Okay then, I will make one last try to explain this.
Of course the information I am now reading in this reply (and the further mention of SX150IS hook location identification in capt_seq.c) is appreciated but not at any price. I'd rather you had replied without mentioning that and then not further replied at all if you find re-explanation tedious. I am not a child.
Bear in mind as you read this that really good sync for 3D stuff requires less than 1 mSec difference between cameras.
Thanks. I know that. I plan my solution, whether ptp trigger or usb remote with / without synch enabled, will be accomplished by flash (or multi-stage flash) in a black room. For all situations, other than what follows, both ptp synch and usb remote (synch not available) would probably be sufficient.
If usb remote with synch is made available I envisage the extra possibility of multi-stage flash capture as follows.
Two banks of cameras:
The first bank to capture 3D Geometry with "normal" flash capture.
The second bank to, as soon as possible thereafter but certainly within tens of mSec, capture 3D Texture in presence of noise pattern projection flash (see some ref e.g's & fmi:
http://www.agisoft.com/forum/index.php?topic=1373.msg10198#msg10198 &
http://www.agisoft.com/forum/index.php?topic=1806.msg10695#msg10695 &
http://www.agisoft.com/forum/index.php?topic=1798.msg9542#msg9542 &
http://www.agisoft.com/forum/index.php?topic=1542.msg8981#msg8981 )
When you use a CHDK USB remote with sync enabled to take a shot, the camera goes almost all the way through its shooting sequence - way past where it checks focus and exposure setting - until right before it releases the shutter. And then CHDK stops it there are holds, waiting for the USB voltage to go away. If two or more cameras all wait at the same point, then they will all tend to release their shutters within 1 mSec of each other once the 5V is removed. If they are not waiting at the same exact place when the trigger signal occurs, then even with focus and exposure locked, they will be off doing something else and will not releases their shutters within 100 mSec of each other (on average).
Thanks, however I already understood this to be the case
However, even that sync is not guaranteed as an interrupt could occur after the final sequence is released, causing one or more cameras to be delayed in releasing their shutter. On many recent cameras, this occasional delay can be over 50 mSec and happens more often than not. So some additional code (precision sync) is available on some cameras that attempts to work around any interrupt and give better sync.
Thanks, however I already understood this to be the case
Sorry, you seem to want to keep quoting me and rephrasing my words to convince yourself that you will somehow get good sync without the sync (or precision sync) code working.
Are you confused ?
My post refered to comparing the shoot command in conjunction with monitoring for a fall on the usb line, in script ( I said:
i.e. just monitoring via get_usb_power then click("shoot_full_only")) to DSLR remote shutter release synchronisation timings in the EOS400D link I posted. Your script disabled usb remote before shooting.
Go ahead then - try it.
I already knew I couldn't get precision synch with DSLR's - so there's no point in doing that.
Just because you read that DSLR's can lock their shutter and focus and get 10 mSec sync does meant that Canon P&S cameras can also do that.
I'll restate the same question, differently:
If the SX150IScamera is in (i) Manual Mode (ii) Manual Focus (iii) chdk usb remote disabled (iv) running a script to (a) do nothing when USB +5V is applied then (b) shoot when USB drops to 0V - why should variability of SX150IS be up to 100mSec in compare to EOS100D 10ms ?
Feel free to hookup an oscilloscope or other test instrument and test for yourself. You asked the question and I answered it as best I can.
It would be foolish of me to buy a scope to do the wrong test, even if I knew which buttons to press on it - I could buy some SX150IS with the money saved.
In summary,
I think it's usually better not to jump to conclusions and be patient with people - my son is currently re-sitting some maths exams. It's hard going but, hopefully, worth the effort
