Suggest you try the search function for extenders or teleconverter - this subject is discussed weekly - it's been addressed on three different threads in the last week or so. There are a variety of opinions on the subject (as you will see), and it varies as to whether you use a zoom or prime, the manufacturer of the extender and whether you employ a 1.4 or 2.0. But to answer your question, it depends on the lens and the teleconverter. In your case, Canon has published MTF charts on the 100-400L MK2 with the Canon 1.4 extender, and the results show a very small degradation in IQ of roughly 5%, which I find very acceptable. You didn't mention which version of the 100-400 you have, but we have at least one member who uses the MK2 version with 1.4 and 2.0 Canon extenders and produces superb results. I use the 100-400L Mk1 with the 1.4 MK2 extender with excellent results, even when enlarged to 16x24. Here are the MTF charts for the 100-400L MK 2 with and without the 1.4 extender so that you can decide for yourself. (I've posted these charts 4 times in the last 2 weeks, so I'm sure some are tired of seeing them) Personally, I would not go to the 2.0x with the Mk1 version of the lens, but as I mentioned, others with the MK2 lens and mk3 2.0x extender have great results. Another consideration is whether your 7D MK2 will autofocus with the extender. I believe that the 7D2 will autofocus at f8, but not beyond, so you would have autofocus with the 1.4x, but not the 2.0x. Hope this helps a little.