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Overrides don't work with external flash

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Offline shawmut

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Overrides don't work with external flash
« on: 13 / May / 2010, 14:17:51 »
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I have an Sx-10is with 100c-0.9.9-890 build firmware on my card.

The aperture, shutter speed etc overrides work fine normally. When I put my flash on the camera (Nissin Di466) the camera no longer will do any overrides. The overrides won't even work if the flash is attached and powered off.

I've tried attaching the flash when the camera is off/on/in playback and it still does not override the cameras values. It shows the override values when the shutter is half pressed but it doesn't follow through.

The overrides work with the cameras internal flash enabled.

Is there something I'm doing wrong, a bug, or a limitation of CHDK?
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Offline sCattleya

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Re: Overrides don't work with external flash
« Reply #1 on: 07 / June / 2010, 15:40:49 »
I have the exact same problem .. no Ev override with external flash (mine is the Canon 220EX) .. even though exposure times of 1/1000 can be used with the internal flash. An explanation is offered here:
http://forums.dpreview.com/forums/read.asp?forum=1010&message=33849915
(It's message 33849915 if the link doesn't work.)
The author states that "shoe flashes" with eTTL have an output function that requires 1/350" for full output and that the camera senses their presence and adjusts accordingly overriding the CHDK override. With "High Speed" synch on, I can get the nominal short exposures (and they are underexposed) provided that I use Tv. In Av, the mode I really want to use, the exposure time reverts to 1/60. 
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Offline sCattleya

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Re: Overrides don't work with external flash
« Reply #2 on: 07 / June / 2010, 15:54:03 »
postscript:
The truth of the explanation provided on DPreview that I cite (i.e. the prolonged output envelope of hot-shoe flashes) is found in tables of the fall-off in GN with HS Synch. For example, moving from 1/125 to 1/650 causes a 2 stop reduction in output. This is somehow not compensated for by eTTL.
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Offline sCattleya

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Re: Overrides don't work with external flash
« Reply #3 on: 08 / June / 2010, 10:53:51 »
As the DPReview post further states, "foreign" external flashes that rely on GN or thyristor should accept Tv override. I have tried this with a Nikon SB800 in GN mode. This more powerful flash has a baseline duration of 1/800" and easily accepts synch speeds of 1/1000 in the Canon native M mode. However no luck at all with CHDK override! In both Tv or Av mode the mere presence of the flash in the hot shoe, whether on or off, "overrides" the override and uses ambient light to set exposure time.
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