Film Camera & Projector Fundamentals

Sprocket Holes - Sprockets - Film Gate - Claw - shutter - douser

In the most simplest of terms, a film movie camera is just an over-glorified still camera that takes a series of still photographs in rapid succession.

Above is a Cine Kodak Model A  16mm   Home Movie Camera
It is hand crank operated and cranking at about 2 revolutions a second, yielded about 16 Frames/sec
Released in 1923

By 1932, spring driven motors started becoming popular for the home movie enthusiast
Later came electric motors - and the rest is history.

Whatever the drive mechanism or level of sophistication, all movie cameras operate basically the same...

First to cover is our perception of motion....

Perception of Motion

If we hold a movie film up to a light, we notice that it is made up from a series of still images...  Likewise in a video camera that has a CCD sensor, the information is read off (scanned) line by line and the lines combined to form a still image.  A series of still images played back in rapid succession is perceived as motion.  That is universal not only to all video cameras, film cameras, but also to the human eye.  Though a bit more sophisticated, even our eyes send a series of still images to our brains to be processed, and it is our brains that give the sensation of fluid motion... How the eye is configured and how the visual information is sent to our brains is rather fascinating.  There are some notable differences...  The image data our brain receives via the optic nerve are not still images as a whole, but are formed by an array of photoreceptors that transfer a series of stills on an individual photoreceptor basis..   That is quite different than saying our vision is made up from a series of still images.  The way it works is that the fluid images are made up of individual stills from each photoreceptor...  Since one fiber can connect  from 5 to many thousands of photoreceptors,  each "bit" is a still, but forms a fluid motion when combined all together..

The Human Eye

The human eye is a marvel of engineering.  No camera, no matter how sophisticated can match the dynamic range and color balancing feats that our eyes and brains do so easily...  It functions somewhat akin to a digital camera  - at least in basic concept... 

Like an 8mm home movie camera, light is focused by the lens onto our retina.  The retina is made up of millions of photo receptor cells.   You could think of them as pixels for a CCD. The amount of light reaching the retina, is controlled by our pupils which performs the same task as does a camera's iris.  They both open to allow more light, or contract to reduce the amount of light...    

In both a film camera and a video camera there is a shutter.  For film cameras, the shutter is a physical one, and for a video camera, the shutter is an electronic one..  The purpose of the shutter is to reduce the apparent movement to form a still image.

Our eyes also have something akin to a shutter.  Not a physical one obviously, but rather in how the information is read from the retina's photo sensors and transmitted down the optic nerve.  The optic nerves is made up of approximately 1.2 million nerve bundles that connect to approximately 125 million photoreceptor cells. 120 million of them are the rods responsible for the luminance information, and 5 million are the cones that supply the color information...  A small area near the center of the retina is called the fovea. It is here where we have the highest density of photo receptor cells and thus the area of the retina that has the highest visual acuity.   In this area, one optic nerve fiber connects to as few as 5 photo receptor cells...   Outside this small zone, we have far less visual acuity...   One optic nerve fiber outside the fovea, connects to many thousands of photo receptors..   That is why to be able to read anything which requires greater visual acuity, we have to be looking right at the subject  to focus the light onto the fovea part of the retina. There is not enough detail outside this small zone...   As a photon of light hits a photoreceptor cell, it triggers a chemical response that results in a small voltage being generated.  But here is the key point; there must be a way to erase that cell's response and return it to black to get ready for the next photon.  If there was no way to erase the past event, all we would receive would be a bright white image..   In any event, the biochemistry of how this works is well known and it's fascinating delving in to how it works.  But after all, this is not a page about the human eye, but of movie film.  The point is that the recharge time acts as a chemical shutter so every signal received by a photoreceptor cell is effectively a portion of a still image.. Unlike a video camera, the image is formed in the brain when all these "pixels" are sorted by our brains to form an image..    You could almost think of the optic nerve as being akin to a coherent bundle of fiber optics only that they are nerves.  I digress: but it is quite a feat of engineering...

Thus Information is read from the retina's photo receptor cells at a certain rate...  You could think of it as sampling...   Unlike video cameras that sample and scan line by line, our eyes send all the information in a parallel manner down the optic nerve, each fiber sending information as fast as it can.  Here is the key point.  when a photo receptor cell or group of cells generates a signal on it's optic nerve fiber at that instant the image is effectively frozen, because every photoreceptor cell only generates a small voltage when a photon strikes it....   While the photoreceptor cell is being erased to establish a new black level, there is no voltage output....  When the brain receives all the information from all the photo receptor cells, it basically has assembled a still image..    Why we don't see flicker is because all the 1.2 million optic nerve fibers are not all clocked  at once..   Every photoreceptor cell operates independently of the others so from a macro point of view, there is always something changing at any given time.  Our brains can parallel process all the 1.2 optic nerve fibers all at once although each photoreceptor cell captures a still and runs independently of all the others..   It makes movie film and video look pretty crude...

What is interesting, is that when we "pan" our heads while keeping our eyes locked and focused straight ahead,  we are unable to discern any detail...  It becomes a blur...  To demonstrate the phenomenon, fix your gaze straight ahead and pan your head...   It literally becomes a blur..  We don't think of it, as it is natural...   The key point is that whenever we pan either our or are in the process of shifting our vision, there is very little detail...  Engineers have taken advantage of that, especially when compressing video.  We are well accustomed to not seeing detail in objects that are in motion...

Still frame and step through any video that has been compressed.  You will quickly observe there is no detail in whatever is moving..  In fact it's quite blurry.   Yet when we are watching the movie, things appear sharp..   We don't even notice...   The reason being that losing detail in the object in motion simply normal for each of us.   We don't even notice or give it a passing thought...

This actually works out quite well when we pan a camera. In the pan the movement is often a bit blurry due to the slow shutter speeds..  But not to worry too much about it, as our own vision is unable discern  much detail for items in motion anyways unless we try and track the object with our eyes to minimize the apparent movement...

Movie Camera Operation

The first movie cameras operated not much differently than today's most sophisticated PanaVision ® cameras. Basically the lens focuses beams of  light, the amount of which are controlled by an iris, onto a strip of film held flat in the film gate. The film gate is nothing more than essentially a flat place with a rectangular hole in it to allow light to pass through that also has a pressure pad to hold the film flat while it is being exposed...

When the film is in position, stopped and stable, a shutter opens and allows light to expose the movie film.  Most shutters amount to slits on a rotating disc placed between the light source and the film...  Once the film has been exposed the shutter wheel continues to rotate and the open slit thus rotates out of the way effectively blocking any light from reaching the film..  An this point, the silver halide crystals have been excited in the film emulsion and now retain a latent image on the film...  At this point, we have exposed one frame of movie film..

What happens next is that a claw inserts itself into the sprocket hole and advances the film ahead exactly 1 frame, since every frame of film has it's own sprocket hole.  Once the film has become steady and stopped in the film gate again, the process repeats..  

Meanwhile the motor is driving the sprocket wheels at a constant speed commensurate with how many frames per second the movie is being shot at.   That is one smooth motion. To keep the film advancing smoothly where it repeatedly has to stop and start in the film gate repeatedly, two loops must be formed by the film: One just before the film gate and it's supply sprocket wheel, and the other loop immediately after the film gate just before the exit sprocket drive wheel.   These two film loops are critical, as they act as a buffer and a free reservoir of film...   Thus the film travels through the projector at one smooth constant speed until it gets to the film gate where it repeatedly stops and starts.  Without the loops, the entire film and reels would have to start and stop transporting the film 16 to 24 times a second...  Just the mass alone of the reels would make that mechanically  impossible at frame rates sufficient enough to convey the sense of fluid motion...  There are different types of claw designs, but each is effectively that inserts itself into a sprocket hole and pulls the film through the gate into perfect position.  (or at least ideally perfect position)...

That is all there is to how a film Camera operates.  The basic concept is pretty simple and straight forward.

Standard Movie Projectors

Movie projectors are not all that different than movie cameras.  It's just that the process is reversed a bit..  Instead of focusing light on the film to expose it, light instead is passed thru the film and then focused by a lens onto a reflective screen.  Just like the camera that exposed the film one frame at a time, the projector does exactly the same... Once the film stopped in the film gate, the shutter opens that allows the light to pass through the film then through the lens that focuses the single frame of film onto the screen...  The shutter then closes at which time the film is advanced one frame. Once that frame is stopped and stable in the film gate, the shutter opens and the process repeats. Thus the transport of the film thru the projector is essentially the same as in a movie camera.  It has a similar film gate, claw to advance the film to the next frame and a shutter, but this time the lens is used to focus the image onto a screen.  Whether one of the first vintage hand cranked movie cameras or the most modern and sophisticated PanaVision ® professional 70mm camera, the basic principal of operation is identical..