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M Vers wrote:
In addition to the above, for your enlightenment, not all TCs are designed with a protruding element and, believe it or not, not all TCs are made by Canon. Such TCs can be used with most lenses without any modification or use of extension tubes.
The reason the high-quality teleconverters/extenders from Canon and Nikon have the protruding front element is to minimize vignetting with telephoto lenses.
The cheaper third-party teleconverters may fit more lenses, but all of the ones I've tried (Tamron, Kenko, Sigma) vignette rather badly on longer lenses.