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[Nikon Shooter]

The decision has to do with two important points:

1. long focal length characteristics
2. AF performance in not too ideal light conditions

Long Focal Length
One of the many characteristics of long lenses is that the longer they
get, the shallower the depth of field — whatever distance to the sub-
ject may be — proportionally to the focal length, that is.

A second characteristic — that applies to ALL focal lengths — is that
the closer the subject, the shallower the DoF… still proportional to the 
focal length.

A third — that applies also to ALL focal lengths — is that the more dis-
tant the BG will be from the subject, the more bokeh is observed.

AF Performance
Since AF performance is contrast dependant and since contrast is best
observed in brighter than dimmer light conditions, the more light comes 
in = more speed and more accuracy may be expected from the AF.

CONCLUSION
I use my 600 ƒ4 — with the D850 permanently combo'ed together for
all wildlife shoots mainly and sometimes landscapes and others but al-
ways together.

Since one never knows what comes next — from a perched bird to a full
wingspan in flight going in whitchever direction — shooting at ƒ8 gives
me a chance to get all the bird in the sharpness zone.

But why the more expensive ƒ4? Because the AF will work better. Simply.

                                      ALL I WANT IS THAT MY
                       COMBO BEHAVES AS CLOSE AS POSSIBLE
                          LIKE A POINT & SHOOT CAMERA DOES.