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Old Mar 2, 2008, 8:37 AM   #1
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I am wondering how the flash exposure control works on compact digitals. I know how my SLR TTL works. Is the compact lighting determined by a light meter that is not monitoring through the lens? How does the camera determine how much flash lighting to provide? Is it measuring the actual light returning to the camera or calculating what it thinks is needed and sending that much flash output?



Of particular interest is how the flash light control works on Canon G9 and A650 IS as I am considering buying one of these.

m wondering how the flash light exposure control works on compact digitals. I know how my SLR TTL works. Is the compact lighting determined by a light meter that is not monitoring through the lens? How does the camera determine how much flash lighting to provide? Is it measuring the actual light returning to the camera or calculating what it thinks is needed and sending that much flash output?




Of particular interest is how the flash light control works on Canon G9 and A650 IS as I am considering buying one of these and am considering buying a supplemental flash unit.
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Old Mar 2, 2008, 8:57 AM   #2
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Most modern digital cameras use a short metering preflash to judge the length of the main flash burst needed. This short preflash occurs approximately 100ms before the main flash burst with most digital cameras.

In a non-DSLR camera model, the camera's main image sensor is also used for metering. Based on how much reflected light is seen from the short preflash, the camera decides how long the main flash burst needs to be (usually taking metering mode and focus point into consideration).

So, if you want a fully compatible flash model that's aware of the camera settings and communicates properly, make sure it's listed as a fully compatible flash for the camera you choose (many older dedicated flash models designed for film will not function on a digital camera unless you use manual power settings when available).

If you don't care about a dedicated flash model, you could also use a non-dedicated flash (for example, a Sunpak 383 Super). Then, you'd use manual exposure on the camera, setting the aperture and ISO speed to match the settings on the flash for the range selected (letting the flash control the output for the ISO speed and aperture set).

A flash like this Sunpak has a built in sensor that can measure reflected light during an exposure, terminating the flash when it sees enough light for the aperture and ISO speed set. So, you can eliminate the metering preflash that way if you want to use a non-dedicated model and use manual settings.

Just be careful about trigger voltage if you go that route (as modern digital cameras can be damaged by many older flash units).

See this thread:

Trigger Voltage Limits with Modern Digital Cameras


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Old Mar 2, 2008, 10:49 AM   #3
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Thanks for the information. This will help.

I take underwater photos and use a fiber optic driven external flash (YS-25)and am consideringadding the Sea and Sea YS-110 withthe 110as the main flash and the YS-25 as a fill flash. These are triggered by the camera flash via fiber optics. The YS-25 using its own sensor and the YS-110 having the choice of its own sensor or TTL.

One issue I have is as the diving continues, my camera gets slower recycling since every shot is using the max. flash capacity on the camera. This happens with new, fully charged batteries but Isometimes shoot 100 photos per dive. I would like to set the camera flash to put out minimal power since its not really doing anything but triggering the other flashes (the camera flash is covered). Sounds like I should be able to use the flash compensation to -2 stops and get some assistance.

I have to use the cameraback display for taking each shot as that is the only reasonableaccess I have while diving. This probably takes up a fair amount of power as well. I have the display off between shots.

Other ideas or suggestionsare highly welcomed.






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Old Mar 2, 2008, 11:05 AM   #4
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Diver Dave wrote:
Quote:
I take underwater photos and use a fiber optic driven external flash (YS-25)and am consideringadding the Sea and Sea YS-110 withthe 110as the main flash and the YS-25 as a fill flash. These are triggered by the camera flash via fiber optics. The YS-25 using its own sensor and the YS-110 having the choice of its own sensor or TTL.
That's probably going to be a problem. What happens with optical triggers that are not "digital aware" (designed to ignore preflashes) is that they fire the external flash with the preflash versus main flash.

So, the external flash doesn't contribute anything to the exposure when the camera is capturing the image during the main flash burst.

Wein makes a "digtal aware" slave trigger that's designed to ignore preflashes. They're about $34.95 each, or B&H sells them by the 3 pack for $74.95

http://www.bhphotovideo.com/c/produc...ut_Slave_.html

I don't know anything about how the YS-25 or YS-110 work, and if you can trigger them by an alternate method (or if you can set them to ignore preflashes).

But, on the surface, you're probably going to have an issue triggering one optically using a camera's built in flash without some method of ignoring a preflash. Some camera models allow manual power settings with a flash. But, that's usually a feature found in more advanced DSLR models (even most of the entry level DSLR models don't have manual power settings with a flash).

With some camera models, you have a way to fire the preflash early via flash exposure lock. Then, with flash exposure locked, the preflash won't fire again when you get ready to take your photo (so you could give the external flashes time to recycle after they trigger on the metering preflash before you take the photo). But, it would take a bit of research to figure out if the models you're interested in would behave that way or not (and many models won't have a Flash Exposure Lock Feature).

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Old Mar 2, 2008, 11:21 AM   #5
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It looks like these models may have been designed for digital. For example, I see this mentioned in the specs for the YS-110:

"Pre-flash control : Equipped with pre-flash cancel mode (ignores one pre-flash)"

So, it may not be an issue. I'd probably try to find someone that's tested one with the camera you choose (or at least a similar model within the same brand), just to make sure though, since you will find some variation in how camera models approach preflashes.

P.S.

You may want to ask about user experience with them in our Underwater Digital Photography Forum

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Old Mar 2, 2008, 3:52 PM   #6
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I have used the YS-25 with success and there are no problems with the digital preflash. I expect no issues with the YS-110 either.THe issue I am trying to get around is the camera recycle rate.

Based on what I have read here and concluded, the preflash will provide no return light to the camera (its shielded and has to be) so the camera signals 100% max flash output each flash.

What I am after is figuring out how to get the camera to only flash a small level of light. I will ask in the underwater section. Perhaps others have figured out this issue. I was hoping to figure it out once I learned how the camera light control was working.

thanks for the input.
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Old Mar 2, 2008, 5:02 PM   #7
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Looking at the G9 and A650IS reviews here, it appears that both of these Canon models have a Flash Output setting (available in most shooting modes), that lets you change it by +- 2EV.

So, you may be able to dial it back for faster recycle times, as long as it's producing enough light for your optical triggers to work. You'll see the Flash Output menu choice go by if you watch the A650 options here:



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Old Mar 15, 2008, 6:46 PM   #8
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I have a Canon A710 IS. I also have a Vivitar 283 flash. I set the camera for no anti-redeye pre-flash. There is a photoelectric trigger on the flash which senses the cameras flash and then flashes the big flash.

However when I look at the picture it looks as though the extra flash hadno effect. The big external flashflashes but the picture looks exactly like it does when I don't use the external flash.

Does anyone have any idea what is happening and how to fix it?
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Old Mar 15, 2008, 7:06 PM   #9
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Your camera is using a metering preflash, just before the main flash burst. The optical trigger in your Vivitar is firing it on the metering preflash versus the main flash. So, it doesn't contribute a thing to the exposure.

You'll either need to use an optical trigger that can ignore a preflash, or go with a radio trigger instead. Here's a quote from an earlier post I made to this thread that's applicable to your problem, with links to an optical trigger solution that you may want to consider:

Quote:
What happens with optical triggers that are not "digital aware" (designed to ignore preflashes) is that they fire the external flash with the preflash versus main flash.

So, the external flash doesn't contribute anything to the exposure when the camera is capturing the image during the main flash burst.

Wein makes a "digtal aware" slave trigger that's designed to ignore preflashes. They're about $34.95 each, or B&H sells them by the 3 pack for $74.95

http://www.bhphotovideo.com/c/produc...ut_Slave_.html
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