Any reliable way to trigger the camera at a precise moment? - General Discussion and Assistance - CHDK Forum
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Any reliable way to trigger the camera at a precise moment?

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Offline RaduP

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Any reliable way to trigger the camera at a precise moment?
« on: 25 / January / 2010, 23:27:50 »
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I want to start doing some high speed photography, like for example a bullet smashing an apple or egg or water balloon.
I will use a PIC microcontroller, with some photodiodes that will detect the precise moment when the picture must be taken.
Now, I was thinking to use a DSLR with an external flash (the flash would be triggered by the PIC). But the downside is that the flash has a relatively long pulse, something like 750 us, and I would need less than that to properly freeze the image.

So I was thinking of using my A570 IS, which can go somewhere like 1/40K of a second (250 us). The problem is, I need to reliably trigger the camera at the precise moment. Some shutter lag is OK, so long as I know exactly how long this shutter lag is going to last. So if the time between pressing the shutter (or issuing an USB voltage) and the time the picture is actually taken varies by more than at 500 microseconds, it's not going to be very useful.

Did anyone try something like that? Any numbers regarding the consistency of the time between pressign the shutter button or USB voltage and the time the picture is taken?
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Offline awdark

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #1 on: 26 / January / 2010, 00:21:31 »
If it helps, I found this http://www.instructables.com/id/Laser-Triggered-High-Speed-Photography/ and there are a few other articles on hackaday which uses a variety of ideas including http://hackaday.com/2009/11/19/axe-your-camera-again/ (which is more for DSLRs but the idea is the same)
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Offline fudgey

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #2 on: 30 / January / 2010, 09:16:42 »
CHDK's USB remote trigger may be precise enough for you in the sync mode using the a570. I believe you'll be working near it's limits, though.

This feature is originally from SDM where it's quite essential to syncing two or more cameras. SDM users have made some measurements, see http://stereo.jpn.org/eng/sdm/crt_synch.htm.

You'll of course have to determine the actual trigger delay yourself (for stereo shooting the absolute value isn't important unlike for your external trigger setup), but it should remain fairly constant once you've got it right.
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Offline Microfunguy

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #3 on: 30 / January / 2010, 09:52:05 »
In Synch mode, the photo is taken after a very precise delay when the switch (or USB signal from other devices) is released.

This device relies on that repeatability to fire external flash equipment :
 http://www.gentles.ltd.uk/gentstereo/options.htm

You have to adjust the delay.

Depending on the camera, I have measured from 11 to 23 msecs delay.
The delay itself is typically repeatable to 0.2 msec.

It is possible to use SDM to do this, I have previously provided details of a suitable method.

If there is no triggering event within ten seconds, a photo is taken anyway.

I am thinking of supporting this method by only saving a RAW image if a trigger occurs, otherwise just save a lowest-quality 640x480 JPG .

If you want to try it, let me know.


David
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Offline RaduP

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #4 on: 01 / February / 2010, 03:16:30 »
I was thinking of the following test method:
Have a PIC send the USB signal to take a shot, and also light an LED. The camera will have the flash enabled. Then I will have two photodiodes, one right against the LED and the other one near the flash, and connect each photodiode to my oscilloscope, one each channel. Then I should be able to tell what is the delay between the actual signal and the moment the photo is taken.
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Offline RaduP

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #5 on: 01 / February / 2010, 03:18:26 »
One more thing: How does SDM do it? Is there a special loop that monitors the USB signal, and if detected it manually calls the shoot() API function? Or it just emulates a shoot button key press?
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Offline Microfunguy

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Re: Any reliable way to trigger the camera at a precise moment?
« Reply #6 on: 01 / February / 2010, 05:22:29 »
Trigger the scope on a high-to-low transition of the USB signal (switch release).
Detect the main flash (the preflash has already taken place).

The Canon image-capture task branches to wait_until_remote_button_is_released() function.

The SDM code looks complicated because of the various shooting-modes supported but essentially it just constantly checks the USB signal has fallen to zero and then lets the Canon capture task continue.

The flash will give you a nice strong scope signal, it will be interesting to know the delay.

The A570 only has front-curtain flash so it will trigger almost as soon as  the shutter opens.
I guess it must delay for a very short time before firing, you could verify that by seeing if setting a high shutter speed altered the shoot-delay value.

If any progress is made with this, it would be interesting to see if the image-capture task can be interrupted even further into its cycle.

That may make the delay even shorter.

« Last Edit: 01 / February / 2010, 06:20:14 by Microfunguy »
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