Measuring A Millisecond Mechanically

If you are manufacturing something, you have to test it. It wouldn’t do, for example, for your car to say it was going 60 MPH when it was really going 90 MPH. But if you were making a classic Leica camera back in the early 20th century, how do you measure a shutter that operates at 1/1000 of a second — a millisecond — without modern electronics? The answer is a special stroboscope that would look at home in any cyberpunk novel. [SmarterEveryDay] visited a camera restoration operation in Finland, and you can see the machine in action in the video below.

The machine has a wheel that rotates at a fixed speed. By imaging a pattern through the camera, you can determine the shutter speed. The video shows a high-speed video of the shutter operation which is worth watching, and it also explains exactly how the rotating disk combined with the rotating shutter allows the measurement.

The marks on the spinning drum move at a precise speed adjusted by a stroboscope. The rolling shutter on the camera shows each horizontal bar as a diagonal line and the exact pattern will show the precise speed. The lines are a bit curved due to the characteristic of the shutter spring.

Honestly, this is one of those things that is probably of zero practical value today. But we never fail to marvel at the ingenuity of engineers who didn’t have access to modern technology. Or materials, for that matter.

Thanks to [zit] for the tip!

23 thoughts on “Measuring A Millisecond Mechanically

  1. I love this mechanical ingenuity… But for me it was easier to use a 6502 Sym-1 development board with a photo-transistor to measure the shutter speed of my analog reflex camera some years ago… Today we have arduinos… but the cameras are digital. So I will probably take a picture of a moving mechanical device to measure it… Just the opposite ;-)

    1. Loved the SYM-1. I used it to replace an analog circuit for computing beats/min from seconds/pulse in and undergrad engr electronics lab. The SYM-1 had been designed a couple years earlier by two people at my college for Synertek. I’ve still got mine, but haven’t powered it up in years.

      1. Still have mine, but when I tried to fire it some weeks ago, it beeped, show me something dim on the display, then didn’t respond… I guess some socket contact cleaning and capacitor replacement is due…

  2. Speedometers in cars actually AREN’T very accurate OR precise.
    Most jurisdictions have laws that account for that inaccuracy/imprecision too, allowing for them to be off by no more than +/- 10%.

    10% is HUGE. But it’s good enough in this case.

    1. It’s more of a rule that all speedometers read 10% too high. The manufacturers set them at the top limit to avoid litigation from people who claim that they got ticketed because the speedo read too low.

      1. I spent a little too much time trying to work out why my GPS project was “wrong” when in fact it was that the electronic speedo in the car which had a weird “hump” in the calibration around 70mph.

        1. Almost certainly not – in my car the average speed display knows my speed pretty accurately, so presumably the odometer does too – its just that the speedo itself clearly has a 105% multiplier on it.

    2. In California it is +/- 2%.

      Although I have owned a vehicle where the previous owner had swapped the stock tires for something larger, since they didn’t adjust the speedo gear(s), this threw off the speed by a good chunk.

        1. The UN/EU/UK regulation is: “The speed indicated shall not be less than the true speed of the vehicle. (…) there shall be the following relationship between the speed displayed (V1) and the true speed (V2).”

          The equation given in the ruling says the indicated speed shall be between +0…+10% + 4 kph. At 100 kph true vehicle speed, the indicator can show between 100 – 114 kph.

    3. Complete nonsense, precise rotational speed is absolutely necessary for today’s vehicle systems and have been for a long time. If you don’t know what you’re talking about DON’T comment.

  3. The good question is, how did they calibrate the stroboscope to sub-millisecond accuracy in the 1930’s without access to reliable frequency sources?

    The video suggests they used the stroboscopic effect itself to synchronize the lamp to something, but didn’t tell what that something was.

    1. It’s probably a pattern on a rotating drum, with the drum being driven by a synchronous motor.

      Certainly sounds like some kind of motor and gearing running inside.

      The rotating mass would smooth out very short-term transients and allow you to be at least as accurate as the mid-term average line frequency, which is probably accurate enough given the precision of the shutters they’re measuring.

      After all, analog film has a pretty wide exposure latitude, so being off a bit in absolute terms is not much of a deal.

      On the other hand, consistency across the fame is an issue you really want to know about because it will show up as exposure variation. The shutter in the topline picture is clearly slowing down and speeding up significantly as it travels across the gate.

  4. This has an advantage over the electronic shutter-speed meter. It can show nonlinearities in the shutter speed- the overall speed can be correct but there can be irregularities in the start and/or stop timings that the older machine can make visible as a distortion of the stripes (or curves).

  5. We used to take a picture of a TV screen, or do it live. NTSC does 15,734 lines per second. You can see consistency of the shutter by the angle of the lines, etc.

  6. As someone who apprenticed in camera repair for 3 years in high-school and then repaired cameras for 6 more years while going to college, I enjoyed this immensely!!! I did not use the mechanical shutter speed tester but I immediately recognized the electronic one used at the end. Rolling shutters can be tricky to calibrate because you have to worry about the 2nd “curtain” overtaking the first (progressive underexposure) or lagging (progressive overexposure).

    I never did work on a Leica. The older technicians pulled rank and got them. They always seemed very fond of them. I did work on all the popular 35mm cameras and some medium format, plus lots of slide and film projectors.

    It’s been 33 years now since I traded in my repair tools for a keyboard and a compiler. Maybe it’s something I can get back to when I retire.

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