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"For the exposure of the foreground, though, the lighting came from the flash only. Therefore, changing the shutter speed didn’t affect the exposure of the subject, since the speed of light is faster than any shutter speed. It was only the amount of light let into the lens through the aperture that determined the exposure."

I think He meant to say The duration of the flash instead of The speed of light

Also the last paragraph "It was only the amount of light let into the lens through the aperture that determined the exposure." as opposed to what?

  • I'd be a smart-ass and point out that there are electronic shutter systems that are in fact fast enough that they can trigger in the time it takes light to travel to a subject a few meters away from the camera and back, but that's primarily just snark. – Fake Name Dec 1 '13 at 13:54
  • "...as opposed to what?" As opposed to letting more of the light from that short duration flash through by using a wider aperture for a higher exposure or letting less of the light from that short duration flash through a narrower aperture for a lower exposure. – Michael C Dec 2 '13 at 11:21
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(1.) I agree that he was probably trying to convey "the duration of the light 'pulse' from the flash" when he said "the speed of light.

As the light pulse is of short duration it will, in all cases when the shutter is fully open, have a duration shorter than the minimum possible opening time. This was not true of ye olde magnesium based flash bulbs which had a duration closer to the fully open shutter times of modern roller blind shutters.

Note that the phrases above - "when the shutter is fully open" and "roller blind shutter" are both relevant to this answer. Most SLRs and DSLRs have roller blind shutters which are only ever fully open below some relatively low speed - mayne in the 1/120s - 1/250s range. Firing a flash when they are not fully open will result in a flash illuminated area with dark areas on one or both sides. A focal plane shutter tends to be much faster - in some cases faster than 1/1000s, but these are essentially only ever found on old or specialist cameras.

(2.) Expand statement along the lines - "It was only the amount of light let into the lens from the burst produced by the flash that determined the exposure - not the time that the shutter was opened for as this was far longer than the time taken for the flash burst, but there was no other light received before or after the burst.

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No, he meant the speed of light. He is essentially saying there is no shutter speed fast enough that the flash could fire and not be captured by the camera.

Also the last paragraph "It was only the amount of light let into the lens through the aperture that determined the exposure." as opposed to what?

The shutter speed.

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    Although it also makes sense with saying "the duration of the flash". Because if the flash duration were longer than the shutter speed, that would have an effect. – Unapiedra Dec 1 '13 at 10:04
  • I don't think this is what he meant. He was pointing out why the exposure of the subject didn't change when he changed shutter speeds. Exposure has nothing to do with the speed of light, but rather with how much light (number of photons) is reflected back to the camera from the subject, which here is fully a function of the flash duration and power (and distance). In other words, if the flash had an unusually long duration (say a second) then changing the shutter speed very well could change the exposure on the subject, even though the speed of light didn't change. – Benjamin Cutler Dec 1 '13 at 21:44
  • Another way to put it is that if the shutter closes before the photon's have a chance to reach the camera, that is a synchronization issue. As a thought experiment say that the speed of light were much slower, so that it takes 1/10 s for light to travel 1 meter. In this case photographing a subject 1 meter away requires one to synchronize their shutter to open 2/10 s after the flash fires, but as long as the shutter is open for the duration of the flash the exposure of the subject won't change. – Benjamin Cutler Dec 1 '13 at 21:58
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With my pedantic physicist hat on: yes, he was incorrect. You cannot compare the speed of light (299 792 458 m/s) to a shutter speed (something measured in seconds). It's the same as trying to compare a kilogram to a metre - they're just different. (Technically, they have different dimensions).

What you'll note if you think about it a bit more is that it's the photographers who are "wrong" here: what we call a shutter "speed" (say 1/500 s) isn't really a speed at all, but an interval of time. If it were a speed, it would involve a distance being divided by a time.

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It is not the speed of light you need to watch when settings your shutter for a flash. It is , as you assume, the flash duration. it sounds like the guy things a flash has no duration. As an example a canon 430ex II (the most common flash for canon cameras) has flash durations of 1/350sec (at full power) through 1/6500sec at minimum. Now at full power all canon dslrs that the flash is meant for can set a shutter that will cut off a lot of that light as the consumer rebels/xxD can go all the way down to 1/4000sec, so only at minimum power you are safe. With prosumer and pro lines (xxD/xD) you can shoot at 1/8000s which also crops off the minimum flash level. Usually you say 1/250sec "Synch speed" , which means you should stay above that (in shutter length), and you see that gives some headroom to 1/350sec flash duration, allowing for variations in trigger accruace and opening up to the sensor.

  • But the rebels wont let you shoot over the synch speed when using flash for the same reason, so you wont get a flawed exposure. – angel rojas Dec 2 '13 at 15:52
  • not even in full manual (cam + flash)? – Michael Nielsen Dec 2 '13 at 16:09
  • Certainly my 550D won't, either with the on-camera flash or with my YN467-II in either TTL or manual on the hotshoe. (I make no comment about Canon flashes or off-camera flashes as I don't have the kit to do any of that). – Philip Kendall Dec 3 '13 at 19:36

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