ISO Limit During Flash

[Poyer]

After much research regarding my EOS 60D, I often find the general statement that when operting in Av (or Tv) mode and flash, the camera will meter for the ambiant light and the flash will be set to fill the subject.  Thus, the camera/flash system will work to properly expose the background and the subject.  This is true enough if one allows, through the custom functions, impossiblely long shutter speeds.  If one limits shutter to the range of 1/60 to 1/250, the ability to properly expose the background is very limited by the apparent fact that Auto ISO is limited to an upper value of 400 during flash photography.  I fully understand the relationship between noise and ISO and have done much testing of the 60D to map this relationship.  But the upper limit at 400 in Auto ISO seems counter intuitive and counter productive.  Any idea why this limit is set?

[Skirball]

That seems low to me, but I would assume it has something to do with the minimum power setting of the flash.  If the exposure is too sensitive (high ISO) then even the lowest powered flash would over-expose the shot.  But I'd have expected it to be a higher ISO than 400, and also variable, as the aperture setting and subject distance will affect the flash exposure as well.

[diverhank]

When using Flash don't limit yourself to Auto ISO - the ETTL will take care of the auto part, crank the ISO up to whatever you like.  Auto ISO will park at 400 for many cameras (my  5DIII does this). Canon apparently chose to fix auto ISO at 400 with flash... it doesn't matter to me as long as I can change it. 

[Poyer]

Thanks for the confirmation of my observation.  Your proposed solution is a good one, but one that should not be forced by an unnecesary and arbittray decision by Canon engineers.  Non-flash auto exposure protocols freely permit ramping up ISO to whatever limit is set by the user.

[Skirball]

As a side note, eTTL allows a wonderful opportunity to learn the use of Manual mode.  The flash takes care of your subject exposure so all you have to worry about is getting your background exposure to whatever you want and let the eTTL take care of the rest.  I think you’ll find the results much more consistent than using Av more with flash.  And unless the background is changing a lot, I think you’ll find M+eTTL just as flexible as a semiautomatic mode like Av or Tv.