Originally posted by Mindflux It'd be nice to have for sure, but as I stated above.. does it matter that it's on the shutter release since a half press is required for SR ? (assuming you use SR).
I think it does.
Here's my
very layman attempt to explain the physiology behind it...
When you flex your index finger your wrist wants to bend ever so slightly. This is because the ligaments between the fingers and the muscles in your forearm that cause them to move run through the wrist and up the length of your hand. The tension from the slack being taken up when your muscles contract causes the wrist to bend. Your palm moves the camera body and your index finger's pressure on the shutter amplifies it. In contrast, the ligaments of the thumb run down the side of your hand and flexing actually causes the wrist to want to bend up slightly, but gravity and pushing the AF button counteract the movement.
As such even the tiny amount of flexing to depress the shutter 1/2 way introduces lateral movement of the body/lens. It's no different really than shooting a rifle on the range, by applying slight pressure to the trigger as you train your sights on the target you flex the muscles of the forearm that move your index finger so that when you squeeze off the round there is no additional wobble induced.
When I'm shooting landscape as I'm composing the shot my trigger finger begins to apply slight pressure on the shutter button. This means that I can then focus (more than once) using the AF button without inducing wobble and when I like the shot I depress the shutter button nice and smooth.
In portrait orientation with the grip you can not do this. Each time you release/depress 1/2 the shutter you introduce wobble as that long muscle in the forearm contracts and relaxes. And each time you let up on the shutter button it takes approx 1/2s to reinitialize the SR.