The Sony a6400, .........
I think that it will be long enough for
a distance of 20-50 feet.
Where "1X" is a FL equal to the long
side of the format, a 1X lens has a
FoV equal to the subject distance.
The long side of APSC is about 24mm
so a 135 is about a 6X lens. The FoV
will be 1/6X of the subject distance.
At 42ft distance, thaz a 7ft FoV. IOW
if you need a portrait, it will be a full
body portrait, subject standing up.
Sooooo, the 135 is too short at 42ft.
At 21ft, the 135 frames a half-body
portrait .... prolly just right with a bit
To get that same half-body framing
at double the subject distance, just
double the FL. IOW this is work for a
100-300 zoom, with minor cropping.