Av-priority not working as expected with E-TTL flash

[hairy_moth]

On a 50D with an old 420EX Flash (which is E-TTL), I am not seeing the expected behavior when shooting in Av-priority mode.

As I understand it, in Av-priority I should be able to set the aperture, and the camera will choose a speed between 1/30 and 1/x (1/x is the maximum flash sync speed) and adjust the flash output accordingly.

But when I shoot in Av-priority; it is setting ridiculously slow shutter speeds (like one half second).

Here are two pictures I took the other night; everything is the same except one was taken in Av-priority and the other in Program mode.

What have I done to allow the camera to choose such a long shutter speed in Av-priority? (I have left the EXIF info in tact. These are right off of the camera, converted to JPEG with DPP and cropped with ACDSee)

http://farm3.static.flickr.com/2769/4077649139_90feb43618.jpg (http://photography-on-the.net/forum/%3Ca%20href=)"> ">http://farm3.static.flickr.com/2443/4078402400_21be195f67.jpg (http://www.flickr.com/photos/44350103@N05/4078402400/)
http://www.flickr.com/photos/44350103@N05/4078402400/

[tkbslc]

AV meters as if there is NO flash present and then uses the flash for fill only. That is why it picked 1/2 second. It was metering for the dark.

It is almost always advisable to use M mode with flash when you want aperture control. Just set the aperture, pick an appropriate shutter speed (1/80 usually works as a starting point for me) and the flash will adjust output. If you just want to snap away, use P mode and it will almost always pick f4 and 1/60.

[hairy_moth]

That is right.. I was mis-understanding the documentation.
Thanks!

[FiveRings]

There should be a custom function that you can set to force it to default to 1/200 (or whatever your max sync speed is) with external flash. Otherwise, you have to go manual.

[Fastfwd13]

There should be a custom function that you can set to force it to default to 1/200 (or whatever your max sync speed is) with external flash. Otherwise, you have to go manual.

Would'nt going in Tv mode do that?

[FiveRings]

Would'nt going in Tv mode do that?
Setting the custom function would allow you to change aperture while maintaining that shutter speed. I don't have it with me, but on my T1i, the settings are something like...

1: choose whatever speed (this is default and causes the camera to pick a very slow shutter speed)
2: 1/60-1/200
3: 1/200

(Obviously not the exact wording.)

So you keep it in Av and the camera chooses a shutter speed based on that setting, vs Tv mode or manual.

[hairy_moth]

I'll be fine with it the way that it is; in fact I like the feature because I was also never quite sure what the best way to get proper fill flash was either.

I was just confused by what I read in my camera and flash manuals. I was under the impression that the E-TTL flashes behave very much as the built in flash; which in this case, it does not. (just jotted a note in the manuals)

So from now on I will shoot flash in manual, starting at 1/125 and adjust the aperture as appropriate.

Thanks!