I was out shooting early this afternoon, and came across this Mallard enjoying today's warm weather and a little swim. I have rented (from BorrowLenses.com) a Canon Extender EF 1.4x iii for a upcoming road trip and thought I better test it out ahead of time. Using the extender with a Canon 80D, 100-400 ii lens, tripod, shutter release, at f/8.0 and 560mm, the images were unfortunately not as sharp as I require. I tested this combination on some blackbirds, pelicans, woodpeckers and cardinals with the same obvious softness.
However, backing off to approximately 500mm, the sharpness improved as seen in the image below. At 480mm, the sharpness was equal to the 100-400 lens alone. Hopefully, the extra 100mm achieved by this extender, over my 100-400 lens will be sufficient for the upcoming road trip. I will lock my 100-400 with the extender at 500mm and hope for the best.