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disabling PREflash?

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Offline spencer!

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disabling PREflash?
« on: 14 / May / 2008, 15:46:05 »
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Hi all,

I'm trying to use an external strobe with an optical slave sensor with a Canon SD700 IS.  The problem is that the canon has a preflash which messes up the strobe's timing.  Is there a CHDK fix for this?  A way to disable the preflash?  Any help is GREATLY appreciated!

Spencer
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Offline PhyrePhoX

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Re: disabling PREflash?
« Reply #1 on: 14 / May / 2008, 15:50:04 »
i have an s3is and an a620. in both cameras i can disable the redeye reduction. the normal flash syncs fine with my slave sensor flash. maybe it is the redeye stuff on your camera as well?
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Offline spencer!

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Re: disabling PREflash?
« Reply #2 on: 14 / May / 2008, 16:01:42 »
it is not the red-eye, the "normal" flash has 2(?) invisible to the naked eye preflashes and then the real flash.  The strobe I'm trying to use can't ignore the preflashes (it is a bit older).  As far as I can tell, a "hack" is my last hope.

spencer
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Offline Microfunguy

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Re: disabling PREflash?
« Reply #3 on: 14 / May / 2008, 17:24:15 »
Checkout the 'Firefly' device, it is popular with cavers and can learn the number of preflashes before firing an external flash.

I find it very reliable.
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Offline ripwink

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Re: disabling PREflash?
« Reply #4 on: 15 / May / 2008, 02:47:51 »
I think that pre flash in your camera is either for red eye reduction or possibly for focus detection if dedicated focus detection light is absent in your model. You try disabling the red eye reduction and focus assist mode.

Also try, if available, putting your onboard camera flash to manual (non auto) mode.

My s3is works ok with external flash with built in slave.
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Offline DataGhost

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Re: disabling PREflash?
« Reply #5 on: 15 / May / 2008, 03:18:21 »
The pre-flash is to determine the amount of light available so the camera can adjust the final flash strength to the current situation (keeping in mind the flash exposure COMPENSATION you might have set). If you use manual mode, you'll probably notice that you can't set the flash compensation anymore, but rather the flash strength. Because of this, there will be no preflash in manual mode.

Not using a preflash in a semi-automatic mode will just screw up your lighting so I don't really see a use for disabling it unless you want to override the flash strength yourself... but you'll be better off using manual mode instead in that case, since (in my experience) the camera always chooses the same Tv/Av (in P, 1/60 and F/2.7 or whichever is the biggest).
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Offline whoever

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Re: disabling PREflash?
« Reply #6 on: 15 / May / 2008, 03:33:47 »
This is a regular metering pre-flash. Building a strobe that only triggers on the main flash of P&S cameras has been the subject of quite some DIY projects, ranging from relatively simple circuits with a timer, used to time out pre-flashes, to microprocessor-based units that can "learn"  the flash sequence of most any camera.

In my IXUS (but probably not in you cam, or you would've found it already) there's an option "Flash Exposure Lock", which, when activated, disables the metering pre-flash. CHDK itself does not help here. I tried to analyse the firmware, looking for ways to control it, but it's too complicated for me to reverse-engineer -- it's not just a single function or variable.
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Offline trintek

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Re: disabling PREflash?
« Reply #7 on: 09 / August / 2008, 19:06:31 »
This is actually the same on many, if not all, canon cameras. press the shutter half down to focus, while holding it half down, press ISO. you should get a metering flash and FEL will display on screen. you are now locked with no metering preflash. this should last until you either shut off the camera, go into review mode, or press ISO again.

my question, of course, is... is there a way to add that sequence of events to the user menu, or as another menu item? something like

press "shoot_half"
sleep 2000
press "iso"
sleep 1000
release "iso"
release "shoot_half"

no clue if that is even correct or supported, just looked at another script to get the basic syntax

edit: spelling
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Offline trintek

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Re: disabling PREflash?
« Reply #8 on: 09 / August / 2008, 22:02:51 »
Code: [Select]
@title Enable FE Lock by Trintek
press "shoot_half"
sleep 2000
press "up"
sleep 1000
release "up"
release "shoot_half"
end

this works on the A560 with no problems. if it doesnt work on yours, change "up" to "iso" and try it that way.

now main question, is there a way to add it as a user defined command or something similar so you don't have to load the script, execute it, then reload the script you use by default?
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Offline dzsemx

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Re: disabling PREflash?
« Reply #9 on: 10 / August / 2008, 03:32:34 »
Quote from: trintek on 09 / August / 2008, 19:06:31
This is actually the same on many, if not all, canon cameras. press the shutter half down to focus, while holding it half down, press ISO. you should get a metering flash and FEL will display on screen. you are now locked with no metering preflash. this should last until you either shut off the camera, go into review mode, or press ISO again.
...
on my A650 and a630 there is no scuh option just exposure lock by pressing +/- on halfpress and locks exposure only for a shoot
in manual mode i can get rid of preflash, but i have to set up flash manually
with slave flashes works perfectly
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