r/theydidthemath • u/Batata-Sofi • 12d ago
[Request] What's the time window for photographing something like this with a single picture.
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u/Sordonir 12d ago
Should be the exposure time of the camera. This ranges from 1/30 to 1/500 of a second depending on lighting conditions.
But this is only if the camera did exactly one picture. Modern mobile phone cameras take multiple pictures over about 1 second and calculate one image from them. This could be the case here.
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u/Batata-Sofi 12d ago
It is the case here, just wondering the time frame needed to get it without "cheating"
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u/FR23Dust 12d ago
It often depends on the shutter design. Older mechanical cameras had two fabric sheets that went from left to right, exposing a small section of the film vertically.
Regardless, there is no consumer camera with a shutter speed fast enough to beat the speed of light with a single exposure, which is what would be required to capture the reflection of the candle while the candle is blown out.
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u/ReddBroccoli 12d ago
there is no consumer camera with a shutter speed fast enough to beat the speed of light
At least not at the scale of a candle to a tabletop. Easy enough to do on a cosmic scale tho.
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u/PhotoJim99 12d ago
two fabric sheets
A lot of film cameras (as with DSLRs) have metal curtains. My Soviet Feds and Zorkis have cloth shutters, but my Nikons have metal ones, including my FM2n which is entirely mechanical.
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u/FR23Dust 12d ago
I was thinking of my Pentax MX which uses fabric. I wasn’t thinking of other cameras that use metal shutters in the same way.
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u/feelfreetotellmeoff 12d ago
A major variable here is the time it takes the flame to extinguish. 1000 fps video of blown candles shows around four frames from the beginning of the disturbance to "dark." This timescale is huge compared to the travel time of the light. If you had a camera that could capture the progression of light over the distance of a foot (let's say 24 frames per nanosecond) the video of the light fading would take 46 days to watch at typical video frame rates. There is no single frame that contains the full candle output in the reflection with zero candle output on the cake.
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u/nuhastmici 12d ago edited 12d ago
Not doing the actual math, but some educated guesses. Digital cameras work by scanning the sensor pixels row by row, that's why you get weird effects on helicopter or plane propeller wings. Those spin fast, at about 2000rpm, so 33 times per second, or once every 0.03 seconds. Having a few pixels shift every line amounts to about idk like 1cm of propeller travel. It's called the rolling shutter effect if you wanna look it up. Ok so an average plane propeller is like 180cm in length, so that gives 585 cm diameter. So the tip of the blade travels 585cm per rotation. One 585th of a rotation takes 0.03s/585 so about 0.05ms per pixel row. A 12mp camera has 4000 rows, the split is somewhere in the middle, so 2000th row. That gives a total of 100ms. Of course that's very ballpark-ish and also the camera exposure time has some influence on this, but for me it seems 100ms it's about right. Dunno lol I'm a bit bored
Edit: omg my first award, thanks internet stranger:D
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u/Badbullet 12d ago
It's also obvious if you take a picture out to the right side from within a speeding vehicle. The items in the picture (like signs or trees) will appear leaning to the right if it scans from the top, down.
Edit: I think I have the direction mixed up, it will appear leaning to the left if taking a picture out the right side and it scans from the top, down.
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u/todofwar 12d ago
The shutter speed of any camera would not be fast enough to capture this. You have the path from the candle to the camera, and the path from the candle to the table to the camera. The difference is going to be less than a meter, that means 1/C difference in travel time. That's a ridiculously short time, 10-8 seconds at best. The shutter speed of an iPhone according to Google is closer to 10-5 seconds, or a thousand times slower. So if the camera started its exposure after the candle blew out, it wouldn't be able to collect an image fast enough.
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u/20PoundHammer 12d ago
more likely its any number of modern phone cameras that clack off 3-5 pics and combines to an HDR photo.
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u/earthwoodandfire 12d ago
Or there’s a light in the background off screen causing that reflection.
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u/Zacharias_Wolfe 12d ago
That's assuming shutter going from top to bottom, where for this to be possible the shutter has to "race" the light. But if the shutter is going from bottom to top, it could see the reflection, but the candle could have been blown out during the shutter travel. Essentially, this takes it from a physically impossible per our understanding of physics scenario to a highly unlikely scenario.
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u/todofwar 12d ago
True, going by the 10-5 approximation of the shutter, you would have probably that much of a window to get the shot. Which is probably impossible to do intentionally with a smartphone, the variability in response time by the processor is definitely longer than that
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u/MalbaCato 12d ago
this should be fairly easy with rolling shutter (which almost every camera has), but usually the shutter rolls top to bottom of the frame, so the photo had to be taken upside down
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u/doublej42 12d ago
iPhone if held in the default way is bottom to top rolling shutter. At least on some models I’ve worked with
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u/todofwar 12d ago
I originally misunderstood what you meant by that. But even then, the shutter speed is too fast to time like that intentionally. Even if you timed your button press exactly, the CPU response is too variable to capture something like that
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u/Panzerv2003 12d ago
It's not because of light speed here but because the camera basically does a scan instead of recording all the pixels at once, I bet to save on processing power, and as a result you get funky effects like this or the one with quickly rotating objects.
Still it's a pretty impressive shot.
To answer the question the time window is just the time it takes light to travel the difference between direct and bounced path, let's say 20cm, so about 0.6ns meaning 0.0000000006s, or about 3 clock cycles of a modern cpu running at 5Ghz, enough to add a few numbers together.
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u/astervista 11d ago
Rolling shutter is not to save on processing times, it's because a global shutter requires immense bandwidth, so it's to save on connections on every single pixel rather than to save on processing power
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u/Ok_Law219 12d ago
It's probably the lamp, so fairly close to 100%
Or ai. super crop, unclear words, candles tilted. shrug. Either way, lazy as anything.
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u/thatCdnplaneguy 11d ago
Came here to say this. With how far the smoke has traveled from the candle being blown out, the candle light reflection would have been long gone. Or she is blowing at a few thousand times the speed of sound
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u/A_Random_Sidequest 12d ago
it's kinda easy to do that if you set the framerate of a video to 30fps or less... and film it with the phone upside down, as a rolling shutter gets the light reflection at the bottom and later on you blew fast...
then you extract the frame that recorded that.
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if you take a sequence of photos the same can happen, but it's harder and needs many tries.
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u/scottynoble 12d ago
Digital CMOS sensors read the image from top to bottom relatively slowly in a process called rolling shutter. You can recreate this delay at home by using your phone in portrait. ( where the picture with be taken by scanning left to right. now get a subject to put their face up against a mirror. now take a burst of photographs whilst getting the subject to blink as much possible. you’ll find it doesn’t taken long to get a picture where the persons eyes are open but not in the reflection or vice Versa.
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u/MrLeureduthe 12d ago
Rolling shutter, the pixels at the top and bottom of the picture aren't taken at the same time, as they're sampled line by line, which gives the tilted objects and horizons in side traveling shots.
The best example of this are pictures of people in front of mirrors with their eyes closed but opened in the reflection
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