In a studio setup with human subjects that are tame and listenand accessible a lens in the 50mm to 135 mm is perfect. however unless you are doing portraits of elephants at the roadside lenses of that magnification will let you down.
Yesterday I was lucky enough to have 2 young male lions in the road in a very quiet part of Addo. The nearest one was slightly downhill at 20 metres (according to my camera) The pics I took with the 70-200 I viewed on the LCD were not good as they were taken top down from my position. I then changed to the 500 and this changed everything ..The background was nicely blurred and the distance compacted to such an extent that it no longer appeared top down but with a far better impact impression of the subject being at Eye Level. Sure I cut paws off etc but this was Head shot portrait instead..I was also blessed with lovely dappled light early in the morning
