Strange. Just tested it (for probably the 1000th time) on my SD940. Works as expected - with or without sync enabled it takes the shot when 5V goes away. I also enabled a 5 second sync delay to confirm it goes through the sync code.
So I checked the source for the ixus220_elph300hs and it appears the necessary hook for sync shooting is not enabled for that camera. The hook to wait_until_remote_button_is_released is not implemented in capt_seq.c for any of the firmware versions. The code hook to disable noise reduction is missing too.
This means you can still try to take pictures with sync disabled but the accuracy of the sync will be suspect.
The above seems it may be relevant to a number of cameras.
In above quote, does this:"hook to wait_until_remote_button_is_released is not implemented in capt_seq"
in "OnePush/Synch enabled" mean that when +5V is first applied then, as per:
In that mode, when the USB power is applied, the code does the equivalent of an immediate "full_press" of the shutter button. I believe that causes the camera to focus and set exposure right away and then proceed to shoot - where it gets trapped by the "wait_untill_remote_button_released" hook.
In two press mode, the initial press looks like a "half_press" the camera. It will focus and set exposure and apparently look for "faces". Quickly releasing and repressing the remote button triggers the additional "full-press" shot and the "wait_untill_remote_button_released" hook. Going there with face recognition events queued up seems to be a bad thing if the shooting task "waits". It's hard to understand why that would be the case as with sync disabled it goes through exactly the same code except for not waiting.
(a) because no trap is encountered it is the variability between cameras subsequently getting focus / exposure that causes the loss of "synch enabled" levels of synch between them ?
(b) with a missing "wait_untill_remote_button_released
" hook should both OnePush with either:
act in exactly the same fashion ?
(c) if the cameras had native Canon Manual Mode / Manual focus, and were in those modes, should the focus/exposure "period" of all cameras be approximately zero and thereby effectively re-instating "synch enabled" levels of synch (even in absence of the hook) ? or is there other "stuff", e.g in "shooting.c", i'm not thinking about yet ?
I'm currently interested to identify a work around to the apparent lack of "OnePush/Synch enabled" on the SX150IS first reported here http://chdk.setepontos.com/index.php?topic=11667.msg118374#msg118374
Fmi, a further description: http://chdk.setepontos.com/index.php?topic=7127.msg112154#msg112154