Sigma 500 DG Super on 5D

Why is it that when my flash is in ETTL mode and pointed right at the subject (not being bounced off the ceiling), the range scale on the back of the flash reads 18m and totally blows out the exposure?

I've got the camera in 1/200 sync mode and using a 50mm f/1.4 in this particular test.

This blow-out doesn't happen when I use bounce, but then the range scale is inactive.

Is there something fundamental that I've missed here?

18 meters is as high as the distance scale goes.
The GN of that flash when zoomed to 50mm is 40 meters.
So the Distance scale should read 18 meters at around f/2.2 and ISO 100, or any aperture wider than that.

If you are getting blowouts it could be that you are too close to your subject, such that the required flash power is less than the minimum output of the flash.

Try a smaller aperture or a lower ISO.

If you are at ISO 100 and for some reason you don't want to use a smaller aperture, you can reduce the range of the flash by pulling out the wide panel or attaching a diffuser of some sort.

Diffusers don't need to cost money. Try toilet paper.

Many thanks for that Curtis, I think I've got a fair bit to learn before I get the flash on board. As it happens, I was set at ISO 800, which probably accounts for the overexposure. I need to get back and read your Flash 101 guide, I think.