Butterfly Shutters

If your camera pulls down one frame before each of the two shutter openings, then the shutter is spinning at 1/2 revolution per frame. That means it is travelling at half the number of rev's per second as the camera movement. That means that the 90 degree opening in the shutter is uncovering the film for the same amount of time as a 180 degree opening would if the shutter and the movement were moving at the same number of revolutions per frame... ...which means that at 24 fps, your 90 degree butterfly shutter will expose the film for 1/48th of a second...same as a 180 degree single shutter would.

Did that help or further confuse?

Mark Weingartner
LA based

Divide 360 by the two openings and you get 180--you're making this too complicated. In the Arri the butterfly is moving at 12fps when the film is being exposed at 24fps. Each frame gets the 1/48sec. exposure you'd get from a conventional 180 shutter disc.

Slowly turn the inching knob and watch how the pulldown begins and ends just as the shutter covers and uncovers the aperture. Each frame is getting the full exposure for as long as it's in the gate, 1/48th of a sec., and is being pulled down for the other 1/48th. The Arri has a cyclical movement, half of the shaft rotation is pulldown, half is exposure.

This was just a trick to make it easier to dynamically balance the relatively heavy Pyrex mirror shutter Arri used, as someone has mentioned. I think you'll find that your Mitchell is also giving 1/48sec. exposure. The Mitchell 16 had an eccentric movement--fast pulldown, longer dwell time--which provided for a 235 deg. shutter giving 1/37th sec. exposure time. But the BNC was a 170.

Wade K. Ramsey, DP
Dept. of Cinema & Video Production
Bob Jones University
Greenville, SC 29614

A good basic visual to understand the butterfly shutter -

Take an ordinary pie - I like pumpkin.

Cut it across twice to create four even pieces.

Give me two opposing pieces -

I don't care which two so long as they are not touching each other in the pie dish.

I eat them.

The two remaining pieces in the pie dish are a fair approximation of a butterfly mirror.

Put the pie dish with the 2 remaining slices on a lazy Susan and rotate.

Pie slice - hole - pie slice- hole -

Every 90 degrees of spin you switch between pie slice and hole.

In a film camera the butterfly mirror rotates similarly at 12 revolutions/sec. that creates 24 film exposures through the two cut-outs.

Because the mirror actually runs at 12 revs, or "half speed" the equivalent actual time for a 90 degree exposure is twice as long - 180 degrees.
90 degrees (hole) of one spin at 1/12th second equals 1/48th second.

The mirror actually is mounted in the camera optical path at an angle.

There is no reflected image ever sent to the film.

The mirror only "slices" the light to the film.

When the film is blocked by either of the two "pie slice" wedges the mirror reflects light into the camera operator's eyepiece while the film is advanced to a new frame.

Thus, light from the lens is time-shared between the film and the eyepiece. When the mirror stops it always does so in a position that blocks the film and sends all the light to the eyepiece.

Pete Fasciano
Co-founder, Avid Technology