550EX vs 580EX zoom

One of the advantages of the 580 is that it uses the sensor size to determine how far to zoom the head. Today I was playing with my 550 and tried to do it manually, 300D, 50mm 1.8, 550EX, I set the apiture to 2.8 and manualy zoomed the head to 80mm in a dim interior room and the flash did not cover the intire area of the pic, I could tell it was too narrow. How does the 580EX determine zoom differently?

I'm not sure where your thinking is on this.

The 550EX reads the lens focal length and sets that to its flash zoom, regardless of any 1.6 or 1.3 factor in the camera body. Therefore, the only error that it makes is that the flash is wider than what the lens requires.

The 580EX does this a little smarter in that it reads the lens and also the body, so it should zoom the flash narrower than the 550EX would.

---Bob Gross---

That is my thinking exactly, the 550 reads the focal length...50mm, then zooms the head to 50mm.........but on the reble 50mm x 1.6 = 80mm, so I manualy zoomed the head to 80mm but the flash coverage was to narrow... my queastion is why. 80mm on the flash head should be exactly right for a 50mm lense on the rebel.