8mm Forum: [ASK] 2 blade shutter, 3 blades and 5 blades, how do you explain

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Topic: [ASK] 2 blade shutter, 3 blades and 5 blades, how do you explain

Winbert Hutahaean
Film God

Posts: 5468
From: Nouméa, New Caledonia
Registered: Jun 2003

posted August 15, 2011 11:15 AM

I am in the middle of discussion with friends (non 8mm collector) about shutter in the projector.

I knew that our normal projector has 3 blades shutter. But to get more light output we can install two blades instead or 5 baldes if we are avoiding flickers (telecine purposes).

My questions are:

1. Is each blade for showing/blocking one frame?
2. If it is "yes", does it mean at 2 blade shutter, we need quicker shutter turn (thus quicker motor)? but how can we install 2 blade shutter without modifying the original motor?

explanation please.

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Winbert

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Joerg Polzfusz
Jedi Master Film Handler

Posts: 815
From: Berlin, Germany, Europe, Earth, Solar System
Registered: Apr 2006

posted August 16, 2011 04:44 AM

No, the blade will turn by 360° every frame (on most projectors). It needs one blade to hide the advance of the film by one frame. The other blades are only needed to show the same frame "again".
Hence 3-bladed shutter @ 24fps = 3*24hz = 72hz (no visible flicker)
Hence 2-bladed shutter @ 24fps = 2*24hz = 48hz (most humans will not see any flicker)
Hence 3-bladed shutter @ 18fps = 3*18hz = 54hz (most humans will not see any flicker)
Hence 2-bladed shutter @ 18fps = 2*18hz = 36hz (most humans will notice the flicker and will be annoyed by it)

(Flicker gets noticed when dropping below something between 50 to 60Hz, depending on your body (eyes, fatigue, ...), the screen's gain-value, the scene, ... .)

Jörg

[ August 16, 2011, 06:26 AM: Message edited by: Joerg Polzfusz ]

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