Confession - I was a college administrator for four years. At most undergraduate programs, a D is a passing grade FOR AN INDIVIDUAL COURSE, however most bachelor's degree PROGRAMS require an overall GPA of 2.0 (C) for graduation (sometimes higher), and also any course with prerequisites will require a C in the previous course (but usually you can take any given course up to three times and have at least a couple semesters of "probation"). It's amazing how much damage a single C, D or F can do to an otherwise great GPA.
Graduate school is much tighter. Generally a graduate program won't award a degree to a student with a cumulative GPA of less than 3.0 (B), and not more than one repeated course. At the graduate level, if you make a C in a course required for your program, you are on very thin ice, and you won't get more than a semester to bring your grades up.
I think it's safe to say that a "bargain" grade lens (from KEH or anywhere else) gets a C grade, and an "ugly" lens gets a D/F. Would I buy bargain gear like this? Maybe, but I doubt I'd use it on a job I was getting paid to complete.