[drewtato]

Reading the title, I thought it would be this photo, which memed around Japan internet recently: https://cloudfront-us-east-2.images.arcpublishing.com/reuter...

These two exposures surely overlapped, with the known photo probably exposing for tens of milliseconds on either side of the flash, but the lighthouse exposures might not have overlapped at all, yet captured a far more exact slice of time regardless.

[dredmorbius]

The old rule for shooting flash photography was to set a relatively slow sync speed with a shutter & film-based camera to ensure that the flash itself occurred whilst the shutter was open, and when the shutter was fully open. So the precise timing of the flash needn't coincide with the moment captured on film.

A focal-plane shutter actually consists of two "curtains", front and rear, which expose the film in a slot at high speeds. Below a sufficiently slow speed, the shutter is fully open for a period of time, which is what is necessary for flash photography.

Shooting either unsyncronised (shutter opens or closes before flash fires), or at too high a speed where the entire frame isn't exposed at once results in a partially-exposed shot (example in the first image in the Wikipedia article below).

That was usually 1/60th of a second, or slower, and some interesting effects can be had when shooting with a flash, or multiple flashes, on a scene that's otherwise lit. It's also possible to sync the flash to the beginning or end of a long(er) exposure, which has the effect of either capturing the beginning or ending of a movement sequence. Rear-curtain sync is popular and shows the subject moving toward its final position which is fully illuminated by the flash.

<https://www.digitalcameraworld.com/tutorials/rear-curtain-sy...>

The two cameras in the Japanese photo (the one capturing the flash of the other, which you show, and the image taken by the camera with the visible flash) likely occurred within 1/60th of a second of one another, but might differ in exposure start/end by as much as twice that interval, or 1/30th of a second. What we can be reasonably certain of is that when the flash fired, both shutters were open, but that's a relatively long period, photographically, at 16.7 1/1000ths of a second.

Some modern phones, and possibly smartphone cameras, may have a faster effective flash synch, particularly if they use an electronic shutter.

<https://en.wikipedia.org/wiki/Flash_synchronization>

[prmoustache]

Would you be so kind to share the context of that picture?

[dredmorbius]

Post-game celebration in Japan: <https://www.reuters.com/sports/baseball/tigers-fans-take-it-...>

The two "simultaneous" images are the one linked, and presumably the one taken by the woman whose camera flash is seen fully illuminated in the first picture, though as I've described, there might actually be a considerable difference the moments exposed due to flash sync speeds.