Kenko TC vs Sigma rear element
Sigma warns against using a TC with the Sigma 50-500mm lens at focal lengths less than 100mm. These images illustrate why this is a problem. At 50mm zoom, the Sigma lens' rear element projects rearward into the TC sitting behind it, resulting in a collision of glass.
The Kenko TC does not project as far forward as the Sigma TC, however.
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