Thoughts on Trailing Curtain Sync

John Sessoms jsessoms002 at
Sat Jul 3 09:48:25 EDT 2010

From: paul stenquist
> On Jul 2, 2010, at 7:05 PM, Adam Maas wrote:
>> > On Fri, Jul 2, 2010 at 4:53 PM, John Sessoms <jsessoms002 at> wrote:
>>> >> The K-10D and K-20D both allow you to set the on-board flash to trailing
>>> >> curtain sync. How difficult would it be for them to make the hot-shoe also
>>> >> fire on the trailing curtain when a non-dedicated "flash" is installed?
>> > 
>> > Very difficult unless certain assumptions about burn time are made,
>> > which won't be accurate. You need to fire the flash at just about
>> > exactly the burn time before the shutter closes for trailing curtain
>> > to work corectly. Most speedlights have a roughly 1ms full-power burn,
>> > but at low power levels it can be an order of magnitude shorter as
>> > output is normally controlled by burn duration rather than intensity.
>> > 
>> > Olympus did allow this on some OM models, but it only worked because
>> > of the low sync speed of the camera's (1/60) so that even if the flash
>> > burn time was much shorter than the assumed ~1ms you don't get much
>> > exposure after the burn ends. This doesn't work so well with today's
>> > high sync speeds.
>> > 
> Of course you can shoot trailing curtain synch at low shutter speeds on either the K10 or the K20. I get good results shooting trailing synch on the K10, K20 and K7 at 1/8th or 1/15th.
> Paul

But you can only do so with the on-board flash or a dedicated Pentax 
flash like the AF-540FGZ.

The regular hot-shoe contact (or the PC contact on the K20) ONLY sync 
with the front curtain. The trailing curtain sync does not work with the 
AF-540FGZ in wireless mode, you have to use the Hot-shoe Adapter F and 
F5P cable to get it use trailing curtain sync off camera.

I was wondering whether the main contact tripping only on the front 
curtain is hard-wired into the cameras or is it something Pentax could 
change in the firm-ware? Could Pentax tell the camera when the on-board 
flash mode is set for trailing curtain, trip the hot-shoe/PC contact 
with the trailing curtain as well, or would it require a physical change 
to the wiring inside the camera?

It would be useful to have the camera trigger the hot-shoe/PC on the 
rear curtain so I could use trailing curtain sync with pocket wizard 
type radio slaves.

Obviously, if the AF-540FGZ can do trailing curtain sync when mounted on 
the shoe, some information is being communicated through one of the 
contacts that tells the flash when the trailing curtain begins to close.

How is the information formatted and what connector does it come 
through? If I knew that, I might be able to bread-board my own adapter.

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