AV Mode & Off Camera Flash

[Sadlybrokeboy]

All,

A rather stupid question (having a VERY blonde day today!).....

I'm trying to use the 580 EX with my 40D in AV mode and the flash seems to fire Differently that when using P or TV or Manual Mode....

Just wanting to understand why - is it because in AV mode it's metering differently to TV mode etc.?

Any simple explanations appreciated!

Cheers,

Matt

[Chappers]

Can you be a bit more specific. In what way is it different .

I can understand things being different in P mode, as the camera is trying to optimise what it thinks is the correct aperture and shutter speed.

Also with the flash attached the camera will limit the shutter speed to 1/200 or less ( I think that's the max flash sync speed on a 40D.) Could this be what's happening

[Sadlybrokeboy]

It's almost like the flash is running in 2nd Curtain mode.

I think I know why now ***' - I think it's because in AV mode it has to work out the shutter speed, which is calculates using the ambient light and it assumes the flash isn't there - it then uses this to time when the flash is required....

[markw08]

Is the flash on or off camera ???

If off what are you using to trigger it...

[Sadlybrokeboy]

On camera :)

[admirable]

what is the shutter speed?

[markw08]

Sorry to ask but your title said off camera....

Is your flash set to TTL mode...

What aperture were you set to...

:thumbs:

my next question was one above...

[Chappers]

If you think the flash/camera is working in Second curtain mode, check that both the camera and flash don't have this option enabled.

Also if you are using the flash in off camera mode how are you syncing the flash. extension cable or remote trigger. If it's remote trigger, i've found that the inexpensive radio triggers can be a bit unreliable and don't sync at some of the higher speeds.

Try the flash on the camera with all flash setting returned to factory default and see if that makes any difference.

[alastairb]

when in av and tv mode the flash exposes for the background ambient light to make that exposed right and uses the flash to expose the forground correctly. if you want a short shutter and flash and dont care about the background chuck the camera in manual on a shutter speed between 1/60 and 1/250 and an f stop for the dof you want and the camera will just expose for the foreground.

[trevorbray]

Could simply be that in AV mode the camera is metering for the AVAILABLE light. Check your custom functions to see if you can change the flash sync speed in Av mode. Should be able to change from Auto to a fixed speed, probably 1/200th. (On the 7D it is C.Fn 1:Exposure) I know you've got the 40D but just to give you an idea where to look.

[alastairb]

yea but why would you turn that off, as then its no different to going into manual and selecting 1/200 and any appature you want when you are using flash and it means that if you want a fill in flash/balance when there is just a touch too low ambient you can still use it for that purpose?

[trevorbray]

You're absolutely right Alistair, just trying to explain to the OP why his 40D is behaving the way it is when in AV. Also offering up a simple solution that worked for me.

[Sadlybrokeboy]

Sorry - been away from the PC a bit and haven't been able to reply....

Anyway Alistair - I think you've hit the nail on the head - I did some playing around and yes in AV mode it meters for the available light as it doesn't "know" what shutter speed to send to the flash I guess!

To be honest, I don't normally use AV mode for flash work but just wanted to understand why it behaved that way - I have to understand things sometime, however useless!! ;)

Many thanks for all for your help - learn something new everyday I guess!!

Matt