The 5DIII currently has a minimum shutter speed setting that you can register to prevent your shutter speed from dropping below a specific shutter speed.
Currently that shutter speed maxes out at 1/250th of a second.
This, in my opinion renders Auto-ISO fairly useless for shooters like myself who shoot frequently with 300mm and 600mm. This is not nearly a fast enough shutter speed for long lenses nor is it a fast enough shutter speed to freeze any sort of actions (sports, wildlife, birds in flight, planes etc...)
I do believe that canon listens to their customers (as is seen with the latest firmware update on the 1DX allowing for f8 autofucs) so for those of us in the community who have a 5DIII and would like to see this "min fast shutter speed" increased... lets make some noise.
I believe that the minimum fastest shutter speed setting should be much faster. At least through 1/2000 sec, but why not the entire range through 1/8000. Also the increments should be in 1/3 stop not just 1 full stop.
I emailed canon about this today as this feature is my only real complaint on this excellent camera. Canon emailed me back very warmly and the representative suggested that I keep watching for future firmware updates which may address this issue.
So: Write to canon or call, or post here that we would like to see 1/3 stop increments of minimum shutter speed registration all the way though the range that the shutter can handle.
Thanks and happy shooting!