how does flash sync work mechanically

OK. I've used my strobe on and off camera and it fires whenever I release the shutter, whether I've selected a "sync" speed or accidentally not

remembered about the limitations.

 

So now I'm really curious. Just what, mechanically, happens when the strobe fires and the exposure time isn't within range? Since the strobe is

generally going to be firing a lot faster than the exposure time, what do the sync limitations actually do?

the shortest sync speed for a shutter is the one where the full area of the frame is

exposed for the shortest period of time measured from the time the first curtain

clears the film gate and before the rear curtain starts to travel across the film gate. At

shutter speeds shorter than that the area of film that is exposed by an increasingly

narrow slit.

If the shutter speed is to fast (faster than the sync speed), you will get a partially (flash) exposed frame. If the shutter speed is slower than the sync speed, you will get flash exposure + ambient light exposure.

The above answers are correct. I'm not sure if "mechanically"

meant having anything to do with a mechanical camera, but the

way flash sync works on mechanical (and many electronic) cameras

is that the sync terminals are wired to electrical contacts

that are arranged so that they complete an electrical circuit

and fire the flash when the first shutter curtain fully opens.

Period. This happens regardless of shutter speed.

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If the shutter is set to a speed faster than the sync speed,

then the second curtain will be partway across the film gate

when the flash fires. If it's slower than the sync speed,

then there will be a lag between the time the flash

fires and the time the second curtain starts to close.

If it's set at the sync speed, there will be very little

time between the flash firing and the second curtain beginning

its trip across the film gate.

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On a simple mechanical camera (and some electronic cameras),

you can connect a flash via a PC cord, open the camera back,

point the flash at the shutter, and fire it at various speeds.

When you select a shutter speed faster than the sync speed,

you'll see the flash illuminate a partly closed second curtain. At

the sync speed or slower, you'll see the flash illuminate a

fully open shutter.

Thanks, Richard, for your info. That's more or less what I was after.

 

Now, just what is it that restricts the top sync speed? Is there some mechanical or electrical limitation to the system, as you described it, that makes it difficult to design a sync at 1/1000 or 1/500th?

It boils down to how fast you can move the shutter curtains. Bigger springs move it faster but require a bigger camera. Lighter curtains can be moved faster with a given spring. Vertical moving curtains don't have as far to travel, so can theoretically cover the required distance faster. How fast do you want to go?

it is practical engineering, and some "acceptance" of design. the fastest vertical travel shutter may be 1/250th sync, which means the first curtain has crossed the 25mm (1 inch) opening just as the second begins its travel across the same gap. So, 25 mm in 1/250th of a second is a sustained velocity of 6.25 meters/second. But, the bigger engineering issues are caused by inertia -- on shutter start-up and end (braking efforts), to have a uniform travel without jolting camera shake. The shutter curtain goes from stationary to that 6 M/sec velocity in, say, 0.05 sec, or accelerates at an average of 120M/sec^2 (much higher initially), and that is a noticeable torque/jerk/jounce and all those other engineering terms I have forgotten.

 

In reality, a fast sync speed is needed to match ambient with flash exposure, and modern films do ok in this range up to 1/250th, which is why camera designers did not need to push this envelope. True faster exposures, say the frozen milk-drop splash, are due to the flash freezing the action, not the shutter-- it only needs be open as long as, or longer than, the flash duration. As well, some flash durations on full output may be in the 1/300th -1/750 range, so a sync faster than that would be messy.

 

I am sure that some digital cameras will have full range "shutter" sync by an electronic gate opening for capture.

Interesting, mechanical limitations. OK. Now this digital revolution. What are the limitations there?

 

The D100 only syncs to 1/180th. Seems a bit on the slow side to me. Any opinions?

The DSLRs have shutters too, so they follow suit.

 

Exception is perhaps the Nikon D70 (or D40? who knows) which, like a crappy digicam, will leave the shutter open for speeds from 1/1000 to 1/8000 and use a "electronic shutter" i.e. it will discharge the photosites early. Since the shutter is open, it will sync even at the top speed (whatever it is, even 1/8000th of a second).