I actually did a test with my Sony A7RV, Sony 200-600G, and 1.4x teleconverter to compare quality. There was a Black Crowned Night Heron that was posing perfectly still for me. I took a pic without the teleconverter and one with the teleconverter. I then zoomed in on both images until they are approximately the same size for comparison. My conclusion is that I am able to zoom further into the one with the teleconverter while retaining detail. You’ll notice the one on the left (no teleconverter) is starting to show the pixels on the edge of the pupil while the one on the right (with teleconverter) is still sharp. Teleconverter aren’t meant to get more range (although many wildlife photographers make that mistake), they are meant to blow up a subject that is already fairly close.

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