Helen, I'm sure you are correct in differentiating the two as various forms of vignetting, but in reviews of lenses for the average person this is normally referred to as light falloff. In Ken Rockwell's review of this lens, (BTW, I have been looking at this lens to buy, that's why I know where these reviews are) he says:

f/1.4: Spherical aberration lowers contrast. A lot of coma in the corners making them quite soft. Falloff

In fact he recommends to bypass this lens altogether and get the 1.8 instead.

Paul B.