Hi,
For macro ports it's easy to know which one to use: take the (real) length of your teleconverter... it gives you the minimal length of your extension ring !
If your kenko 2x teleconverter is the MC7: you'll need the EXR-33 - with the Kenko PRO 300, it could be the EXR-40 but the EXR-50 will be fine if you want to add a filter in front of the lens.
Claude
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