Auto iso: why does my speedlight choose ISO400 ?

I use a 60D, at clubs I will go manual with my 30mm sigma, set the lowest shutter I can handhold comfortably, nice low aperture and let auto ISO do its thing. It works exactly how it should going from 100 to 3200 (locked at 3200) to obtain proper exposure. I would much rather use a flash but I don't always carry everything.

Auto ISO with a flash mounted wont go above 400 but it should drop to 100 if enough light is available. If you want higher ISO with a flash manual is the only way I know.

boerewors wrote in post #13089762
Is this ISO thing for speedlights only intended for shady areas or something. Because if thats the case i can understand why is because you can use a lower flash power, but when going against the mid day sun does it make any difference?
Thanks for taking your time to read this. Any enlightment will be apreciated.

1. Assuming ISO 400 and the use of a 'normal' lens in conjunction with 580EX...
   Sunny 16 for ambient light says 1/400 f/16...so we choose 1/200 f/22 permutation in order to be able to use flash. At ISO 400, the 580EX has a GN260 rating. So with f/22 on the lens we can shoot to about 12' while equalling the power of the sun.
   
2. Assuming ISO 100 and the use of a 'normal' lens in conjunction with 580EX...
   Sunny 16 for ambient light says 1/100 f/16...so we choose 1/200 f/11 permutation in order to still be able to use flash. At ISO 100, the 580EX has a GN130 rating. So with f/11 on the lens we can shoot to about 12' while equalling the power of the sun.

The 40D has a very limited auto ISO function. The range is ISO 400-800, unless the conditions will give you overexposure with ISO 400. If so, it will go down to ISO 100 (200 if highlight tone priority is active), to avoid overexposure. But it will first try the smallest aperture and/or the shortest exposure time available. In M mode it's always ISO 400.
This is all in the user's manual.

If you compare with the 7D, which is today's most advanced APS-C camera, then Auto ISO will start from ISO 100, and then go to anywhere between 400 and 6400, according to a setting you make.

On the 7d with a speedlight attached auto iso will only range from 100-400, same as 60d. With no speedlight they will range from 100-6400 as stated.

My guess, canon assumes if you have flash to loght the scene you won't need anything higher than 400, they are wrong, but that's my guess.

Set it in P mode and tilt the flash and you'll get ISO 1600. Normal is 400. That's of course because they have to get some consistency between the ambient and the flash, and it's too late to change the ISO when the pre-flash has been emitted, and before that you don't know which ISO you want. So they go for something reasonable, and selected 400 as "reasonable".

Thank goodness cameras still have M.