From: gaer@fellini.harvard.edu (Art Gaer)
Newsgroups: rec.arts.movies
Subject: Re: 24 frames a second: technical query
Date: 12 Jan 93 22:55:49 GMT
Organization: Harvard University, Statistical & Data Analysis Center

In article <1993Jan7.033153.16652@Princeton.EDU> spencer@phoenix.princeton.edu (S. Spencer Sun) writes:

>In article <C0Gr6q.FHJ@ccu.umanitoba.ca>, feld@ccu.umanitoba.ca (Michael Feld) writes:
>>Some one told me once that a film screen is lighted for less time than
>>it is non-lighted, during the showing of a film; that the blank,
>>black, spots that separate the individual frames occupy the screen for
>>more time than the lighted frames do.
>>
>I've been told by someone that I consider fairly reputable that each
>individual frame is actually projected TWICE... i.e. for each frame
>the shutter opens twice.
>
>Actually, it's a disk with pieces cut out of it, so that as the disk
>spins, it passes or blocks light.  So, for each frame, the shutter goes
>pass-block-pass-block and then the film advances in the second block
>period.

Yes, the image is blocked (at least) twice, once for the frame
pull-down, the other to increase the flicker rate.

The necessity for this is because the human threshold for noticing
something like film flicker (there's a better technical term that
escapes me at the moment) is about 40 times per second.  Only block
the frame during the film pull-down and you're only flashing the image
24 times per second--quite noticeable.  Do it a second time while the
film is in the gate and you're up to 48 times per second and therefore
can't see the flicker at all.

Actually, dual bladed shutters are something of a compromise.  Silent
projectors had triple bladed shutters: since the silent film speed was
supposedly 16 frames per second, multiply be three blades and you're
up to the same 48 flashes per second.  However, triple bladed shutters
were pretty much standard until the advent of wide-screen films.
I don't understand the optics well enough to be precise about this,
but apparently the more sharply the light is cut off going to the
lens, the sharper the perceived image is, and triple bladed shutters
were able to have a sharper angle of cut-off.

When wide-angle and CinemaScope lenses were added to projectors,
however, it was necessary to increase the amount of light pumped
through the film frame, since the same size image was being spread
across a much wider screen area, so dual-blade shutters (which allowed
more light through) were substituted, sacrificing image sharpness (a
much gentler angle of cut-off) for image brightness.

Actually, many projectors at that time (the early '50s) had a *second*
shutter in front of the lens to help ensure the sharp cut-off of the
projected image.  It was necessary to remove this outside shutter in
order to be able to fit the much larger CinemaScope lenses onto the
projector head.  In the process of modifying the shutter system, a
dual bladed shutter would be substituted behind the lens.

I don't remember the exact angles, but I believe the actual image
on-screen is actually dark for a longer amount of time than it is
illuminated.

----------

Art Gaer
Unix Systems Analyst    gaer@sdac.harvard.edu    (617) 432-2521
Statistical & Data Analysis Center, Dept. of Biostatistics 
Harvard University, School of Public Health



Newsgroups: rec.arts.movies