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Sometimes it would be better to up the ISO for better exposure and a cleaner image.
+1 High ISO gets a rap for noise, but underexposing a lower ISO (imo) can be a worse rap than a proper exposure/ISO combination even if the ISO is a little higher than you'd want for a "fine art" image. You really weren't flirting much with "high ISO" ... one more stop could have made a bit of difference. High(er) ISO with enough light is better than low ISO with too little light ... your 5D can easily handle ISO 800.
+1 @ BrianO ceiling height, etc. The GN of your flash (for a given ISO, and given AOV) is for your flash to subject distance. While your camera to subject distance is a straight line, your flash to subject bounced off the ceiling is going to be a longer light path.
There are three things that seem to be working against you on this.
1. Longer light path of the bounce (reduces aperture calculated from a given GN)
2. Wide beam setting for group coverage (reduces GN)
3. Bounce absorption/diffusion (reduces effective GN) +1 FEC assumed as attempt to account for this
While the 580 EX has a rated GN of 58m (at 105mm FL coverage), it is rated @ 36m for a 35mm FL coverage
You didn't say what FL coverage you were using, camera to subject distance or ceiling height, so I'll make some assumptions to illustrate.
At ISO 100, a GN of (appx) 100 ft (35mm zoom head setting) and a camera to subject distance of 12 feet, your aperture should be f8. (i.e. 100 / 12 = 8.5)
Take that same camera/subject distance and bounce your flash off a 12 foot high ceiling and your distance increases by a factor of 1.4 ... so now, instead of GN 100 / 12 ft = f8, it becomes GN100 / 17ft = f5.6
While you can tell your flash to add in +1 FEC, it can't add in more than it is capable of producing. So, even though you've told it to compensate for the loss caused by the bounce difusion loss (in addition to the extra distance), the amount of light available to do so isn't really increased because we are calculating from MAXIMUM to begin with ... so we still have to somehow account for that loss ... which takes us down from 5.6 to 4.0.
So, at MAX POWER the 580EX will deliver correct exposure for a camera to subject distance of 12 feet, with 12 foot high (assumed white) ceilings, with the flash set to a 35mm FL coverage will yield an aperture of roughly f4.
However, if your shooting distance is farther than 12 feet, or your flash zoom head setting is even wider than 35mm (or both) your effective GN / respective aperture will be even lower. I'm guessing that you may have been farther away than 12 feet for a group of 7 people wide and were using your flash's zoom head at wider than 35mm setting ... thus lowering your GN below the 36m we started from, suggesting that f2.8 (or f2.0) is probably more likely the appropriate aperture at ISO 100.
Obviously an aperture of 2.8 would seem to shallow of a DOF for your group, so the increase of 2 stops from ISO 100 to ISO 400 gains you an effective aperture of 5.6. Shooting at f8 would then be a 1 stop underexposure ... and your attempt to compensate using the +1 FEC is a good thought ... but you've already exhausted all the light your flash has to offer ... so even though you are asking it to compensate for the bounce loss ... it simply can't make up the difference if it doesn't have anything left to do so with. If f5.6 is where you want to be, ISO 400 is all the "higher" you need to go. But if you are wanting to shoot f8 or f11, you'll need to be at ISO 800 or 1600 unless you make other changes.
1. Raise ISO to 800 and gain that stop
2. Move flash off camera, closer to subject or closer to ceiling (or both) to reduce light to subject distance
3. Position group in 3 rows or arc to allow for a narrower FL for lighting coverage
4. Set your flash zoom head to a longer FL setting (it is traveling farther via the bounce and will disperse wider than if it were aimed directly at subject)
5. Add more light (i.e. second flash, increase ambient)
After all that ... it simply sounds like you were underexposed because your flash was "maxed out" from being asked to spread a "wide beam" ... a "long way" ... off a "soft/textured/diffuse surface". Bounce has a nice effect regarding your "quality" of light ... but it does come at a price to be paid regarding the "quantity" of light reaching your subject.
NOTE: While unlikely for this type of shot (i.e. future reference) ... it is also possible that somehow your camera's metering (mode ??) picked up a strong specular reflection that trigger the flash to power down some. (Another good reason for shooting manual.)
Also, while the "calculations" are theorectical, many digital cameras ISO ratings are slightly inflated from reality by 1/4-1/5 stop as they are "rounded up" to the standard ISO increments, i.e. ISO 400 might only be ISO 360 or so ... not really a biggie for most situations, but when you are already short on light, it doesn't help matters any either.
Edited on Dec 24, 2011 at 03:21 PM · View previous versions