Underexposed flash pictures: question re a specific scenario

[kevinma]

Today I was taking pictures with my 20D and built-in flash at a kids birthday party held in a bowling alley. I took pictures of the kids at around 12 feet with the dark bowling alley in the background and they are all underexposed. The shots were on P and the maximum aperture of the lens 3.5; ISO 100. All other flash pictures (all using FEC+1) were OK. The kids are all in focus. The subjects were within the range of the flash but somehow the flash got fooled and underexposed - I assume. What went wrong? What should I have done differently?
Kevin.

[robertwgross]

What I would have done differently is to fire one or two test shots, then review the histograms. If they need some brightening, then I correct that in FEC and try again.

---Bob Gross---

[Tom W]

12 feet in a very dark setting could cause problems with the built-in flash at slower ISO settings, say ISO 100. Make sure you're at ISO 400.

Also, make sure that the camera's battery is fully charged if you're going to make extensive use of flash.

[scottbergerphoto]

The 20D's internal flash has a Guide Number of 43ft at ISO 100. At f/3.5, the maximum distance it can cover is 43/3.5 = about 12ft. So if your lens was wide open, you were just about at the limit of the flash. You could have doubled the range by going to ISO 400. In addition, there is a tendency of Canon's ETTLII to underexpose. See the link:
http://www.robgalbraith.com/ubbthreads/showflat.php?Cat=&Number=247684&page=&view=&sb=5&o =&fpart=all&vc=1
Regards,
Scott