So, butterfly shutters are 2-sided in their mirrors, meaning there are 2 mirrors and 2 shutters openings in one full revolution. If you divide 360 by four, you get 90.
So are the shutter openings of butterfly shutters 90??
The Arri is, which is a butterfly shutter, has a "nonadjustable" shutter that is "equivalent to 180?", as quoted from my Carlson book.
Isn't the Arri IIc the same? Is the frame exposed twice on these cameras to allow for a 1/48th shutter speed? does the movement somehow travel slower?
This comes from the BNC thread, and my mitchell has a butterfly shutter. Inching the movement reveals a full frame movement between the shutter openings.
Following the logic above, does that mean my shutter opening is 90?, which is 1/96 shutter speed?
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?
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