[scoundrel1728]

I can understand why teleconverters are less likely to be sent in for alignment:

1. Optically, they are much simpler, with perhaps 6 elements instead of 15. These devices are thus much simpler to build, align, and test.

2. No moving parts to speak of, so they can be mechanically rugged. No focusing or zooming rings or moving internal elements. This also contributes greatly to their ease of manufacture and testing.

3. Confounding of the lens' failure as opposed to the teleconverter's. If a bad image occurs, is it the teleconverter's fault or the lens'? And is it a problem with all teleconverters of this particular design or is this just a defective copy? This isn't an easy question for the consumer to answer, who would probably chalk up imperfect perfomance to the design rather than a manufacturing defect, or perhaps even blame the lens the teleconverter was being used with.