From the horses mou...erm, manuals...pages:
Pg. 56:
Auto ISO Range: Minimum 100; Maximum 12800
Pg. 125:
ISO Auto
If the ISO speed is set to "A" (Auto), the actual ISO speed to be set will be displayed when you press the shutter button halfway. As indicated below, the ISO speed will be set automatically to suit the shooting mode:
| Shooting Mode | ISO Speed Setting |
| A+ | Automatically set within ISO 100 - 12800 |
| P/ Tv/ Av/ M | Automatically set within ISO 100 - 26500 *1 |
| B | Fixed at ISO 400 *1 |
| With flash | Fixed at ISO 400 *1 *2 *3 |
*1: The actual ISO speed range depends on the
[Minimum] and
[Maximum] settings set in [Auto ISO Range].
*2: If fill flash will result in overexposure, ISO 100 or a higher ISO will be set.
*3: When using bounce flash with an external Speedlite in the
<A+> and
<P> modes, the ISO speed will automatically be set to ISO 400 - 1600.
As I get page 125, notes 1 and 2 indicate ISO will be set automatically within the full range, or if you have set Auto ISO Range limitations...within the range you have selected. In the case of bounce flash, it defaults to ISO 400 for third-party flashes, defaults to an ISO range of 400-1600 when using canon-brand Speedlight external flash, and WILL automatically select any ISO from 100 on if the camera metering decides the scene will over-expose.
From all that, it appears "Auto ISO" is pretty much indeed COMPLETELY automatic within the FULL RANGE of possible native ISO settings. The only case that seems limited outside of the context of exposure issues is Auto ISO + Flash when using a third party (i.e. non-Speedlite) brand.
Pg. 128
Setting the Minimum Shutter Speed for Auto ISO
When Auto ISO is set,
you can set the minimum shutter speed (1/250th sec. to 1 sec.)
so that the automatically set shutter speed is not too slow. This is convenient in both <P> and <Av> modes when you use a wide-angle lens to shoot a moving subject. You can minimize both camera shake and subject blur.
As I understand page 128, Canon is ALLOWING YOU to SET the minimum shutter speed to SOME SETTING between 1 second and 1/250th of a second. The goal would be to prevent a shutter speed that is TOO SLOW when using P or Av modes, where both the shutter speed and ISO setting would be automatically selected. I see no reason whatsoever to use a shutter speed slower than 1 second. A
minimum shutter speed of 1/250th of a second seems entirely reasonable to me. I would rather have a slightly lower minimum shutter speed than be unable to get a shot because the camera can't set a higher ISO and can't set a shutter speed long enough to capture the shot.
According to page 224, there are some additional restrictions on Auto ISO Range, Min. Shutter Spd. and ISO Speed Settings WHEN SHOOTING VIDEO. This makes a lot of sense since you pick a specific shutter speed when you choose your video frame rate (i.e. 24/23.9fps). It also makes sense to limit auto ISO, as it cannot be varied smoothly...it can only be varied in 1/3rd stops. Having ISO constantly jitter around in 1/3rd stop jumps would be incredibly annoying for those watching your videos. It would be a hell of a lot better to set ISO manually as needed, and if necessary apply an exposure ramp during post processing to normalize exposure smoothly.