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Old Aug 1, 2008, 9:21 AM   #1
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Im a little confused with guide numbers and setting my 580exii for correct manual flash output. (GN= 58m @ full power)

At the moment I own a 580exii a 40d, and have just bought a flash meter. Having recently taken a class in studio lighting, I understand the process of how to measure a studio strobe with flash meter, take the reading and dial in the fstop and/or shutter speed into camera. I also understand the value of incident readings Vs reflected readings, and so I would like to improve my on-camera flash lighting technique using my flash meter.

1) Can someone please explain what the workflow/starting point of how to get a correct exposure with flash output for a very dim room (so all lighting will be flash, no ambient lighting).
2) Do i start by keying in a desired shutter speed for starters?
3) Do i choose 100 ISO? (would 100 be ok, because I will use flash?)
4) How does my guide number come into the workflow?
(ie how do I come up with appropriate F/stop to enter into the equation of GN divided by f/stop = distance flash to subject.)
5) Do I just alter the power on the flash maually? ie start at 1/1, take a flash meter reading, key in whatever the f/stop says?
(Isnt there a better starting point of approx output based on GN?)
5) Basically where do i start here, what should i do first?

Please, please help if you can! Thanks guys! :?
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Old Aug 3, 2008, 3:07 PM   #2
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hi gerryb,

well we use the 430ex, but i'm sure they are the same. anyway, if you don't want the background ambient light, which usually you bump the iso up, slow shutter and big ap. try 125 shutter, anywhere between f8-10, and depending on the distance to your subject, start with 1/2 power on your strobe, then you can move from there. thats why i like the target, because it gives you the correct exposure for your individual situation. because once the subject moves out of distance(steps back or forward) you will either over or underexpose. i haven't really used a light meter, i mainly use the target or the cameras meter. then adjust from there.
i hope this helps, good luck, john
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Old Aug 6, 2008, 4:54 AM   #3
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Thanks AirbrushJohn,

I'll continue playing around with the flash and start at 1/2 power, go from there. After experimenting i see the higher aperture exposure/flash output collapses the bkgd compared to say f/4.5 exposure & flash output which lets in alot more ambient lighting. I think i might write up a mini chart of "distance of flash-to-subject", and appropriate f/stops to use at 1/2 power. Hopefully after a while I'll be able to recall these in the heat of battle, on a reception dancefloor for instance. where the bride's white dress might underexpose with an ETTL metering system.

if anyone has any further tips to add, please reply! thanks folks.
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