[HN Gopher] How to measure milliseconds mechanically for camera ...
___________________________________________________________________
How to measure milliseconds mechanically for camera shutter
calibration [video]
Author : ChuckNorris89
Score : 109 points
Date : 2023-03-03 08:21 UTC (1 days ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| ShakataGaNai wrote:
| Part of what makes Destin's videos so good is his obvious and
| pure love of learning. Do I care how to measure a microsecond of
| a mechanical shutter? Not really. Even though I'm a photographer,
| who cares, that tech is long gone. But he is just so excited to
| learn about these things its truly infectious.
|
| Also helps that he's covered some awesome topics.... like Coast
| Guard jetboats, nuclear submarines and Saturn V rockets.
| adrianmonk wrote:
| Something fairly similar, called a strobe tuner[1], is used for
| tuning musical instruments. (Before electronic tuners, these were
| more popular, but they're apparently still used.)
|
| There's a mechanical spinning disk that turns at a precise rate.
| It has patterns on it, and the sound of the instrument controls
| the strobe light.
|
| Your goal is to adjust the pitch of the instrument until the
| wheel appears to stand still.
|
| Here's a video about how that works:
| https://www.youtube.com/watch?v=U8KHeyT6xB0
|
| That video claims accuracy as good as 1/10th of a cent.
|
| There are 12 notes in an octave, and a cent is 1/100th of the
| distance (in frequency) between two adjacent notes. If it can
| tune an A440 within 1/10th of a cent, then that means it can get
| the frequency within the range between 439.975 Hz and 440.025
| Hz[2].
|
| If I've got the math right, that means it's measuring way more
| precisely than a millisecond.
|
| ---
|
| [1] https://en.wikipedia.org/wiki/Electronic_tuner#Strobe_tuners
|
| [2] 440 * 2^(-1/12000) and 440 * 2^(1/12000), respectively.
| lightedman wrote:
| I have something like this built-in to a modified cigarette
| pack guitar amplifier. I have two LEDs wired in a circular-
| series circuit (I have the positive of one attached to the
| negative of the other, repeat for the other end) and I have
| those on the speaker output in parallel with the speaker. I can
| watch the harmonics visually via the LEDs when I play and tune
| the guitar.
| dekhn wrote:
| See also the way the speed of light was originally measured:
| https://en.wikipedia.org/wiki/Fizeau%E2%80%93Foucault_appara...
| karavelov wrote:
| I was using the same technique back in the day when I was
| maintaining my Pentax MXs but using a fluorescent lamp as a
| source - they blink with the AC power cycle - 50 or 60Hz
| depending on your country.
| nyanpasu64 wrote:
| Reminds me of trying to take pictures of CRTs with a smartphone
| camera, setting the shutter speed to 1/60 of a second to capture
| (just about) a single frame's worth of exposure even if the
| camera shutter and CRT scanline aren't lined up. I didn't know
| that mechanical rolling shutters accelerated along the path,
| rather than sweeping linearly across the image.
| jayceedenton wrote:
| The way this technique combines knowledge of mechanical and human
| optics is really fascinating.
|
| Of course the calibration could also be done by recording an
| image to film and developing it, but it would be very slow and
| inconvenient. This technique is literally using the film-like
| qualities of the retina, which has just enough permanence to
| record the required pattern. Genius.
| flipthefrog wrote:
| Film isn't linear the way digital is. 30% under exposure
| doesn't give an image that is exactly 30% brighter, but may
| show up more as less shadow detail, compressed mid tones, etc.
| Also, negative film can handle a huge amount of over exposure
| and still look good. You can get a better idea with
| positive/slide film, which is much more sensitive to exposure,
| but that is very expensive.
|
| Slower shutter speeds (1/30 second and lower) ar usually the
| speeds that are the most incorrect with old shutters. That can
| be tested more or less reliably by filming it with a high speed
| camera (I use a Fuji XT3 at 120 fps). You can also measure the
| speed by recording the sound of the shutter.
|
| Filmomat have a free app for measuring shutter speeds using
| audio, as well as a light sensitive doodad that you plug into a
| smartphone, which is accurate to up to 500th of a second.
| https://www.filmomat.eu/shop/photoplug
| kqr wrote:
| I think what they mean is exposing a frame with the
| stroboscope-and-shutter combination, not just shooting
| anything at a known light level.
|
| So the same thing as here, but instead of shooting with the
| back open, doing it with film inside and back closed.
| tiffanyh wrote:
| What I didn't understand in the video is, it appears to an assume
| the rotation of the light source box is known/constant/exact.
|
| How is the light box calibrated?
| wolf550e wrote:
| He shows that between 9:50 and 10:50 in the video
|
| https://youtu.be/Q07CTj4fUeY?t=592
| helsinkiandrew wrote:
| But doesn't that rely on having a strobe flashing at the
| specific frequency - I thought modern record players do
| anyway.
|
| I'm not sure how this worked on the 1920s machine - unless it
| assumed 50H mains frequency, or have I missed something?
| hotpotamus wrote:
| Yeah, I would assume they're using mains frequency as the
| reference. It was a pretty common thing back in the old
| days. I remember reading one electrical system engineer
| talking about how they had to tweak the grid hz in order to
| make sure that peoples' clocks stayed fairly correct. The
| grid would typically slow down a bit during times of peak
| load, so they'd run a bit faster at other times to force
| clocks to catch up. I think most of those clocks are gone
| now that there are much better sources, but they were still
| fairly common when I was a kid in the 90's (though I didn't
| learn about how they worked until much later).
| kqr wrote:
| No, many mains connected clocks still use the power
| frequency for timing. As recently as four years ago we
| had the same problem: https://www.nationalgeographic.com/
| science/article/european-...
| hotpotamus wrote:
| I wonder where they are still in use? In my world, NTP is
| king, but there are also radio transmitters in various
| parts of the world that send out time signals, and
| satellite navigation systems that broadcast time so
| accurate, it can be used as a stratum 0 source for NTP.
| Any time I've checked my phone, it's had essentially 0
| error that a human could perceive, and I assumed it was
| getting that from the cell network. Even my
| decorative/fashion quartz wristwatches only drift a few
| seconds a month at most. In my world, tech has certainly
| moved on, and I'd assumed that was the way for most
| people too.
| kqr wrote:
| I have personally only experienced it with bedside alarm
| clock radios, and large wall clocks e.g. in schools.
| hotpotamus wrote:
| Yep, I was thinking of the bedside clocks, but my phone
| does that job for well over the last decade now. When I
| was a kid in the 90's, the school clocks were already
| centrally controlled - you could see them all change in
| unison sometimes as they corrected them. I'd imagine they
| might have generally used the 60hz reference and then
| been corrected as needed, but I wouldn't consider that
| quite the same thing.
|
| It's also interesting to look at Swiss railway clock
| homages. The design is iconic and quite readable, but the
| more interesting ones have what are marketed as "stop to
| go" movements that mimic the actual Swiss clock system.
| They tick as you would expect, but when the second hand
| reaches 12 o'clock, it pauses, and the minute hand
| advances one full minute. It's a strange effect, and then
| of course you start asking about what happens during that
| pause and how it was actually counting time around that
| dial (it would have to be a bit fast, wouldn't it?). From
| my limited understanding, this is because the system
| synchronized every minute at the top of the minute, and
| the system had been around since at least mid-century.
| Obviously it's just an amusement on a wall clock or
| wristwatch.
| junga wrote:
| Using an iPhone with continuity camera as webcam for
| work. Can't remember the exact reason why the iPhone can
| not receive network time in my setup but it effectively
| can only rely on its local circuits to meassure time.
| Time is ten minutes off just after a few weeks. It really
| blew my mind when I first saw this.
| netsharc wrote:
| Hmm, but how do they make sure the strobe is strobing at the
| right frequency?
| beAbU wrote:
| Most likely the 50/60hz mains frequency?
| kqr wrote:
| As someone else said, the trick is that you can use a
| slower source to line up the rotation of a faster source to
| high accuracy. The idea is similar to that of the Vernier
| scale.
| helsinkiandrew wrote:
| Very inventive, electronics makes things so much easier. It
| reminds me of the high speed mechanical cameras used in the 1950s
| Atomic tests that can take 200K - 10M images per second (although
| not for very long).
|
| https://collection.sciencemuseumgroup.org.uk/objects/co83584...
|
| https://gizmodo.com/filming-monster-atomic-blasts-requires-m...
|
| > At these high framing rates the recorded event must be
| illuminated by an intense source of light. Just like an atomic
| flash during the first milliseconds of a nuclear bomb test
| dylan604 wrote:
| aside from the light requirements, how long does it take to
| roll out a 1000' of film at those frame rates? at 24fps, that's
| typically 10-12 minutes. 200,000 / 24 = 8,333x faster. 10min x
| 60s = 600s / 8333 = 0.07s
|
| i'm guessing the military could afford to have a different type
| of film load than standard motion picture film rolls, but
| that's still ridiculously fast at 200k fps so maybe 10M fps
| qualifies for ludicrously fast. what fps would need to be
| achieved before the picture just goes plaid?
| teaearlgraycold wrote:
| I love when a difficult engineering task has a built in
| solution to a sub-problem. You want to capture an image of a
| nuclear blast immediately after the explosion for only a
| minuscule time slice. That means you're worried about getting a
| viable exposure because the film's only under light for
| microseconds. Solution: nuclear bombs are really fucking
| bright.
___________________________________________________________________
(page generated 2023-03-04 23:00 UTC)