Mark K wrote:
I carried my 100-400L out this morning. Even in the dim dawn light.....the iso was pushed up to 12800 and I kept thinking, what if the f stop was as small as 6.7
So what? Why are you comparing a $1700, 100-400 to a $1070, 150-600? Your 100-400 is f5.6 at 400mm. The Tamron gives you the extra reach of 400-600mm at f6.3, not 6.7.
Use a tripod or shoot in brighter light.