[davidmanze]

With MFD of four and six meters don't expect a good magnification
However who wants to use a 800mm lens for macro?
Dave correct me if I am wrong but in real life, framing a bird at 800mm within seconds is a very difficult task forget about framing it in flight...

  The idea of these new lightweight telephoto lenses is they are easy to carry ....... so it will be aimed at photogs that are out taking nature shots  ....... and coming acroos butterflies and the like ...... unfortunately these new lenses 1:0.14 ratio won't cut it for critters that small ..... 
 ....... they are limited usage lenses ...... especially shooting at high shutter speeds when images will get noisy.



  Ah real life .... yeah that's where we want to be ..... never mind the spec sheets .....

 I'm trying to get closer images of the "lesser kestrel" at the moment ...... which are nesting under roof tiles in a village 30 kms from here ...... they fly very fast wheeling and catching dragonflies ..... so I resort to a tripod and gimbal!!
....... 600mm hand held on APSc is possible but difficult ...... 800mm would be next to impossible. ..... just finding this aerobatic bird in the OVF and tracking it results in very few keepers. ...... however less erratic bird flight patterns are perfectly possible.
 ........ this is where the wider field of view of the D850 would come in useful preventing wings being chopped off with little other penalties .....