HELP! 580 EX exposure differences

I just got a 580ex! Yay except, Tonight, while working I noticed that the flash tended to underexpose shots when it was pointed directly at people and exposed better when pointed at a 45 degree angle. All pictures were through a soften diffuser.

Why am I getting such drastic differences? My 430ex seems to expose fine no matter what!

UPDATE: Post #8

Directly at people:

What lens? One thing I could think of, is that the distance information (If the lens supports that) is taken into account when the flash is used straight on.

PEBKAC. Someone please explain FEC and histograms, I don't have the time right now. Add brightness and lots of contrast and they'll be fine.

My 580 needs to have the FEC boosted up at least a stop. Try +1 or 2 FEC and make sure the zoom on the flash correspondes to the focal length on the lens (or put the flash zoom in auto).

You could try a bounce card rather than the stofen.

martinsmith wrote in post #5542385
My 580 needs to have the FEC boosted up at least a stop.

That's the way they're meant to work. Speedlites aren't automagic, they rely on information from the photographer to make a good exposure. If the photographer doesn't understand how the metering system of the speedlites works then the exposures will suck.

This isn't a comment about Martin, it's just information. Understand your tools. Martin knows how to get good exposures.

Rules of thumb:
- Dark suits FEC-1
- People FEC+1
- White wedding dress FEC+ 1 2/3 (varies a lot)

tim wrote in post #5542471
That's the way they're meant to work. Speedlites aren't automagic, they rely on information from the photographer to make a good exposure. If the photographer doesn't understand how the metering system of the speedlites works then the exposures will suck.

This isn't a comment about Martin, it's just information. Understand your tools. Martin knows how to get good exposures.

Rules of thumb:
- Dark suits FEC-1
- People FEC+1
- White wedding dress FEC+ 1 2/3 (varies a lot)

Yes, speedlights are not automagic. It is unfortunate, however, that Canon ETTL seems to be much more prone to underexposure in general, whereas TTL flash control or even Auto flash (using flash photosensor) was fooled far far less often. That seems to be true that dSLR flash is far less accurate more frequently, than TTL with film SLRs ever was!

General practice with ETTL is to dial in FEC of about 2/3EV or +1EV. Another thing is that ETTL is very often fooled by high reflectance surfaces which bounce back a lot of the preflash, leading the camera to think the scene is brighter than it really is!

The main thought that came to my mind that might be going on has to do with where you're metering. In my new venture (and Speedlite-heavy) into performance photography, I've had to really keep in mind where I meter so I know how to adjust the FEC to get a proper exposure.

Meter on something black or dark and the flash will try to compensate to make it 18% gray by brightening up the black.. esentially overexposing the rest of the image. FEC needs to be adjusted DOWN to make for a proper exposure.

Meter on something light and the flash will try to compensate to make the white or light-colored item 18% gray by easing up on the power of the flash.. essentially underexposing the rest of the image. FEC needs to be adjusted UP for a proper exposure.

Some more experienced photographers on this board may be able to explain things better and with more knowledge than I can, but so far.. keeping this in mind has really helped my exposures when I use my 580EXII. Also, I'm not sure if this has anything to do with how your images came out.. but it may help keep all the factors that have to do with small flash photog in better perspective as it has for me.

I wanted to follow up on this post. The next night, I was at another club taking pictures, heeding some things learned in here.

First: I set my 580ex to +2/3 exposure
Second: I angled the flash 45 degrees for most of my shots
Third: I played with the metering modes finding pretty good results in spot and partial metering

I had much better results. I think the biggest help was the +2/3 FEC on the 580ex.

Bounced with FEC about +1 is probably my most typical setting, but I really do go all over the place based on experience and what my histogram says.

I go into the custom functions and change the flash metering to average instead of the default evaluative. I found anything shiny or white fools the flash metering into thinking the whole scene has been properly exposed. In average metering I get more consistent results.

George