Focal Plane Shutter: Closing Curtain Speeds Up

[wjrl]

Hi. I'm trying to teach myself vintage camera repair, working on a Canon FTb. So I am a newbie. I have shutter test results that indicate at high speeds the closing curtain is speeding up. That is, less time is spent traversing the second half of the frame. Is this something that adjusting the closing curtain brake to increase the torque would likely address? Thanks.

 

[ic-racer]

Usually the closing curtain brake is there to eliminate curtain bounce from brief stretching of the curtain fabric and snapping back.

Usually one would want equal exposure on the opening and closing side of the film gate. If the curtains are speeidng up, they gap needs to increase as it travels across (to keep the exposure constant).

 

[wjrl]

Thanks for the info. Understanding that the purpose of the brake is to reduce or eliminate bounce, and that the overall desire is to keep constant slit width during the length of the exposure. So if opening curtain speed is constant and the closing curtain is speeding up, the requirement is to slow down the closing curtain in the second half of travel to maintain the constant slit width. My understand is, that with a clean mechanism, I have two degrees of freedom to influence curtain speed: curtain tension and curtain brake. If curtain tension sets the overall curtain speed, how does one control a speedup or slowdown during travel? Can the brake provide that? Is the brake designed to slow down the curtain down as it comes in to land, or is it designed to just provide an overall dampening influence of a bouncy system?

Since the brake pads on this camera are felt material, I would think that with use the felt would perhaps become slipperier and the brake would eventually underperform?

I have not torn the shutter system down yet to clean it up and re-lubricate, and perhaps that will solve my problem. But I am trying to understand what tweaks can address a speed change during curtain travel.

Thanks again!

 

[wjrl]

Having done some more studying of the topic, I figured I would provide my results here and answer my own question.

1) On a Canon FTb, the shutter brake indeed only engages in maybe the last 10% of total travel, and thus does not appear to be something that would produce much of an effect when tackling curtain speed changes.

2) Far and away the best solution to these problems is to clean the shutter curtain mechanisms, first flushing the bearings with solvent and then applying shutter oil. The vast difference in opening shutter curtain slowdowns shown below in the figures is simply due to cleaning.

3) In this case, cleaning had some effect on closing curtain speed changes, but not much. It is clearly less stable overall in curtain speeds across the shutter speeds spectrum than the clean opening curtain. Note the three distinct regimes in the curve: (1, 2, 4), (8, 15, 30), and (60, 125, 250, 500, 1000). I think this demonstrates how the closing curtain is interacting with the pallet/speed governor, since those regimes reflect the different modes of operation of the pallet. The design looks ingenious: the worst speedup occurs at the slow speeds when it really doesn't matter too much, but it is very tight (a consistent 10% speedup) when maintaining curtain slit width is critical.

4) So while you can change overall speed with the curtain tension ratchets, it looks like there is no way designed into the system to adjust curtain speed changes, since a clean mechanism is really not in need of any adjustment.