I already replied to this thread but it got wiped out by the LNers because I was getting too close to home. 
Chris Davidson is on the right track. There are only 2 questions needed to be answered to resolve this one:
1) Where is the "original" Z-film? Life got a COPY.
2) Why was the film "edited"?
There were at least 2 splices in the film, which the FBI must have created and they removed an entire section of the turn onto Elm. Zapruder said he never recalled letting his finger off the trigger at any time after he started filming. So why was this footage removed? And why are there splices in the film? And most importantly, where is the original film and why did the FBI keep it?
Any editing done to the film to remove contradictions to the LN scenario would have been done with an optical printer like they used for practically all movies at the time for FX. This would include:
1) Speeding the limo up by removing frames before and after the head shot. Easy peasy.
2) Blackening the back of JFK's head for a few dozen frames. Piece of cake.
3) Cut out the turn onto Elm where the limo barely navigated the turn. Just another splice, which they botched.
So to ans the OP of how long does it take to alter hundreds of frames of film is...a few hours max. Which is exactly what they did. Otherwise, why did they modify it at all?
I already replied to this thread but it got wiped out by the LNers because I was getting too close to home.
This is what I was talking about, delusions of self grandeur are rife in your community.
Any editing done to the film to remove contradictions to the LN scenario would have been done with an optical printer like they used for practically all movies at the time for FX.
This was explained to you in the other thread, as soon as images are combined in an optical printer you have a degraded image with additional layers of grain. The special effects in movies at the time were locked off matte shots or blue screen work and not suitable for Zapruder type alterations.
The following two frames were filmed directly on set but the robots fighting picture was processed with multiple passes through an optical printer for added special effects like lasers and starfields which creates a much softer grainier image whereas the Zapruder film was just exposed once on the correct film stock.
The Zapruder frames reveal no build up of Film grain and thus the film was exposed in Zapruder's camera.
1) Speeding the limo up by removing frames before and after the head shot. Easy peasy.
As soon as even one frame is removed you introduce an obvious stutter but throughout the Zapruder film is silky smooth. In the following gif only one frame was removed and the resulting lack of fluidity is plain to see.
2) Blackening the back of JFK's head for a few dozen frames. Piece of cake.
You do realize that blackening one frame requires the exact correct pigment of black to match the surrounding level of black and then when you start a series of frames then you have to consider how your black is affected by the movement and constantly changing lightsource that have a direct effect on your black, so in other words without advanced computer cycles to calculate the various intensities of black you're not going to provide anything close to photorealistic.
3) Cut out the turn onto Elm where the limo barely navigated the turn. Just another splice, which they botched.
Or maybe because an 8mm camera only takes a limited amount of film and Zapruder had already used some of the film so was simply saving film for Kennedy.
JohnM
