click on the thumbnail image to change  and then hover your mouse over the main image to the left to zoom in
When I previously used a third-party 1.4x teleconverter with my 80-400mm f5.6 zoom lens the results were quite disappointing to the extent it was rarely used as I found equally good, if not better, results could be achieved by enlarging the raw image the extra 40%.  However,  upgrading to a full frame sensor and 500mm f4 prime telephoto lens it wasn’t long before I began to hanker after extra reach for bird photography.  So when the brand of my camera/lens released an updated version of their 1.4x converter, mark III, boasting minimal loss of detail I succumbed.  Since acquiring one in September 2014 I’ve been quite impressed with the image quality and it became a permanent fixture with my 500mm lens as it has become with my present 600mm lens.  I first tried the converter on my 500mm lens taking a series of images of the moon varying the aperture  between f8 and f22 and then repeating with just the prime 500mm.  The best image for the 500mm was at f11 and with the converter attached was at f13,  and of the two images I felt the one with the converter (700 mm equivalent) just shaded it,  see fig 2.
fig 1 (opposite)- full frame comparison illustrating the 1.4x magnification provided by the converter . fig 2 (below) - comparison between 500 mm image enlarged an extra 40% against a 500 mm with a 1.4x TC attached.  In my opinion the 700 mm equivalent (in the lower right) provides slightly better clarity.  Hoover over the main image on the left to zoom in for greater detail.  
fig 3a - virtually full frame image of a Little Stint  4:3 crop fig 3b - same image enlarged & cropped of the Stint's head fig 3c - further enlargement showing feather details captured Conclusion - as mentioned above the converter is now virtually a permanent fixture with the 600 mm f4 lens.  Below are some recent images with the converter on 600mm lens.
fig 2       fig 3a      fig 3b      fig 3c click on the thumbnail image to change  and then hover your mouse over the main image to the left to zoom in
Review of 1.4x teleconverter
Review of 1.4x teleconverter When I previously used a third-party 1.4x teleconverter with my 80-400mm f5.6 zoom lens the results were quite disappointing to the extent it was rarely used as I found equally good, if not better, results could be achieved by enlarging the raw image the extra 40%.  However,  upgrading to a full frame sensor and 500mm f4 prime telephoto lens it wasn’t long before I began to hanker after extra reach for bird photography.  So when the brand of my camera/lens released an updated version of their 1.4x converter, mark III, boasting minimal loss of detail I succumbed.  Since acquiring one in September 2014 I’ve been quite impressed with the image quality and it became a permanent fixture with my 500mm lens as it has become with my present 600mm lens.  I first tried the converter on my 500mm lens taking a series of images of the moon varying the aperture  between f8 and f22 and then repeating with just the prime 500mm.  The best image for the 500mm was at f11 and with the converter attached was at f13,  and of the two images I felt the one with the converter (700 mm equivalent) just shaded it,  see fig 1. below. Fig 2 is a virtually full frame image of a Little Stint taken with the converter on my 500mm lens;  and Fig 2a is an enlarged section of the same image illustrating the feather detail captured.
Fig 1. Fig 2 Fig 2a