aberrations)