How to use 580EXII in Aperture Priority mode?

Flash newbie question.
I just got my 580EXII and experimented in a near pitch black room in Av mode. Using f/2.8, resulted in a painfully slow 2 second shutter speed! In Program mode with the same aperture, it was a more typical 1/60 seconds. How do i get my flash to work properly in Av? I know the flash is powerful enough to light up the whole room (as it did in Program mode) but why not Av mode?

It is working properly in Av.

Camera settings are for ambient light (but in P mode, it locks in minimum values) and the flash provides its own exposure for the subject.

If you want control over shutter speed and aperture, use M.

https://photography-on-the.net/forum/showthre​ad.php?t=138907

Go to M or lock the synch speed at 1/250 in Av which gives you the same functionality as M except that you have fixed the shutter speed at 1/200-1/250 depending on the camera.

cdifoto wrote in post #6937179
https://photography-on-the.net/forum/showthre​ad.php?t=138907

THanks cdifoto! Im digging in and doing the reading.... lots to learn! I thought ETTLII would make things easier

One thing nobody mentioned is hidden in the Custom Functions section of my 450D is this mode:

'Exposure Flash sync. speed in Av mode 0:Auto 1:1/200sec. (fixed)

When I set it to 1/200 sec, everything works perfectly! I get the Aperture i want and no blur from slow shutter speed. I know there must be something Im 'giving up' for this but i dont know what and it seems to work great!

If you're fixing your shutter speed at 1/200 and only adjusting your aperture while maintaining that constant speed aren't you doing the same thing as just putting the camera in manual with 1/200 and only adjusting aperture? Is there a difference in how the camera meters for the flash in Av vs M?

cueball wrote in post #7334651
Is there a difference in how the camera meters for the flash in Av vs M?

The camera never meters for the flash. Av, the camera is trying to meter for the ambient, in Manual, it's not metering anything -- you are in control.

purpletiger wrote in post #7334246
One thing nobody mentioned is hidden in the Custom Functions section of my 450D is this mode:

'Exposure Flash sync. speed in Av mode 0:Auto 1:1/200sec. (fixed)

When I set it to 1/200 sec, everything works perfectly! I get the Aperture i want and no blur from slow shutter speed. I know there must be something Im 'giving up' for this but i dont know what and it seems to work great!

You want your flash to do the work with ETTL. So, set your camera to manual mode with the settings you want (e.g. 1/200; ISO 200; f/5.6). The flash will determine the proper power for the exposure.

It will also using the camera fixed at synch speed.

The functioning of the camera/flash works exactly the same in M or Av... the difference is that in M you can change the shutter speed, in Av, you have fixed it at 1/60 or 1/250, for a 40D.

Gatorboy wrote in post #7335025
The camera never meters for the flash. Av, the camera is trying to meter for the ambient, in Manual, it's not metering anything -- you are in control.

The camera has to meter for the flash. How else is ETTL going to work if the flash is not being metered?

I think what cueball was getting at is that if the OP is going to fix the shutter speed at 1/200 when using a flash and manually adjusting the aperture in Av mode, then why not just set the camera to manual mode, unless there's a difference in the way ETTL behaves between Av mode and Manual mode. And, AFAIK, if the shutter speed is fixed at 1/200 via the custom function, then there will be no difference in how the flash is metered in either modes.

Gatorboy wrote in post #7335035
You want your flash to do the work with ETTL. So, set your camera to manual mode with the settings you want (e.g. 1/200; ISO 200; f/5.6). The flash will determine the proper power for the exposure.

Actually, it's the camera that determines the proper power for the exposure, not the flash. The ETTL (or ETTL-II) algorithm is built into the camera body, not the flash. That is the reason the 10D, 300D and earlier EOS models use ETTL for flash exposure whereas the 1D2, 20D and later models use ETTL-II, using the same flash unit.

Thank you PacAce that is exactly what I was getting at, or rather asking about myself in a round about way. So what I'm taking away from this is that the camera meters the same whether it's in M or Av and it will calculate the necessary flash power the same in either case (i.e. fixed SS of 1/200 and say aperture of 4.0).

Lets say you are in a room lit bu low powered lights. In Av your meter tells you that you need 1/60th f2.8 ISO800

By setting the flash to 1/200 and leaving the other settings as they are you are under exposing the background by almost 3 stops, Subject may look well lit but the background will be pretty dark.

Now set your camera to M and set the camera to 1/60th f2.8 and ISO800, The background will be properly exposed but will not be completely dark and your subject will be properly exposed. Move the SS to 1/125 and you'll darken the background slightly.

You can play around to get a look you like. By getting more ambient light in, your flash will use less power and will recharge quicker.

SS controls the amount of ambient light your camera lets in. Aperture controls the power.

The simple answer is that in M, Av, Tv mode your camera does not know the flash is there. In P mode it does and sets itself accordingly.

In other modes you have to set the camera to what you want.