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multicam: synch problems when using scripts

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Offline Alarik

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Re: multicam: synch problems when using scripts
« Reply #30 on: 21 / July / 2012, 06:39:38 »
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Would that not be expected as all you need is for all the shutters to be open when the flash goes? The short duration of the flash burst provides the sync.

actually not... its clear that if i can shoot in a controlled environment (studio) I would shot in open flash with shutter speed at 1 sec or higher and a fast flash... this is easy and already tested...

What I need to know is: in a non-controlled environment (any public place where I cannot have complete darkness for open flash shots), which is the maximum shutter speed I can use and be sure that the shot is synchronized (so that I won't have the "vibrato" effect in the resulting video)?
And futhermore how much I can push the shutter speed + camera delays in order to create a super slow motion video?
There are many variables involved in this and I assure you the answer is not trivial... If you look my last tests, for example, you can see that I can have synch at 1/60 but not at 1/100...(meaning that all shutters are NOT open when flash goes...even if this was expected...;-)
This leads me to think that there's about 100ms maximum of non-controllable delay on each cam, so I can do slow motion only shooting with delays > 10ms...
Also (since max synch is at 1/60) I will never be able to shoot super fast motion, etc...
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Offline waterwingz

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Re: multicam: synch problems when using scripts
« Reply #31 on: 21 / July / 2012, 11:01:53 »
Quote from: Alarik on 21 / July / 2012, 06:39:38
This leads me to think that there's about 100ms maximum of non-controllable delay on each cam
Now we are at the root of the statistical problem.  On average, the random delay is much smaller than 100 mSec and thus with a two camera setup you can usually get almost perfect sync. 

But sometimes you won't. 

As you add cameras,  the probability goes up that one or more cameras will not be in sync on any particular shot until you get to the situation where you have so many camera that  the chance of them ever all being in perfect sync approaches zero.

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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline Alarik

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Re: multicam: synch problems when using scripts
« Reply #32 on: 21 / July / 2012, 12:06:37 »
well,
when talking about sync, in my opinion, is not a "yes or not" question but a "at which shutter speed" i have sure sync at 100%...
If:
d = max random camera delay
f = flash duration
then
max shutter speed = d-f

From my tests I saw that i have sure sync at 100% when shooting at 1/60 with a high speed flash.
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Offline lapser

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Re: multicam: synch problems when using scripts
« Reply #33 on: 18 / November / 2012, 14:58:35 »
Are you still working on this problem?

What if the CHDK gurus added this Lua function:
wait_click_full_only(delay) -- delay in microseconds

The function would first run a tight loop checking for the USB trigger. It would delay the required amount (different for each camera), again in a tight loop for microsecond accuracy. Finally, it would click the shutter to take the shot, the same as:
click("shoot_full_only")
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EOS-M3_120f / SX50_100b / SX260_101a / G1X_100g / D20_100b
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Offline waterwingz

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Re: multicam: synch problems when using scripts
« Reply #34 on: 18 / November / 2012, 17:54:40 »
Quote from: lapser on 18 / November / 2012, 14:58:35
Are you still working on this problem?

What if the CHDK gurus added this Lua function:
wait_click_full_only(delay) -- delay in microseconds

The function would first run a tight loop checking for the USB trigger. It would delay the required amount (different for each camera), again in a tight loop for microsecond accuracy. Finally, it would click the shutter to take the shot, the same as:
click("shoot_full_only")
That's exactly  what the USB sync delay function does now.
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline lapser

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Re: multicam: synch problems when using scripts
« Reply #35 on: 19 / November / 2012, 01:04:11 »
Quote from: waterwingz on 18 / November / 2012, 17:54:40
That's exactly  what the USB sync delay function does now.
Great! I see you used my idea before I even thought of it  :D

So you're starting the shot in close sync, but the actual time the shutters open has some variation between the cameras of a little less the 1/60th second?

The next idea then is to calibrate the individual cameras somehow. You could repeatedly trigger a flash and sync signal for each camera, using a fast shutter speed. You would then vary the delay until the flash appeared in the the image. Using get_shot_histo() might help. Maybe the delay from trigger to shot is constant in a single camera but varies between cameras?
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Offline Alarik

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Re: multicam: synch problems when using scripts
« Reply #36 on: 19 / November / 2012, 05:03:15 »
Quote
Maybe the delay from trigger to shot is constant in a single camera but varies between cameras?

I dont think so,
look at these tests I've made:
http://www.experience-3.com/PROJECTS/timefly/neorTest.html
(rows = cameras, columns= different shots)

here you can see exposure differences (due to the cam's unpredictable delay) that is quite random and not the same for each cam

look also:
http://www.experience-3.com/PROJECTS/timefly/neorTest2.html

here the "white frames" indicate shot out of flash: again it's very random...
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Offline waterwingz

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Re: multicam: synch problems when using scripts
« Reply #37 on: 19 / November / 2012, 10:26:07 »
Quote from: lapser on 19 / November / 2012, 01:04:11
The next idea then is to calibrate the individual cameras somehow.
Pointless exercise.  The issue here is that the delay between when the USB remote power is removed and when the actual exposure is taken varies within each individual camera. 

You might want to read carefully through this thread : 

testing sync among more cameras

starting with this posting :

http://chdk.setepontos.com/index.php?topic=8312.msg92385#msg92385

Reproducing what was done for the S95 on the sx130 would go a long way to completely solving Alarik's issue.
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline andrew.stephens.754365

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Re: multicam: synch problems when using scripts
« Reply #38 on: 26 / August / 2013, 16:08:35 »
Quote from: waterwingz on 19 / November / 2012, 10:26:07
Reproducing what was done for the S95 on the sx130 would go a long way to completely solving Alarik's issue.

Hi waterwingz, hope it's not a silly question ? has your splendid work http://chdk.setepontos.com/index.php?topic=7127.120 made any difference to the quoted S95 / sx130 statement ?
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Offline andrew.stephens.754365

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Re: multicam: synch problems when using scripts
« Reply #39 on: 26 / August / 2013, 16:20:57 »
Quote from: Microfunguy on 14 / July / 2012, 14:58:50
The other problem is flash.

If this is for 3D modelling or similar, you need a single light source to avoid confusion.

Hi David, i hope the snow earlier in the year didn't cause you too many extra problems !

I might plan to use something like 6 of these http://www.elinchrom.com/product/D-Lite-RX-ONE.html ...so i'd [really] appreciate your thoughts:

in what way might a single light source avoid confusion ? [so long as light production starts/stops (from all units)  within a small number of microseconds of each other ?]
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