Part Nine - Converting from film shutter angles to digital workings

Some cameras such as the Blackmagic Cinema Camera and 4K camera do not express their shutter speeds as you may currently be used to, such as in fractions of a second, like 1/30th, 1/50th, 1/100th of a second, etc. They are instead expressed more like the traditional motion picture standard of shutter angles like 180 degrees, and the normal 172.8 degree shutter.

The history

To understand how the shutter speeds are calculated we need to first take a look at how film cameras used the rotating camera shutter for two different reasons. Film cameras used optical through the lens viewing by using a 45 degree angled rotating mirror with sections cut out.

A film camera has a spinning shutter/ mirror disk that rotates in front of the film gate, and it has a section cut out - (much like a slice cut out of a pie). This will allow light to pass through and expose the film, and then when the solid portion of the mirror/ shutter rotates in front of the film, it gets blanked off and the camera can then advance the film to the next frame. When the shutter was closed the mirror showed the camera operator the image through the lens to operate the camera.

The rotating mirror therefore acts as a shutter to blank the film off from being exposed when the film was moving on to the next frame. It really was an inspired design, as the operator can see the action through the lens through the reflection on the mirror. As it rotated it blanked out the eyepiece and the film frame  was then exposed. The camera operator obviously saw a flickering image as the shutter rotated yet this was a great way to know the camera was running!

The standard movie camera shutter 'angle' is set at 180 degrees, and some cameras did not have an option to change the angle at all. The rotating disk is therefore a half-circle (a circle being 360 degrees, of course). So at 24 frames per second, the film is actually in the camera 'gate' for 1/24th of a second but it is only being exposed for half that time, thus the exposure is 1/48th of a second.

Shutter speeds

When the camera ran at 24fps on a movie, the standard shutter angle was 172.8 degrees and this gives 1/50th second exposure. I will show you how that is worked out in a short while. However, the clever part about the film camera shutter was its ability to be mechanically altered on some cameras whilst the camera was still running! This was useful when changing the frame rate speed of the film cameras ( ramping up or down ) in shot. This creates an interesting effect when the normal speed scene starts to slow down in shot. This is really all done in post production now, so this art has generally been lost.

However, as the camera speed rises the shutter rotates quicker, and the exposure would become shorter, so the shutter angle was changed to compensate. By altering the shutter angle when the frame rate rises, the actual exposure can be kept constant, and no compensation is needed by opening the aperture.

Blackmagic Cinema Camera

This is how the shutter speeds are expressed in the Blackmagic Cinema Camera menus, and there is no current way to change this reading to fractions of a second. This may be somewhat puzzling for you if you have never experienced this way of measuring shutter speeds, so I am here to explain how to convert, and what it all means...

Most film cameras for cinema normally ran at 24fps, so the equations are quite easy when you know how. Calculations are all based upon a camera speed of24 fps.

Firstly Default Exposure Time - Speed vs Angles

When you want to convert a shutter speed to a shutter angle the maths equation looks as follows:

Frame rate x 360 / the time fraction (24 x 360) / Time Fraction or 8640 / xx where xx is the number from the fraction 1/ xx sec)

So as an example the equivalent shutter angle for 1/50 sec shutter speed is:

8640 / 50 = 172.8. So a 172.8 degree shutter angle

Here are calculations with fractions of a second and the equivalent conversion in shutter angle degrees:

1/32 = 270 1/48 = 180 1/50 = 172.8 1/60 = 144 1/96 =  90 1/120 = 72

To find a shutter speed that relates to a known shutter angle, you simply do the maths in reverse.

Again at 24 fps the equation would be: (24 x 360) / Shutter Angle or 8640 / xxx      (the Shutter Degree angle) So the shutter speed for 144 degrees for our American cinematographers would look like: 8640 / 144 = 60.  So this is a 1/60th second exposure.

Degree of shutter and the equivalent conversion to fractions of a second:

270 = 1/32 180 = 1/48 172.8 = 1/50 144 = 1/60 90 = 1/96 72 = 1/120 45 = 1/198 22.5 = 1/348 11 = 1/696 8.6 = 1/1000   - So there you go.

Posted on November 4, 2015 and filed under basics.