5D3 and an original 580ex flash. not working so well

talico wrote:

You can't control the flash through the menus in the camera, but you can control it via the options on the back of the flash. Only the 580EXII and later have that feature.

I have always had best luck in manual mode. There is definitely a learning curve, but it isn't too bad. The flash duration is so short, there is little effect of changing shutter speed, but keep it below 1/200 of a second or else you will get a black bar on the photo where the shutter passing through blocks some of the flash exposure. If your ambient is too bright for a setting of 1/200th Shutter speed, then make sure you are at ISO 100 and you may need an ND filter.

Set your Aperture and Shutter speed(below 200) and ISO so that you get they ambient that you want. Then use the flash exposure compensation to adjust the lighting on your subject. The two(ambient and subject exposure) work independently in Manual mode.

Hope that helps.

Tom