Does the 8:1 shaft on the Bolex H16/H8 allow accurate crystal sync?

Johnny Liu · February 11

I had read this person's comment about the 8:1 shaft on a Bolex H16/H8 in relation to crystal sync motors:

"...The Bolex MST motor was the first motor designed to utilise the 1:1 drive shaft included on all Bolex cameras after 1966. This extra drive shaft was designed for the express purpose of accurately synching external drive motors to a tape recorder. Thus enabling you to shoot picture and sound in perfect synchronisation. The earlier motors were unable to do this because they connected to the 8:1 drive shaft.

The 8:1 drive shaft does not allow the motor to drive the film through the gate directly but instead the torque is fed through the internal governor of the camera via the fps dial and then the internal mechanisms of the camera to the gate.

Thus the film speed varies ever so slightly from camera to tape because of the internal mechanisms and therefore the speed may not be exactly what you have set it to but may be something just slightly above or below this..."

Does anyone know if this is true? Is it true that the 8:1 shaft does not allow true/accurate crystal sync? Is it true that only the 1:1 shaft would allow true crystal sync?

I just want to be able to have the Bolex run at 24fps using a crystal sync motor. I assumed that as along as the 8:1 shaft can be rotated 3 times in a second (3 x 8 frames each rotation = 24 fps), that that would give me a true/exact 24fps. Or is it not that simple?

Edited February 11 by Johnny Liu

Dom Jaeger · February 11

I think the author was confused by the fact that some earlier 8:1 motors like MC17 utilised the camera governor to control the speed. That motor was supplied with different voltages to increase the speed roughly, with the camera speed governor being the fine control. Later 1:1 motors had their own internal electric or crystal speed controls.

But he is wrong in that the 8:1 shaft does directly connect to the pull-down drive shaft via an 8:1 gear, and you can essentially disable the camera governor (like you do with 1:1 motors) by turning it up to 64fps. The governor is always connected to the mechanism, whether you drive from the 1:1 or 8:1 shafts. I believe there were early 8:1 regulated motors that ran a Bolex at 24fps, though I haven’t come across one.

That site has a few errors, the author also propagates the mistruth that RX lenses compensate for the prism light loss. It’s unfortunate, because there is also a lot of good information.

Simon Wyss · February 11

It’s nonsense. The centrifugal speed governor stays engaged and part of the mechanism all the time. When using an electric motor one turns the speed knob to something above the desired pace so that the governor is ineffective.

To drive an H via the 8-1 shaft is possible but I doubt one finds a suitable electric motor that makes only three revolutions per second or 180 per minute for 24 fps. So you’re up to a reduction gearing.

Johnny Liu · February 11

Dom, Simon,

Thank you so much to the both of you for your knowledge and experience on this matter and for helping to dispel that claim! I thought that the quote from that website might not be accurate, but I didn't know enough about the design of the H16/H8 to say for sure.

In the past I have seen two crystal sync motors sold for use with the 8:1 shaft - Tobin Cinema Systems TXM-26B - and the NCS Revolution - but you would have to buy them used now (if you can find them). But, recently Aapo Letitinen is planning on making a new crystal sync motor for the 8:1 shaft:

Edited February 11 by Johnny Liu