Canon 580 EX II - Stop Down -1 1/3 ?

In manual mode how do you stop it down -1 1/3 I am not sure how to read or set the display to this?

If you are going from full power. 1/1 the display will read 1/2 -.3. If turning the power up from 1/4 power it will read 1/4 +.7
1 stop is cutting the power in half. Then each 1/3 stop will read plus or minus .3 or .7

thanks, some reason I am still confused what would the display show if I wanted say minus a 1 stop and 1/2?

The adjustment steps of the flash are in 1/3 stops. I do not remember if you can change to 1/2 stops or not. The difference between -1/3rd and -1/2 is not going to be very noticeable.

These are really just "steps"of power, not "stops" of light. They could equal stops of light, but that would depend on the ISO and aperture you use as well.

What you are doing is just setting the manual power from 1/1 to 1/1 -.3 to 1/1 -.7 to 1/2 in three steps. On the way back up from 1/2 the steps are 1/2 +.3 to 1/2 +.7 then 1/1.

I would assume that 1/2 +.3 is really 1/2 +.3333 which is equal also equal to to 1/1 -.7 which is really 1/1 -.6666 (confused yet?)

There is no custom function to set the flash to 1/2 steps. It always changes power in 1/3 steps if in manual mode.

From the manual: "You can set the flash output from 1/128 power to 1/1 full power in 1/3stop increments."

Canon's instruction on reading the output is as confusing as the reading itself. Fortunately, they add to 40D and later DSLRs a "Flash Control Menu", which displays on the camera's LCD a very clear and easy picture.

If you really want to split hairs on terminology I guess you can argue steps and stops. That still does not change the fact that when you go from 1/1 to 1/2 you reduce the light by 1 stop. Since there are 2 intermediate steps between those two values that works out to 1/3 stops. Just like when you click the wheel on your camera 3 clicks it changes the value by 1 stop. Each click is 1/3rd stop. The flash is the same.

Of course you are right. I rarely use manual mode flash and I just forgot that going from 1/1 to 1/2 power also does equal 1 stop of light.