Basic Flash Question about Shutter Speed AV Mode

[Hector1970]

Apologies for asking such a basic question for which I should really know the answer but its something I get caught out every so often over.
Usually when I use flash I'm in Manual or Tv.
Every so often I'm not paying attention and I'm in Av Mode.
Frustratingly in low light the camera goes to a very low shutter speed.
The photos look fine until you see them on a PC and you get movement blur

Some real examples recently
24-105mm at 24mm at ISO 400 F4 went to 1/10.
24-105mm at 56mm at ISO 400 F4 went to 1/5
The ISO wasn't fixed but I'm not 100% sure and can't confirm

If I accidently fixed the ISO I can understand the camera has no option but to reduce shutter speed because two elements are fixed.

If I didn't fix the ISO how does a Canon camera decide between ISO and Shutter Speed.
How low does it go on shutter speed and can you preset a lower limit based on the focal length of the camera or a fixed shutter speed.
Does it 50:50 the changes between ISO and Shutter speed or is it biased to changing one more than the other.

Ideally for me I'd like to set the camera so that regardless it doesn't drop below 1/100 shutter speed.
Can I set that as a custom function on the dial ie: C1 , C2, C3?

The camera was a 5DSR (which limits the ISO faster than the 5DIII).

 

[neuroanatomist]

There's a setting for Min shutter speed in AV mode. It's full stops so you have to pick 1/60 s or 1/125 s. It's on the same settings screen where you set the ISO range.

Edit: you mean with flash, when ISO is set to Auto and you mount a flash, it fixes ISO at 400. There's another setting (same screen, I think) for Flash sync shutter speed, default is auto which gives you a slow shutter (camera exposes for background, flash exposes for subject), other options force shutter speed to 1/60 - max sync or fixed at max sync (1/200 s for 5-series).

 

[Mt Spokane Photography]

Its called Fill Flash when you are use a flash and AV. The camera is trying to expose not only the foreground, but the background as well. The slow shutter speed is to expose the background, while the flash exposes the foreground.


Here is a more detailed article.

http://www.learn.usa.canon.com/resources/articles/2011/fill_flash_use_EOS_article.shtml

 

[Hector1970]

Thanks Mt Spokane.
I'll read that article.
I didn't know I could set a min shutter speed in Av.
I assume them I could set it in a custom too.