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<blockquote data-quote="Clovishound" data-source="post: 844911" data-attributes="member: 50197">It looks like you may have the same issue, as the extension tubes have a rectangular mask that may interfere with the element of the converter. It's possible there is clearance, but I wouldn't count on it. Even if it did have clearance, you would likely have to be VERY careful when mounting the tube to avoid damaging the converter. Even if this worked, I doubt you would be happy with the results. The 28-400 has an aperture of F/8 at 400mm. You would lose 2 stops with the 2x converter and end up with F/16 as maximum aperture. I seriously doubt that your autofocus&nbsp; would operate at F/16. The other issue is a 2x converter will degrade the image quality of a lens that is already suffering from a high zoom ratio design to begin with. <img src="https://static.bhphoto.com/images/multiple_images/images500x500/1589285785_IMG_1356250.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></blockquote>

[QUOTE="Clovishound, post: 844911, member: 50197"] It looks like you may have the same issue, as the extension tubes have a rectangular mask that may interfere with the element of the converter. It's possible there is clearance, but I wouldn't count on it. Even if it did have clearance, you would likely have to be VERY careful when mounting the tube to avoid damaging the converter. Even if this worked, I doubt you would be happy with the results. The 28-400 has an aperture of F/8 at 400mm. You would lose 2 stops with the 2x converter and end up with F/16 as maximum aperture. I seriously doubt that your autofocus would operate at F/16. The other issue is a 2x converter will degrade the image quality of a lens that is already suffering from a high zoom ratio design to begin with. [IMG]https://static.bhphoto.com/images/multiple_images/images500x500/1589285785_IMG_1356250.jpg[/IMG] [/QUOTE]

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