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Slave flash causes dark pictures


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Hi, I have been trying to use an off-camera slave flash with both a

Canon s300 and a Sony P100 point & shoot digicams. The results have

been the same in both cases - the camera has reacted to the extra

flash by underexposing dramatically.

 

I have used the same technique with film P&S (a Contax T3 and an

Olympus Stylus Epic) with good results - using the slave to brighten

up the room behind the subject or adding a 'modeling' light. I varied

the intensity of the slave by having it various distances away from

the subject so that I get an overall good exposure. I try this with

the two digital cams and they both deliver very underexposed pictures.

 

I have turned off the red-eye flash so there is only the single flash

for the slave to recognize - which it does and fires ok.

 

How does the digicam sense the flash? Is it like the thyristor

control on the flash itself - shutting off the exposure time when it

has seen enough light through the lens? Or is the light meter on the

camera not TTL and it just fires the on-board flash just long enough

for the distance that the AF has sensed? In which case I would expect

the extra, slave, flash to cause the pictures to be washed out

instead of dark?

 

This is not a question about fill ratios - but about a more basic

question how the digicams react to the extra flash in such a way that

it seems like it is over-compensating for the extra light.

 

What is happening? Any ideas?

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It just sounds like the metering preflash is firing your slaves before the shutter even opens.<p>Are you sure the redeye function is the only preflash? Digital cameras (at least Canon) have a preflash for exposure metering, which happens just prior to the shutter opening, then immediately flashes a second time after the shutter opens. This happens so quickly that your eyes cannot tell it flashed more than once, but it still screws up optical slaves.
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Your assumption that there is only one flash is almost certainly wrong. Most of these cameras judge flash exposure by measuring a pre-flash that precedes the actual exposure so fractionally that you don't notice it. However, the first flash will trigger your slave, which has therefore discharged before the picture is taken. You need a "digital slave" that can be set to ignore the pre-flash.
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Perhaps it would be helpful to explain what actually happens. When the camera fires its (normally weak) pre-flash, your slave is triggered, throwing out much more light. The camera meters the combination of its own weak pre-flash and the full output of your slave and deduces that only a very weak flash is required to illuminate the scene for the actual exposure - which ends up even darker than if you hadn't been using the slave at all in consequence.
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I rarely use flash but did one little experiment where I masked off the cameras' flash [the object was to get a single off-camera light source with reflectors as fill]

 

I have a feeling, since some of the exposures were dark, very dark, that by masking off the camera flash with some cardboard and just letting a little leak out of the side .... the preflash mentioned above was too weak to trigger the slave and so it only fired when the real camera flash fired. I was working in Manual with a 5700 so the camera had no control over the 'slave' flash being used as Key Light.

 

I understand there are some slave triggers which are designed to ignore the pre-flash but have not bothered to investigate.

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I would like to say "Thank you" to everyone that responded. Your explanation makes perfect sense.

Last night I was reading the instruction manual, looking for something else, when I did come across the information buried in the middle of a paragraph. It does use a pre-flash for metering.

So I have learned two lessons,

1 Photo-net contributors are very knowledgable and responsive.

2 The information is usually buried somewhere in the manual.

 

Many thanks again. I will consider buying one of those 'digital' flashes that can be trained to ignore the pre-flash and work out how to do fill-ratio by trial & error (the beauty of a digital camera is that I won't burn up a ton of film before I get it right!)

best regards, Tony Armstrong

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