The problem is:

We have a mapping camera which a hot shoe output synchronization speed of 1/250 and during data acquisition its shutter speed is 1/1000, which is very fast compared to the synchronization speed.

Once the camera triggers the image it sends capturing feedback through the hot shoe to the mapping module and the mapping module logs the coordinate.

If my aircraft speed is 17 m/sec then my mapping module will note a 4cm forward coordinate each time.

is this assumption correct, Or there is something else? Can someone help me to figure it out?

and, does this difference acceptable in mapping?

  • \$\begingroup\$ Your assumption is basically correct (but 17m/s in 1/250s is 6.8cm). It all depends on the terrain resolution of your pictures (so how big 6.8cm is compared to one pixel in the picture), which will depend on altitude and focal length. And also how accurate is the positioning system... \$\endgroup\$
    – xenoid
    Jan 7 at 12:22
  • \$\begingroup\$ Thanks for your quick response. I got your point. \$\endgroup\$ Jan 7 at 12:51
  • \$\begingroup\$ The assumption that the entire frame is captured in 1/1000 sec is also probably grossly incorrect. If the camera's X-sync is 1/250, it probably takes around 4 milliseconds for the shutter curtains to cross the focal plane, regardless of the Tv used. That is, one side of the frame (or the top or bottom) is exposed 4 milliseconds before the opposite side (or bottom or top) is exposed, each for 1 millisecond, respectively. \$\endgroup\$
    – Michael C
    Jan 7 at 18:54
  • \$\begingroup\$ Whether any difference is acceptable depends on the use case of the data collected. It may well be within parameters for some uses (such as geotagging photos), and well outside the parameters of others (like targeting precision missile strikes). \$\endgroup\$
    – Michael C
    Jan 7 at 19:58

2 Answers 2


It seems to me your issue is of flash signal timing from the hotshoe. That is not really related to the X-sync speed.

X sync relates to the shortest duration where the entire sensor can be exposed (or read out for electronic/rolling shutter). At shutter speeds faster than that a mechanical shutter records via a moving slit opening across the sensor resulting in a rolling shutter effect. And both will not record all of the flash due to a portion of the sensor being blocked/disabled... But you are not actually using flash.

Instead what you are concerned with is if the flash signal is sent during the exposure, and it is; regardless of the shutter speed. You probably want the flash timing set for "front curtain" where the flash timing signal is sent at the beginning of the exposure, but rear curtain (end of exposure) may also be viable. The duration of the trigger signal itself is extremely short.

You will still have the issue of shutter travel time (which does correlate to x-sync). And as Michael noted it is around 4MS for a 1/250 x-sync. What that means is that there will be a 4ms difference in travel distance (6.8cm) during the exposure. That may result in a blur effect in the image if the lens (aperture/sharpness) and sensor (pixel pitch) are able to resolve down to 6.8cm at altitude (I rather suspect it's not)... i.e. is the angular position change greater or lesser than the angular resolution of the camera/lens...

And your mapping module will not be forward of the image location; because that distance is in/part of the recorded image.

Of course there could also be some communication delay between the flash signal and the mapping module reading/recording the position. If it is that critical that the recoded position is exactly image center you can use a camera that is HSS capable in HSS, which moves the sync timing forward (may require a trigger w/ adjustable sync timing in the hotshoe before the mapping module).

  • \$\begingroup\$ Even with front curtain, the sync signal is NOT sent at the beginning of exposure, it's sent once the first curtain is fully open. When using a Tv of 1/1000 with a camera that has an X-sync of 1/250, that's roughly 2/3 of the way through the total time it takes to fully expose the entire sensor for 1/1000. \$\endgroup\$
    – Michael C
    Jan 7 at 19:55
  • \$\begingroup\$ Front curtain and rear curtain are both offset from actual shutter travel somewhat... but they are still typically described/understood as beginning/end of exposure. And that's not really relevant because the flash timing will correlate with the image's position either way... the mapping program will not be offset. \$\endgroup\$ Jan 7 at 20:06
  • \$\begingroup\$ Based on the wording of the OP, I suspect the geolocation is not being extracted from the image metadata, but rather from an external device independent of the camera. \$\endgroup\$
    – Michael C
    Jan 7 at 20:07
  • \$\begingroup\$ 1st/2nd curtain flash sync can be typically understood as beginning/end when the exposure duration is long enough for both shutter curtains to be fully open at least long enough for the flash to reach T.1. Most cameras, even those with 1/250 X-sync, limit 2nd curtain sync to exposure times longer than 1/30. At Tv's much shorter than X-sync, the sync signal for first curtain is not sent until the exposure is almost over as the second curtain is already an appreciable percentage of the way across the frame by the time the 1st curtain is fully open. \$\endgroup\$
    – Michael C
    Jan 7 at 20:11
  • 1
    \$\begingroup\$ @Michael, your are right, geolocation is not extracted from image meta data. \$\endgroup\$ Jan 9 at 9:24

Or there is something else?

Your question indicates a lack of awareness between how the exposure time (i.e. "shutter speed") can differ from the shutter transit time (i.e. amount of time it takes to expose the whole picture):

Just because a camera is set to 1/1000 sec does not mean the entire photograph is captured in 1/1000 sec. It means any particular spot on the film/sensor is exposed for 1/1000 sec. But usually the entire frame is exposed sequentially from one side to the other or from top to bottom.

For modern digital cameras with mechanical shutters this typically requires around 2.5-3.5 milliseconds plus the exposure time. (Most current digital cameras with "electronic shutters" take even longer to fully scan the sensor from top to bottom for one cycle, resulting in more severe "rolling shutter" effect.)

Cameras with focal plane shutters have two shutter curtains. The first one opens to expose the sensor/film to light, the second one follows it in the same direction to cover the sensor/film to end the exposure. The shutter curtains move at the same speed regardless of the exposure time used. The exposure time is determined by the interval between the beginning of the movement of the first curtain and the beginning of the movement of the second curtain.

Exposure times shorter than the curtain transit time are accomplished by beginning to close the second curtain before the first curtain has fully transited. The only difference between 1/1000 and 1/2000 is the width of the slit between the two shutter curtains as the second one chases the first across the focal plane.

shutter transit times vs exposure times

For a camera with a shutter transit time of 2.5 milliseconds and using a Tv of 1/1000 (1 ms), exposure on one side frame is begun 3.5 milliseconds before exposure on the other side of the frame is ended.

The reason cameras have a flash sync speed is to insure that the first curtain is fully open and the second curtain has not yet begun to close when the flash is fired, and that both curtains remain fully open long enough for the flash to release most of its light energy over a short time interval. Cameras with a flash sync of 1/250 sec typically have a shutter curtain transit time of around 2.5-3 milliseconds and allow the flash another 1-1.5 millisecond to release its energy before the second curtain begins to close. The flash sync signal is sent to the hot shoe a miniscule fraction of a millisecond after the first curtain is fully open. But if you're using an exposure time of 1/1000, then the second curtain is already covering over half the sensor by the time the sync signal is sent.

In your case using a Tv of 1/1000 with a camera that has a 1/250 X-sync, the signal is sent at approximately 2.5-3 milliseconds after one side of the image has begun to be exposed and approximately 1 millisecond before the other side of the image will finish being exposed.

  • 1
    \$\begingroup\$ thanks for you in-depth explanation. \$\endgroup\$ Jan 9 at 8:51

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