Newbie question here - when I have the omnibounce on the 580ex ii and point it directly at the subject, it is usually at least 1 stop underexposed (possibly more). When I tilt the head up 45 degrees, I get near perfect exposure. Any ideas? I am shooting E-TTL.
When the flash is in the tilted position, the fresnel lens goes into the wide-angle position, and the camera calculates the exposure from the metering preflash based on that assumption. With the flash in the direct-fire position, the camera assumes that the beam will be focused at whatever focal-lengh the attached lens is, and bases the output on that, but the actual beam is spread out by the Omni-Bounce. If you're using a long lens the output can be off considerably.
I never use direct flash when I have the Omni-Bounce mounted (that would kind of defeat the purpose, really), but if you want to do it you can use your 580EX II in Auto mode rather than TTL mode; it'll measure the actual reflected flash at the time of exposure that way.
One thing that might seem obvious, using the omnibounce outside will do nothing but reduce your flash range. I am amazed at the number of people I have seen doing outdoor portraits with nothing to bounce their flash off of.
btw I am not saying your are doing this, but just incase you are.
When flash used individually is tilted it zooms the 50mm zoom position, not the wide angle 24mm setting. When flash is set to Master or Slave to zooms to 24mm regardless of head orientation. These automatic actions can be overridden manually via the 'zoom' button.
FEC = 0 is not "correct" exposure so get past that notion. FEC=0 is simply the camera's best guess based on its stock configuration. When modifiers are added it can't guess correctly. If as in your case you find exposure is consistently off by -1 stop when using the cap pointing forward simply move your starting FEC baseline from 0 to +1 when using it that way. For example I adjust the FEC baseline when using my DIY modifiers, starting from a baseline of +1-1/3 stops because that's what usually produces correct exposure.
In terms of cause and effect what cap diffusers do is split the light into key and fill components:
The mix and net % of output reaching the subject will vary with room characteristics and shooting distance but it usually is in the range of 40-50% vs using direct non-diffused flash. Outdoors where there is nothing to bounce the light about 80% of the light will be wasted if the flash head is aimed up.
Less is wasted if the flash is aimed forward outdoors (about the same 50% loss you are now encountering) and the cap does have one redeeming function in that situation: to will warm the color temp of the flash so it is a closer match to Daylight WB.
I find my approach of using a forward bouncing diffuser on a bracket most effective because the bracket at all times gives the flash a more naturally downward flattering angle. By moving the position of the top flap on the diffuser I can choose to direct all the flash output forward (which I do when using dual flash or shooting outdoors) or open the top flap and bounce most the light off the ceiling as the directional "key" component, with a much larger surface than a StoFen or LightSphere bouncing the fill forward and producing less specular reflections on skin and larger catchlights.
A characteristic of ETTL flash is that it is known to UNDERexpose when a softbox is placed on the front of the flash. But in testing by a forum moderator (not fredmiranda.com) and myself, we have also proven that when (variables uncertain as to which lens does what) the ETTL flash might OVERexpose in exactly the same situation. Tests were conducted with four different models of Canon dSLR, and using Canon and Metz ETTL flash units, and a number of different Canon lenses.
In any event, a flash bounced (with no softbox) works fine for exposure. Yet if you use a softbox, even if you elevate the flashhead to 15 degrees upward (so as to eliminate the distance information on the back of the flash), it does NOT affect the degree of underexposure (or the overexposure in those other circumstances). So in spite of ETTL preflash thru the softbox, ETTL does not appear to correctly factor the light modifier in place.
