Photographing wildlife - particularly birds - you never seem to have enough lens...ie magnification for far off objects. So the temptation is to add extenders, in this case a 1.4x, so here is a little test for me to see if the quality and reach is worthwhile. See what you think, with this night heron in Shatin Park, and similar crops on the images.
First shot, 700mm f11
This brighter shot above is at 500mm, f7.1
This big crop of the 700mm shot appears to win, but actually I can see some loss of details in the feathers and also some blown out highlights, and it is definitely more contrasty.
The 500mm does a great job allowing for lower ISO and an overall brighter image with more detail.