ed rader wrote in post #18607067
rarely am I happy with a TC. I guess it comes down to expectations or threshold for pain. usually cropping works better for me or crop camera. I will use my 1.4 III with 80d and 100-400L II for shooting the moon tho.
I will voice a counter opinion to above, that it all depends upon WHICH LENS is being used with the telextender. For many years photozone.de did testing of Canon 70-200mm zooms in various incarnations, with and without 1.4x telextender, and published MTF ratings for the tests. What they found was that the Canon 70-200mm zooms all performed remarkably well with the 1.4x telextender, and the MTF values for 200mm vs. 200mm+1.4x only fell by about -10%. Considering that pixel zoom (cropping) to achieve 40% increase in subject size in frame was -40% in pixel count (in one direction) vs. the -10% loss of detail resolution via optical magnification increase, the 1.4x telextender was a good trade-off compared to pixel zooming.
Nearly a decade ago, I did some tests of my own to look at subjective differences, and found that subjectively one could not 'see' a loss of 10% in detail resolution. I altered camera position to keep the subject size the same from shot to shot, and these are CROPS of the full area of the frame, and these shots were posted on POTN previously:
Canon 70-200mm at 200mm FL...
Canon 1.4x on 70-200...
Canon 1.4x and Kenko 1.4x stacked
teleconvertors on 70-200
Some lenses, like the 100-400mm old version have been shown to suffer with the simple addition of a FILTER, which is why I point out the results vary by lens.