Why I can't use my 580 flash on AV mode?

[Samsen]

Why I can't use my 580 flash on AV mode?

I need to have higher DOF by narrower F number.

[Tim_S]

I use my 580 in AV mode a lot. What are the settings and the situation you are trying to use it in?

[JasonTanaka]

You can but make sure you shutter speed does not exceed your shutter sync speed or set the speedlight to high speed sync. Note that hi speed sync will eat more power. 

[Samsen]

I am using two 580s.

The one on camera is set as master. Both same channel.

I set the apparture high so I would think sync speed should not be a problem but then let me rechech and will post more info with this in consideration.

My main reason to use on AV is to have a better DOF as set on Auto, Flash TTL everthing works well but very shallow deepth due to mostly chosen highest F stop. 

[stevepow]

should work - we shoot AV often -  high f should cause lower shutter speed - no high speed sync needed unless you are in blazing sun.

One thing to realize is that in most all shooting modes (AV, TV, etc) Canon wants to do fill flash - i.e., the camera tries to set the exposure so that the background is correct and uses flash to fill-in.   Thus, problems can occur in dark conditions where setting a high f would not be possible (shutter speed cannot go low enough to compensate for the narrow aperture) for correct background exposure.  You can over-ride this with manual settings, but will probably end up with a really dark background - that or you have to use to high ISO settings to allow for a correct aperture/shutter speed combo.