Charles, do you think a very loud bang causing what Phil Willis said “the reflex from the shot caused me to take one of these pictures” would only affect his camera trigger finger, i.e. so no part of his hand or wrist or arm or shoulder would shutter so there would be no blur in the picture? If Zapruder at a noticeably further distance (and sound intensity falling off as 1/r^2) jiggled his camera at the gunshots, was it because he was a significantly less stable photographer compared to what Willis could do up much closer to the noise?
For what it’s worth, an approach I looked at awhile ago related to first shot testimonies (which would be expected to have lower than average variability when interpreting for the first shot timing) does appear to agree with your comments regarding those Willis photos.
https://sites.google.com/view/anchored-first-shot-testimony/home
sites.google.com/view/anchored-first-shot-testimony/home
There are a lot of variables involved. First, unexpected sudden loud noises, like a gunshot, affect some people more than others. Personally, I am very sensitive to them. And believe that I would be affected much more than just a finger twitch. Someone else might not be affected at all and be rock steady. Secondly, photography has variables such as film speed, aperture, shutter speed, that will affect the image. Unless the shutter speed was very fast, I would expect that the limousine and occupants should be clear. And the background blurred due to the tracking motion of the camera.
I will have to review your link when I have access to a computer tomorrow. Thanks