Film frame rate

The frame rate of the motion picture specifies the frequency at which sequential frames are captured. In a film camera, this is the number of frames per second that pass through the camera gate and get exposed. In the days of silent film both motion picture cameras and projection cameras were hand-cranked. Cameramen took pride in their ability to crank at a steady pace. Common knowledge is that 16 frames per second (fps) was the prevalent rate of cranking. But this is not exactly so. Cranking speed varied wildly between 10 and 26 fps. Cranking speed varied even across the reels of a single movie. Projection was an art in itself. The projectionist had to watch the action closely and correct cranking speed when needed. On top of that, theater owners sometimes required projection speed to be increased in order to squeeze more showings in a given time frame – a practice that could lend a slapsticky feel even to the most serious drama.

Standardization came with the arrival of sound and the need to have sound in sync with picture. And ever since, the accepted standard for cinema frame rate is 24 fps. There wasn’t anything special about this number, other than being the approximately average projection speed from a bunch of sampled theaters in 1926. Nevertheless, it has come to define some of the prominent characteristics of the cinematic look.

Shutter angle and shutter speed

The second temporally important property is the period of exposure for each frame. In a motion picture film camera the negative is not exposed continuously. A window of time, with light shut out, is needed in order for the sprocket wheel to pull the next frame in the film gate and prepare it for exposure. In a film camera this is what the rotary shutter does.

Schematic of a half-moon rotary disc shutter positioned beside the film gate

The rotary shutter is an arc shaped mirror (called “half-moon”) that rotates in front of the gate. Its pivot is placed either beside or underneath the gate. When the gate is covered, the mirror serves to reflect the image from the lens into the ground glass, so that the cameraman can see it in the viewfinder. At the same time the next frame is being loaded into position behind the shutter. When the shutter is rotated away from the gate, light exposes the negative. The exact shape of the arc defines the shutter angle. The shutter angle is specified in degrees and describes the size of the cut out part of the shutter disc.

In simple cameras the shutter is shaped as a semicircle (and the shutter angle is 180 degrees). In more advanced cameras the shutter angle (and thus the shape of the shutter) can be changed. The shutter rotates with a constant speed and makes one revolution per frame. So for the standard cinema frame rate of 24 fps that means 24 revolutions. Bigger angle means longer exposure time. Smaller angle means shorter exposure time. In photography and cinematography the exposure time is often called shutter speed, because the exposure time is the time the shutter stays open per frame. The shutter angle can easily be made small, but large angles are harder because of the need for next frame advancement.

Various shutter angles and their respective exposure time as a fraction of total frame time

There is another popular rotary shutter type, called “butterfly”. It consists of two segments (90 degrees each, for 180 degree shutter angle) positioned opposite of each other. Butterfly shutters rotate at half the speed of a half-moon shutter. That is, a full revolution exposes a couple of frames. Panavision Panaflex cameras usually have this design.

Early cameras usually had a fixed shutter angle. The angle itself varied between cameras. For example, Bell & Howell Eyemo 71k used a 160 degree shutter; the 16mm B&H Filmo 70 DR used a 204 degree shutter. Eventually a shutter angle of 180 degrees ended as standard or “normal”. For a frame rate of 24 fps this equals shutter speed of 1/48 sec. The shutter angle can be converted to shutter speed with the following formula:

The following table conveniently lists some useful shutter angles converted to shutter speeds.

Motion blur and strobing

There are a couple of artifacts that arise from the rotary shutter and the 24 fps frame rate.
As the camera only ever sees half of the time (for a typical 180 degree shutter), it doesn’t capture the scene continuously. This means that fast moving objects, and especially objects moving across the frame, will exhibit jerky movement. This is called strobing. The defect is also very noticeable during pans.

The relatively slow shutter speed and fast motion in the frame result in motion blur

The other artifact is also related to motion. Because of the relatively slow shutter speed (1/48 sec for 180 degree shutter angle and 24 fps), fast moving objects blur in the frame, because the longer the exposure, the more movement is captured. This is what we call motion blur. Note that while strobing is in essence an artefact related exclusively to the no full time shutter, motion blur results from the (relatively) long exposure of each frame.

Smaller shutter angles (shorter exposure) exhibit more pronounced strobing effects. Bigger shutter angles (longer exposure) increase motion blur. Faster frame rates can smooth out the perception of strobing, even with shutter angles smaller than 180 degrees. Faster frame rate also decreases captured motion per frame and decreases motion blur per frame. Note that small variations in shutter angle (and shutter speed) are imperceptible for most of the audience. Most people won’t notice any difference between a 180 degree shutter angle (1/48 sec shutter speed) and a 144 degree (1/60 sec shutter speed).

Top: 180 degree shutter will produce typical motion blur and strobing. Middle: decreasing shutter speed increases blur and decreases strobing. Bottom: increasing shutter speed decreases blur and increases strobing.

Motion blur and strobing are the main ingredients of what people call the dream-like effect of cinema. They set apart the cinematic image from the crisp and fluid reality and reinforce the unreal feel of cinema. This makes them an important characteristic of the traditional cinematic look.

Digital cameras

In digital video cameras (and DSLRs) there is no real need for a rotary shutter as there is no film negative that needs moving around. Nevertheless, some high-end digital cinema cameras like the Arri Alexa Studio and the Sony CineAlta F65 use rotary shutters to closely simulate the exposure process of film cameras while letting light on the sensor. But the vast majority of digital cameras only have electronic shutters. The camera simply reads out the sensor (or parts of it, for CMOS sensors) simultaneously ending its exposure and resetting it for the next frame.

One advantage of the electronic shutter is the ability for full time exposure (or 360 degree shutter angle equivalent). Rotary shutters need to be closed for some time after each exposure so that the sprocket wheel moves the next negative frame into position. No such movement is necessary in digital cameras. This allows for exposure times as low as 1/fps seconds. A disadvantage is the jello effect (image skew) that may happen with fast pans or fast moving subjects. This is a characteristic of the rolling shutter – typical for CMOS sensors – and is the result of partial sensor readout: parts of the sensor continue exposing while other parts are being read. High-end sensors with their fast readout times tend to minimize this defect.

Setting the digital camera to 24 fps and 1/48 sec shutter speed emulates the way film cameras work pretty close. Some digital cameras (especially DSLRs) don’t have an option for 1/48 sec. But setting the camera to a shutter speed of 1/50 sec will give results virtually indistinguishable from 1/48 sec.

The higher frame rate debate

Recently there has been a push towards higher frame rates, especially in connection to 3D movies. Peter Jackson is shooting The Hobbit in 48 fps and James Cameron will allegedly shoot Avatar 2 and 3 in 60 fps. The reasoning behind this is that higher frame rates result in more fluid and crisper image compared to 24 fps and the typical 180 degree shutter angle. This proposed change has been met with polarized reactions. Many find this image bland, TV-like and lacking the dramatic feel of 24 fps (at 180 degree shutter). Nevertheless, a higher frame rate will probably be accepted as standard alongside 24 fps. If high frame rates take off this may lead to a shift in perception about what is considered cinematic in terms of temporal characteristics. But for now, 24 fps and a 180 degree shutter angle define the traditional cinematic look. This is the look games (in cutscenes) and video emulate when trying to be cinematic. You can read more in the articles on high frame rates and 3D movies and frame rate as artistic choice.

The next part of the Cinematic Look series is on Dynamic Range.

Cinematic Look, Part 2: Frame Rate and Shutter Speed

To better illustrate this we need to go back in time. Since the arrival of sound cinema frame rate has been fixed to 24 fps. As such, the choice of frame rate was not an option for filmmakers. There were tries to introduce higher frame rates but these never really took off, mostly because higher rates require more film stock. And this obviously means the price of filmmaking would go up. With digital movie making this is less of a concern – storage is cheap.

But we should actually go further back in time. Before talkies. It is a common misconception that silent films were shot at 16 fps. Cameramen claimed that they had hand-cranked at this speed; some cameras even had indicators for 16 fps to help hand-cranking. This myth was busted by Kevin Brownlow in an article from 1980. During his work on restoration and conversion to tape of silent films he found that 16 fps was not the norm. Cranking speed varied widely between 12 and 26 fps. And higher (than 16 fps) speeds were common. There was also a tendency towards high frame rates mostly based on the habit of theater managers to have their projectionists cranking at higher speeds in order to squeeze more shows in the busy evening schedule. This would inject a slapstick feel even into the most serious drama. To counter that, directors and cameramen would increase speeds to ctach up, hoping that projection would look about right.

It is popular knowledge that the frame rate of talkies (24 fps) was selected as the minimum rate that can yield a decent optical track. This is not exactly so. When Western Electric had to select the frame rate for their process they conducted a survey in a bunch of movie theaters. 24 fps turned out to be about the average projection speed in these theaters. So they selected it. In fact, rival sound processes had the frame rate as low as 21 fps. The point is, there was not anything really scientific or special about the choice of 24 fps. It was largely arbitrary. So it is to a large degree coincidental that the established look of cinema has the dream-like qualities it happens to have. This was not intentional. But all this by no means diminishes these qualities. One way or another, they are here, and we’ve come to love them.

Home Sweet Home has the frame rate rising from reel to reel. And then the last reel is very slow.

Now, how conscious were the directors of the silent era of the artistic side of frame rate is debatable. Maybe they had external reasons for specific choices. For example, low fps means less film stock used. Which leads to a lower price and extended scenes in a reel. Maybe the variations were unintentional. Nevertheless, this provokes additional thought.

So back to the topic at hand. Democratizing frame rate choice can be a good thing. While too many available frame rates may lead to chaos, having available 24 fps plus a higher rate (48 or 60 fps) is something that deserves consideration. Frame rate can be used as a mean to further an artistic vision. The obvious example is with films striving for perceived realism, or films going for a documentary feel. No doubt some of them can benefit from more fluid and crisp action. This could help make the viewer a part of the scene. To make them experience it in a more visceral way. Other (most?) films are better suited for the traditional cinematic illusion. And I believe that existing movies should be left alone and not upconverted to high fps. But having the option to go for a higher frame rate with a film increases the creative possibilities. The same way a filmmaker can choose film or digital, a specific film stock, lighting style, framing, etc. they would be able to choose a frame rate that promotes their vision in the best way possible. It is also good to mention that speeds lower than 24 fps are being used in 24 fps movies for artistic effects. This is achieved by either undercranking the camera, or shooting 24 fps and then dropping frames. In editing, frames are duplicated as many times as needed to achieve correct speed when projected at 24 fps.

But then again, having too many variables can confuse people and may also lead to wrong or arbitrary choices. Still, the latter is not really a good argument. The lack of restraint or of understanding of a specific variable of filmmaking is not an excuse. Confusion amongst the audience on the other hand is something that should be considered. The audience of the silent film wasn’t conditioned to a specific frame rate in the way that we are with 24 fps. Getting used to high frame rates may lead to 24 fps movies being rendered unwatchable for some viewers. But perhaps this is a risk worth taking.

Then there is the option of variable acquisition frame rate. Even the critics of Jackson and his 48 fps endeavor admit that his aerial, scenery and establishing shots look spectacular. Think of Discovery or National Geographic. Variable frame rates can give us the best of both worlds: motion blur and strobing for dramatic impact, and crisp, fluid images for landscapes and panoramas. This is easy to achieve technically once theaters start supporting high frame rates. For example, a 48 fps master can simply double frames for the 24 fps sequences. This will fully preserve low frame rate aesthetics where necessary.

Frame Rate as Artistic Choice or What Can We Learn from Silent Films

But 48 fps actually comes with benefits. Well, for 3D at least.

Cinema and reality

Motion blur and strobing are the main ingredients of what some label the dream effect of cinema. These artifacts of relatively low frame rate and less-than-360 degrees shutter angle separate the cinematic image from reality and their abnormality reinforces the unreal nature of cinema. This is something cinema viewers have come to appreciate. This characteristic of cinema has stayed more or less the same after the arrival of sound, after the introduction of the wide screen and after the advance of CGI.

The Hobbit breaks new ground with its 48 fps presentation

Quake players would often run the game with a frame rate into the hundreds for maximum fluidity and responsiveness

3D movies

3D changes things for cinema. Most importantly, it blurs the boundary between the fantasy of cinema and reality by tricking the brain to think that what it sees on the screen is not a screen at all, but deep and three dimensional. This is in direct contradiction with the otherworldliness of cinema, as defined by its most famous image artifacts. So, in a sense, in the moment we put 3D in the equation, we clash with the dream-like nature of cinema. And even more: one might say that with 3D we give up the dream nature of cinema. But there is more than just philosophical reasoning.

Problems with non-coinciding eye convergence and eye focus aside, 3D simply does not coexist well with strobing and motion blur. The already strained eyes get additional load while trying to discern depth plans because of the lack of crisp object edges and the jerky movement. Selective focus does not help either because the eye can’t wander freely through the scene (but that’s unrelated to frame rate). So 3D tells the brain “This is real!”. On the other hand, the motion blur and strobing artifacts obstruct this perception.

So we can argue that 3D simply is not cinematic. With its life-like aspirations 3D is more suitable for interactive representations of reality like games and, ultimately, virtual reality. But – like it or not – 3D is here anyway.

So there is really only one logical direction to go from here in the 3D cinema case. Motion blur and strobing need to go. 48 fps will no doubt help tremendously in terms of both fluidity and crispness. And 3D cinema becomes reality – for good or bad – thus resolving the contradiction. And if we are lucky, 2D movies will stay 24 fps so that we have our opposition to reality.

The familiar fluidity of TV and video games is the main reason younger generations will most likely embrace 48 fps without questions. That, and probably being more open to changes. They have sunk so many fluid images through their TV and games entertainment, that actually it won’t be surprising if the strobing film look is going to be considered weird and severely outdated in a few years. And the hyper-realism of the “be in the scene” factor will no doubt sell the concept to many others.

(There are some further thoughts on the topic and an interesting reference to silent films in my follow-up article on frame rate as artistic choice.)

Cinema and Reality, or Why 48 fps is Good for 3D Movies