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THE GREAT BLANK SCREEN MYSTERY by S.P. Rocket

"You've gotta be kiddin'!" My friend Jack was indignant.

"Are you trying to tell me that every time I go to the "flicks", if it's three hour programme, for 45 minutes of that
time, I'm looking at a blank screen?"

"That's right Jack!"

"Argh! If that was true, I'd be entitled to a quarter of my money back!"

"Well - it is true, so you'd better start asking."

Jack is a good mate, but not mechanically minded, unless it is to do with the amount of energy required to lift
a glass of Fosters from the counter to his lips. So I knew it was really an impossible task to explain the Great
Blank Screen Mystery to him. I dropped the subject.

But I'll do my best to explain it to any of our Cinemarecord readers who don't already understand the basics of
film projection, and the strange mental faculty that we all possess, called "Persistence Of Vision~~.

Persistence Of Vision
Did you know that when you look at something, and then shut your eyes, for a fraction of a second you can still
see the image? That is called Persistence of Vision. If it were not for that phenomenon, we could not enjoy
motion pictures or television. (Did someone say, ''We don't enjoy the television much anyway!}

Let's get down to basics. If you look closely at a length of movie film, you will see that it is made up of a number
of small pictures, all of which at first glance appear to be the same. But they are not the same. Normally, each
picture differs slightly from the one preceding it and the one following it.

The Camera
The camera which has photographed the scene or action has taken 24 separate photographs every second on
a roll of film which remains still in the camera while the photograph is being taken, and then moves on quickly
to allow the next picture to be photographed. While the film is actually moving in the camera to expose the
next frame, a shutter blanks off from the film the image the lens is seeing.
So we finish up with a roll of film containing a succession of tiny pictures measuring approximately 30mm x
22mm, in normal 35mm professional cinema format, each picture slightly different from its neighbours.

The Projector
A movie projector, no matter what size, is virtually a very sophisticated 'magic lantern". The important principle
behind the production of a moving image is that the film does not move continuously through the camera or
projector.

The film moves intermittently ... Move - stop - move - stop - move - stop. It stops and moves twenty four times
every second. When it is moving, a shutter masks the projector lamp or light source from the film. If the film
moves ever so slightly while the light is passing through it onto the screen, all we see is a blurred picture.

So the cinema screen is receiving Picture - Blank - Picture - Blank- Picture - Blank twenty four times a second.
In other words, the screen is receiving and reflecting a series of "rock-still" images. As it takes approximately
a 1/4 of each "cycle" to move the film down in the Projector Gate to the next frame, that means that for a 1/4
of the total time of each frame, the screen is black or blank.

But, because of our inherent Persistence of Vision, our brain tells us that the picture is still on the screen. What
a deceiver is our brain! By the time our Persistence of Vision has faded, the next frame of the film is now being
projected onto the screen- and we think we are seeing a continuous "moving picture".

But what we are really seeing is a succession of "rock-still" images on the screen, each image separated by a
black screen which our convenient but deceitful brain tells us is a continuation of the image we have just seen.

So, in a three-hour film, for approximately 3/4 hour, you are looking at a black or blank screen.

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