Back and to the right?

[Joe Elliott]

https://www.jfk.org/the-collections/abraham-zapruder-film/abraham-zapruder-film-timeline/
The specs were rated 16 FPS so 18.3 is really pretty close.

The Zapruder camera did have a setting for 16 frames per second. It had no setting for 18.3 frames per second. But cameras varied from camera to camera, even of the same model. Alvarez studies of people clapping in the Zapruder film indicate that the filming rate had to be close to 16 frames per second, so it was set for 16 frames per second. However, the FBI tested Zapruder camera, not one of the same model but that very same camera, and found that at the 16 frames per second setting it actually filmed at 18.3 frames per second.

So, 18.3 frames per second is the better estimate for the film rate of the Zapruder film, not 16 frames per second.

[Gary Craig]

Does that logic hold up though? Wasn't the limo almost stationary at the time of the head shot? Some witnesses thought the limo even stopped completely.

The Limo slowed down. Based on the film I've seen it didn't stop. IMO

[Gary Craig]

Yes, but the limousine was moving at a pretty steady speed of 8 mph, with only a little acceleration, a fraction of the acceleration that we see in JFK’s head, mild as it was. The movement (from Physics) inside a vehicle moving at a constant speed are the same as if the vehicle was stationary, relative to the vehicle.

"The movement (from Physics) inside a vehicle moving at a constant speed are the same as if the vehicle was stationary, relative to the vehicle."

Unless an outside force intercedes. JFK's brain changes speed at Z-313 when a bullet(s) goes through his skull.

That same bullet(s), from the front, impeded the forward motion of his head and upper torso.
Everything else in the vehicle kept moving forward for that split second.
The combination of the loss of momentum and force from the bullat creates the head snap. IMO

[Royell Storing]

 "Head SNAP"? There was No SNAP. There was a Head EXPLOSION.

[Bill Chapman]

 "Head SNAP"? There was No SNAP. There was a Head EXPLOSION.

A head snap preceded the explosion of the head

[Royell Storing]

A head snap preceded the explosion of the head

   What you guys are referring to as a "SNAP" is actually a very Minor "Tilt" or "Tipping" of JFK's Head.  A tree limb "SNAPS". Or, "You can "SNAP" Into a Slim Jim. Oh Yeah"! We see Nothing close to this with regard to the head of JFK.

[John Iacoletti]

The Zapruder camera did have a setting for 16 frames per second. It had no setting for 18.3 frames per second. But cameras varied from camera to camera, even of the same model. Alvarez studies of people clapping in the Zapruder film indicate that the filming rate had to be close to 16 frames per second, so it was set for 16 frames per second. However, the FBI tested Zapruder camera, not one of the same model but that very same camera, and found that at the 16 frames per second setting it actually filmed at 18.3 frames per second.

So, 18.3 frames per second is the better estimate for the film rate of the Zapruder film, not 16 frames per second.

How would Alvarez know how fast people clapped?