Aperture blinking???

Guys, I attached my 580EX onto my 20D (fitted with 16-35 2.8L) set to 35mm used in shutter priority Tv mode.

No matter what shutter speed i set (between 1/60 to 1/250), the aperture value of 2.8 keeps blinking. The ambient lighting in my room are those ring-type daylight fluorescent light.

Is this normal? Is the camera trying to say that the ambient lighting of the room is still too dark?

Yes, it is pretty normal.

The camera can sense the flash unit, but it has no idea at all what distance the flash must cover, so it does not try to count on anything specific about the flash result. Then when you push the button, the pre-flash will happen, the reflected light from it is measured, and then the main flash happens. Normally, there will be a decent flash exposure.

---Bob Gross---

does the preflash always occur? i don't remember that any preflash occuring when i half-press the shutter.....

The preflash occurs when you fully press the shutter, and it's too fast for most people to see. If you press the * button it'll go off right away and lock the exposure. This is in ETTL/ETTL2, in manual it's just a dumb light.

I never said that the preflash happens when you half-press the button.

---Bob Gross---