I want to point out this excellent background document on MTF, written by a Dr. Nasse from Zeiss. The document is full of rich information, you really should set aside some time to read it. The introduction on the point spread function was a very nice way to start.
I won't summarize the document, I think it's already very dense as it is, but I did find two surprises inside which I'll relay to you.
Surprise #1: wide-spectrum MTF versus monochromatic MTF
The two charts above pertain to the same lens, a 300mm telephoto in this case. The chart on the right looks only at monochromatic green light, while the one on the left uses the spectrum of white light. Among other things, this makes comparing Zeiss MTFs with other manufacturers very difficult. Imagine the case where two lenses appear to be close competitors but their MTFs use different light spectra, one of them would be far superior to the other.
Surprise #2: wavelength-dependent MTF
This chart says two things. The first one is that the distance to the projected plane of focus is wavelength-dependent, i.e. the red image doesn't form at the same distance as the green image. This is normal for all lenses that aren't apochromatic (APO). I knew this, and it is a concept I observe in daily shooting with fast telephotos.
The second thing is that the contrast of the lens itself is dependent on the wavelength, and the contrast of white light (the black line) follows the lowest common denominator. This is the same as saying that if you could find a B&W sensor camera and mount a green filter in front of it, then the useful resolution of the lens could basically double with all else being equal.