A huge front element, not hidden especially deep inside the barrel, inner focusing and inner change of focal length, along with as many as 20 elements positioned in 15 groups mean the standards are raised very high when it comes to damping down ghosting and flares. Such a high number of groups means as many as 30 air-to-glass sufraces. Even if the producers employed coatings of the best quality, which lose just 0.2-0.3% of light on one air-to-glass surface in the wide range of the spectrum, it still would mean that, overall, the whole optical construction might lose about 10% of light, and you should also take into account light absorption in glass.
What's more, the situation gets even trickier after adding the teleconverter which consists of 7 elements positioned in 3 groups. That way you get six air-to-glass surfaces more and of course more chances to catch light reflections in your photos.
As you see the scale of the problem was serious and, taking it all into account, you have to admit the sample shots, shown below, are not that bad. We are also hardly surprised that the weakest performance you get after attaching the TC 1.4x III.
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When you compare the tested Canon to its direct rivals you find out it fares much better than the Tokina and slightly better than the Nikkor and the Tamron.
