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I have a Sony NEX3 camera and an old external flash. Since Sony NEX3 doesn't have a hotshoe I use the optical sensor in the flash to be triggered by my camera's flash.

Unfortunately the camera uses two flashes (double flashes even when the red eye is off) so it triggers the off-camera flash before the shutter captures it. What can I do regarding this problem?

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The first flash is probbaly for light metering, but the external flash doesn't know that of course. You should be able to get an external optical trigger that can handle this (a hotshoe with a tripod thread on the bottom). I'm sure I've seen one with an e-ttl mode but I can't find it for now (hence this comment not an answer). – Chris H Mar 15 at 10:12
    
@ChrisH Exactly, Isn't there there any other tweaks? I was thinking about slowing down the shutter time so that the flash will have time for recharging but again it will make the image blurry, Are there any info on Sony's flash connector's configuration? Maybe I could connect the cable to hotshoe connector in my flash?! – adel rahimi Mar 15 at 10:24
1  
@ChrisH I encourage you to make it an answer even without that bit about an ettl external optical trigger (which isn't the main point anyway). – mattdm Mar 15 at 10:27
    
@adelrahimi that wouldn't help, or not much. A flash+long exposure photo is sometimes done, but is quite a specific look. And you might well find that the external flash doesn't fire the second time even if you can use second curtain sync on the NEX3. – Chris H Mar 15 at 12:16
1  
@mattdm, I've tracked down some info worth having. – Chris H Mar 15 at 12:32

One good solution is to get a Hot Shoe Flash Adapter so you can use your old external flash right on the camera, or get a smaller Manual only flash to trigger your external optical slave flash.

$10 Amazon Hot Shoe Flash Adapter MSA-10 for Sony NEX 3

enter image description here

An even better solution is to get this Wireless radio transmitter/receiver:

$10 Wireless Flash Trigger Hot Shoe Adapter with Receiver for Sony NEX3

enter image description here

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Oh... How I did not think of that? :o) – Rafael Mar 15 at 17:16

The first flash is probably for light metering (Canon call their version e-TTL, Nikon iTTL if you want to read about how it works), but the external flash doesn't know that of course.

You should be able to get an external optical trigger that can handle this (a hotshoe with a tripod thread on the bottom). Optical triggers are cheap so this would be a viable solution. Slave triggers that will do what you want apparently exist: "For example, I have a very-inexpensive underwater optical slave that can be set to count various numbers of pre-flash pulses before arming for the "real" flash" (photocentric.com).

Here's a list (dpanswers.com) of some models that apparently ignore a preflash, or attempt to smartly ignore a string of preflashes. The brand reviewed there is Wein (models HS-XL D, PN-XL D, L8 D and XL8 D -- might be out of date now).

Apparently the Seagull SYK-5 Trigger can be set to ignore red-eye reduction flashes -- it might work for you (Amazon in the hope that the link won't die too soon).

Some Yongnuo external flashes can be set to an optical slave mode (davidpartington.com) if you're shopping for a new flash.

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2  
Chris, I think you're right. I thought about AF assist light. – Agent_L Mar 15 at 16:00

If you're using a disconnected non-dedicated external flash like this, it'll be unmetered - you'll have to set all the exposure and flash parameters manually to get the image you want.

The first flash from your onboard flash is a metering flash for auto flash exposures. With manual exposures like this, it's neither needed nor even useful.

So, set your onboard flash to a manual exposure mode (and the rest of the camera, too). It'll only give a single flash, at the right time, which your external flash will pick up at the right time.

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2  
The Sony NEX3 onboard flash doesn't have an option for Manual flash. – Mike Sowsun Mar 15 at 14:07
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You're probably right (so +1) but I'm sure there are cameras that fire the preflash even when they don't need to. Also metering off a main subject lit mainly by the internal flash, with the external flash diffused and used to soften shadows or illuminate the background may still be better with TTL metering. – Chris H Mar 15 at 14:09
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@MikeSowsun Well that's a bit rubbish! Ah well - thanks for confirming. – eftpotrm Mar 15 at 14:54
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@ChrisH There's two reasons I'm aware of for a preflash - metering and redeye reduction. While I agree with you that the external flash may produce an attractive effect, that'll be by luck because in the scenario the OP described it's definitely unmetered and so rather more likely to just overexpose the subject. – eftpotrm Mar 15 at 14:56
    
If the external flash is illuminating the subject to any significant extent I agree. – Chris H Mar 15 at 15:37

An alternative to the optical triggers or a new flash that Chriss H recomended.

If your external head allow it, lower the power to 1/2 - 1/4, so the flash has enough power to trigger twice.

The problem is that your camera can "think" there is enough light so it won't fire for the exposure.

You can not have all on life.

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There will be a minimum cycle time even if there's enough power left in reserve for the next flash -- this may or may not be too long. It may also confuse the metering. – Chris H Mar 15 at 15:38

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