"It is my belief that, where possible, it is better to put the left hand on the far end of the lens, between the hood and the focusing ring (but keeping the hand off the focusing ring!), or on the hood/lens "joint""
Thanks Neil: that absolutely makes sense!
As I now see it, putting your hand above the foot of the lens (i.e. the centre of gravity of the lens/camera) can help if the main problem is that your tripod vibrates, which it does somewhat, and is the reason a tripod with little or no flex is desirable. But the other problem, which a gentle hand pressure can best help with, is that the lens is long and heavy, and its (mostly vertical) vibration is largest at the far end.
If I am right, would you agree that the type of Long Lens Support made by RRS would be a real help?
Photography is a way of shouting, of freeing oneself, not of proving or asserting one's own originality. It's a way of life. (Henri Cartier-Bresson)