this is a not so smart question probably, but i cant figure out how to make the 580exII flash work on my 5d with a 24-70l lens. i have literally never used flash before, and i am doing what the manual says, but i can tge tit to flash automatically. any ideas as to what i might be doing wrong?
TIA
amnon

Isn't there a camera setting for internal vs external flash, where external can be disabled?

in the 5d? or on the flash?

In the camera...if that is set wrong, the external flash will never fire.

There are a lot of variables, but start with the basics. Put batteries in the flash. Turn on the flash -- without putting it on the camera just yet. Make sure it comes on, and the red ready light turns on. Hit the Mode button and cycle through the modes -- E-TTL, Manual, and Multiflash (stroboscopic). With the flash set on Manual mode, hit the ready light/test fire button and make sure it fires. (Don't look directly into the flash when you do this. Bad idea.)

If all that works, put the flash on the camera. Set the camera to Manual mode, say at 1/125 at f/5.6 at ISO 100. Turn the flash Mode dial to E-TTL. Focus on a nearby indoor subject and fire the camera. You should get a properly exposed flash photo. (Well, given the limits of E-TTL, maybe not properly exposed, but the flash should fire and you should be able to see the results on the camera.)

Note that the Canon world, there are large differences between Camera Mode settings when using an on-camera flash. Program, Av, Tv, and Manual all meter and act differently. Odd, but true. For a comprehensive look at the system, check out:

http://photonotes.org/articles/eos-flash/

There's a lot more to learn about flash photography -- I would highly recommend reading the Strobist blog:

http://strobist.blogspot.com/

You should start with Lighting 101:

http://strobist.blogspot.com/2006/03/lighting-101.html

Hope this helps.