From crop to full-frame & expanding my collection of lenses. Help requested

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The Canon Extenders are not compatible with the 17-40mm. From Canon's specifications page for the 2x Extender:

"... is only compatible with fixed focal length L-series lenses 135mm and over, as well as the EF 70-200/2.8L, EF 70-200/2.8L IS, EF 70-200/4L, and EF 100-400/4.5-5.6L."

Same goes for the 1.4x Extender. They both have a protrusion that prevent them from being mounted on other lenses.

Thought so... although, that wording implies it can be used on the 1200mm monster?

Just read a review today that says you can use the 1.4 and 2x on the 1200mm.